Investigating the Interaction Mechanisms of Clostridium difficile Toxins with Host GTPases: A Bioinformatic Approach

Investigating the Interaction Mechanisms of Clostridium difficile Toxins with Host GTPases: A Bioinformatic Approach | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Investigating the Interaction Mechanisms of Clostridium difficile Toxins with Host GTPases: A Bioinformatic Approach Mohnad abdalla, Maaweya E. Awadalla, Uwem Okon Edet, Reham M Alahmadi, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6870900/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Background Clostridioides difficile is frequently implicated in colitis and antibiotics-induced diarrhoea in both community and hospital settings around the world, and there reports of resistance to the antibiotics of choice used in the management of its infections. Yet, there is limited information on the structural dynamics of its toxins TcdA and TcdB that could guide potential therapeutic candidates. Aim We undertook a structural insights study into the glucosyltransferase domain (GTD) of Clostridioides difficile toxin A (TcdA). Methods Structural analyses and molecular dynamics simulation (250 ns) were carried out for the various TcdA glucosyltransferase domain of 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY. Results our findings highlights conserved catalytic residues (e.g., Trp101, Glu514), Mn²⁺ coordination, and substrate-binding motifs. Key residues (Lys428, Glu381) were shown to mediate RhoA engagement, while small-molecule and antibody inhibitors targeted both active and allosteric sites. Molecular dynamics revealed RMSD increases to ~ 3.6 Å across five TcdA structures over 250 ns, indicating intrinsic conformational flexibility. Complexes with RhoA or inhibitors showed altered contact profiles and dynamic behaviour, supporting functional plasticity. Principal component analysis (PCA) revealed that the ternary complex 7U2P exhibited the highest global motion, suggestive of catalytically relevant conformational changes. Contact frequency analysis confirmed stable ligand engagement in active complexes and disruption in inhibitor-bound states. Conclusion These findings underscore the structural adaptability of TcdA’s GTD and reveal potential therapeutic targets through inhibition of conserved residues or conformational states essential for substrate recognition and catalysis. Biological sciences/Cell biology Biological sciences/Computational biology and bioinformatics Biological sciences/Drug discovery Clostridioides difficile Toxins Bioinformatics Simulation Therapeutics Conserved amino acids Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Introduction Clostridium difficile , now known as Clostridioides difficile , is a Gram-positive, spore-forming bacterium. The bacillus is an obligate anaerobe that inhabits the gastrointestinal tract of humans 1 , 2 . The bacterium is the leading cause of diarrhoea following the administration of antibiotics in either community or hospital settings in developed nations, and its infection presents with high morbidity and mortality, as well as a significant societal and financial burden 3 . In addition, it is also implicated in colitis globally. C. difficile is well known for its ability to secrete two large exotoxins, toxin A (TcdA) and toxin B (TcdB), which are considered its virulence or pathogenic factors 1 , 2 . Both toxins are glycosyltransferases capable of inactivating small GTPases within host cells via monoglucosylation, which leads to the functional inactivation of Rho GTPases and causes disruption of the actin cytoskeleton, leading to cytoskeletal breakdown, cell rounding, and eventual cell death 1 , 4 , 5 . Recently, surveillance, prevalence and epidemiological studies collectively indicate an increase in cases of C. difficile infection (CDI) across the globe 6 , 7 . The 21st century has witnessed a marked increase in CDI cases across the world with cases now reported in African countries where it has been shown to drive mortality in patients diagnosed with tuberculosis 8 , 9 . The changing epidemiological landscape of the bacterium is characterised by increased incidence and severity, occurring at a disproportionately higher frequency in older patients 9 . Although its infection was previously considered to be nosocomial, cases are now reported in community settings that were previously labelled as low risk. There has been an increase in the prevalence of CDI 10 . The increased prevalence and severity associated with CDI correlates with the emergence and rapid spread of a previously rare strain, ribotype 027 or NAP1, which shows high resistance to fluoroquinolone antibiotics 11 , 12 . Raising public health concerns. Several antibiotic classes, including the first and second generations of fluoroquinolones, have been shown to induce CDI. The current treatment options include the administration of antibiotics such as metronidazole, vancomycin, and fidaxomicin. However, even for these first-line treatment antibiotics, resistance has been reported 13 , 14 . This underscores the pressing need for new therapeutic strategies and vaccines to effectively manage the pathogen 15 , 16 . To achieve the development of newer and safer therapeutics, studies have aimed at understanding the structure and functional mechanisms of TcdA and TcdB 17 . The TcdA and TcdB toxins are structurally composed of four domains, namely the N-terminal glucosyltransferase domain (GTD), a cysteine protease domain (CPD), a delivery and receptor binding domain (DRBD), and combined repetitive oligopeptides (CROP) 1 . The glucosyltransferase domain of the protein is responsible for the catalytic inactivation of small GTPases 18 . The GTD component of the toxins drives the glucosylation of host GTPases, which impairs critical signalling pathways that regulate cellular functions 19 , 20 . Structural studies have revealed that the GTDs of TcdA and TcdB share a similar overall architecture, including a conserved glycosyltransferase fold 1 . Furthermore, significant differences in their surface properties and substrate-binding regions suggest divergence in their enzymatic activities. For instance, while both toxins glucosylate Rho family GTPases, TcdA has also been shown to modify Rap family GTPases, which is not the case for TcdB. These differences in target specificity may underlie the varying clinical manifestations associated with infections caused by different strains of C. difficile 1 . The necessity for effective therapeutic interventions against C. difficile infections has spurred interest in the development of small-molecule inhibitors targeting the GTD. Inhibitors can potentially block the enzymatic activity of these toxins, thereby preventing the glucosylation of host proteins and mitigating the toxic effects of the bacterium. The identification of such inhibitors requires a thorough understanding of the structural dynamics and interaction mechanisms of the GTD with its substrates 1 . Recent advances in structural biology have facilitated the elucidation of the co-crystal structures of TcdA and TcdB GTDs in complex with their respective substrates 21 . These studies have provided insights into the conformational changes that occur upon substrate binding and have highlighted the role of various residues in substrate recognition and catalysis. For instance, the identification of critical residues involved in interaction with GTPases has paved the way for targeted mutagenesis studies aimed at further understanding the functional implications of these interactions 22 . Moreover, understanding the structural and functional differences between TcdA and TcdB can provide insights into their distinct pathogenic mechanisms, specifically their substrate specificity and activity levels. For instance, TcdA has been shown to glycosylate a broader range of GTPases than TcdB, which may have implications for its role in disease severity and the development of potential therapeutic interventions. The studies also highlight the importance of accessory factors such as the cofactor Mn²⁺ and the substrate UDP-glucose in modulating the activity and stability of these toxins. To assess the potential differences in structure and functional reactions that can arise when TcdA interacts with diverse substrates and proteins, we examined five structural complexes. These were 5UQL 23 , 3SZA 24 , 7UBY 1 , PDB 7U2P 1 , and 4DMW 22 . Specifically, we assessed the interactions between human ALDH3A1 (aldehyde dehydrogenase) and the TcdA glucosyltransferase domain (3SZA), the interactions between the TcdA domain bound to Mn²⁺ and UDP (4DMW), TcdA in complex with human RhoA (7U2P), a host or human GTPase, and TcdA bound to a VHH antibody (7UBY). Methodology Retrieval of Study Protein and Ligands The proteins 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY were retrieved from the RCSB (Research Collaboratory for Structural Bioinformatics) database hosted at https://www.rcsb.org/ . Specifically, the proteins were: 5UQL, the TcdA glucosyltransferase domain ( C. difficile ); 4DMW, the TcdA domain bound to Mn²⁺ and UDP; 7U2P, TcdA in complex with human RhoA; 7UBY, TcdA in complex with a VHH antibody; and 3SZA, human aldehyde dehydrogenase ALDH3A1. The selection of the various proteins was done to allow for the exploration of diverse reactions of the TcdA with different substrates. Molecular Simulation (MS) The retrieved proteins were subjected to a 250 ns molecular simulation using the Desmond tool, with parameters set to default as previously reported 25 , 26 . The Desmond suite was chosen as it supports long molecular simulation runs between proteins. To achieve solvation and neutrality in the system, the TIP3P water model and a sodium chloride (NaCl) concentration of 0.15 M were set, respectively. To attain equilibrium, the temperature and pressure were set at 310 K and 1.013 bar. The boundary of the simulation was defined by an orthorhombic box, while the force field utilised was the Optimised Potential for Liquid Simulations (OPLS). The results of the simulations were visualised and analysed using the Desmond Virtual Molecular Operating Environment (DVMOE) and in PyMOL 27 . Results Structural interactions of TcdA and various substrates, cofactors, and inhibitors The molecular interactions between the glucosyltransferase domain (GTD) of C. difficile toxin A (TcdA) and various substrates, cofactors, and inhibitors revealed diverse interactions (Fig. 1 ) revealed by molecular docking. The various panels represent different high-resolution crystal structures obtained from the Protein Data Bank (PDB), with key amino acid residues, substrate analogues, and coordination of manganese ions (Mn²⁺) highlighted to illustrate the enzymatic mechanism and potential sites for therapeutic targeting. As shown in Panel A (PDB ID: 5UQL), the GTD of TcdA bound to the uridine-based substrate analogue U2F. Key interacting amino acid (AA) residues include Ile102, Trp101, Val286, Asn138, Trp519, Asp285, and Arg272. Mn²⁺ is coordinated by Asp285 and Glu514, while substrate stabilisation is facilitated by hydrogen bonding with Asn138 and Trp519. Panel B (PDB ID: 3SZA) shows the GTD bound to uridine diphosphate glucose (UPG), a natural sugar donor. Key interacting AA residues were Asn383, Asp287, Glu514, Val286, Tyr283, Arg272, Trp101, Leu518, and Asn138. The manganese ion (Mn²⁺) is coordinated primarily by Asp287 and Glu514. There is also the presence of Hydrogen bonding with Asn383 and Asn138. Panel C (PDB ID: 4DMW) displays the glucosyltransferase domain (GTD) in association with uridine diphosphate (UDP). Critical AA residues implicated in this interaction include Asp287, Val286, Tyr283, Arg272, Trp101, and Ile102. The Mn²⁺ ion is coordinated primarily by Asp287, facilitating stabilization of the UDP molecule through surrounding residue interactions, especially with Tyr283 and Arg272. Panel D (PDB ID: 7U2P) illustrates the GTD bound simultaneously to UPG and its target protein RhoA. The interaction involves a broad network of AA residues that includes Ile102, Asn138, Trp101, Leu518, Trp519, Ser520, Asp462, Lys432, His431, Lys428, Asp65, Glu40, Glu381, Val381, Phe39, Thr491, Asn41, Gly379, Gln29, and Lys27. Coordination of Mn²⁺ is mediated by Glu514 and Asp285. Panel E (PDB ID: 7UBY) captures an alternative conformation of the GTD–UPG complex, shedding light on structural dynamics during the catalytic cycle. The key interacting AA residues include Val286, Leu518, Ile102, Trp101, Trp519, Tyr283, Arg272, Gln384, and Pro470. As in other complexes, Mn²⁺ coordination by Asp285 and Glu514 is preserved. Also noted was the presence of hydrogen bonding between UPG and residues such as Asn138 and Trp519. Also, panel E (PDB ID: 7UBY) shows the GTD interacting with AH3, a small-molecule inhibitor. The inhibitor binds within a pocket comprising AA residues Tyr189, Tyr112, Ser110, Gly113, Lys114, Gln248, Asn194, Asn198, Val193, Ala98, Pro196, Lys195, Asp115, Arg100, and Asp203. Structural interactions revealed by x-ray crystallography Figure 2 presents the structural interactions between the glucosyltransferase domain of C. difficile toxin A (TcdA) and various ligands, including its natural substrate, a small-molecule inhibitor, and a protein inhibitor. The panels display crystallographic structures obtained from the Protein Data Bank (PDB), highlighting the molecular mechanisms underlying substrate recognition and inhibition. Panel A (PDB ID: 7U2P) illustrates the ternary complex of TcdA glucosyltransferase (green) bound to uridine diphosphate glucose (UDP-glucose, UPG; magenta) and its substrate RhoA (pink), with a catalytic manganese ion (Mn²⁺) positioned at the active site. The UPG molecule serves as the glycosyl donor, while RhoA functions as the glycosyl acceptor. In Panel B (PDB ID: 7UBY) the structure shows TcdA glucosyltransferase (green) in complex with a small-molecule inhibitor, AH3 (cyan), which occupies the UPG-binding pocket. The positioning of AH3 overlaps with that of UPG (magenta), indicating competitive inhibition. The Mn²⁺ ion is also present in the active site. Panel C (Protein Inhibitor Complex) represents TcdA glucosyltransferase (green) bound to a proteinaceous inhibitor, along with a chemical inhibitor, likely similar to AH3. Summary of the AA residues involved in the various interactions Table 1 presents the presence or absence of selected amino acid residues in five crystallographic structures of the C. difficile toxin A (TcdA) glucosyltransferase domain, as indicated by their respective Protein Data Bank (PDB) entries: 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY. Residues marked with an asterisk (*) denote its presence in a given structure, implying potential involvement in structural stability or enzymatic function. Conserved residues Trp101, Ile102, Arg272, Val286, Leu518, and Trp519 were consistently identified in all five structures. Their invariant presence shows their potential role as components of the catalytic core. Partially conserved residue Asn138 was observed in four of the five structures (5UQL, 3SZA, 7U2P, and 7UBY). Similarly, Tyr283 appeared in three structures (3SZA, 4DMW, and 7UBY), potentially indicating its involvement in non-essential but relevant interactions. Residues Gln384 and Pro470 were found only in the 7UBY structure. Less conserved residues Asp258, Asp287, Asn383, and Glu514 were detected in one or two structures. For example, Asp258 was exclusive to 5UQL, whereas Asp287 was found in 3SZA and 4DMW. Partially conserved residues included Asn138 and Tyr283, while Gln384 and Pro470 was present in a single structural state. Table 2 presents a comparative assessment of specific amino acid residues identified in two crystal structures of the C. difficile toxin A (TcdA) glucosyltransferase domain, corresponding to Protein Data Bank (PDB) entries 7U2P and 7UBY. The comparison focuses on residues presumed to be functionally or structurally significant within each conformation. Residues marked with an asterisk (*) indicate its presence in the respective crystal structure. The following residues were observed exclusively in the 7U2P structure: Lys172, Gly379, Val381, Lys428, His431, Asp432, Lys448, Arg462, Thr491, and Ser520 close to or within its active site. In contrast, residues Ala98, Arg100, Ser110, Tyr112, Gly113, Lys114, and Asp115 were observed solely in the 7UBY structure. There was an absence of overlapping residues between 7U2P and 7UBY implying distinct conformational states of the TcdA glucosyltransferase domain. Table 1 Summary of the presence or absence of specific amino acid residues in various crystal structures of C. difficile toxin A (TcdA) glucosyltransferase 5UQL 3SZA 4DMW 7U2P 7UBY Trp101 * * * * * Ile102 * * * * * Asn138 * * * * Arg272 * * * * * Asp258 * Tyr283 * * * Val286 * * * * * Asp287 * * Asn383 * Gln384 * Pro470 * Glu514 * * Leu518 * * * * Trp519 * * * * Table 2 Comparison of the presence of specific amino acid residues in two different crystal structures of C. difficile toxin A (TcdA) glucosyltransferase, represented by PDB IDs 7U2P and 7UBY. 7U2P 7UBY Ala98 * Arg100 * Ser110 * Tyr112 * Gly113 * Lys114 * Asp115 * Lys172 * Gly379 * Val381 * Lys428 * His431 * Asp432 * Lys448 * Arg462 * Thr491 * Ser520 * Molecular simulation dynamics evaluations of the various interactions Figure 3 presents Root Mean Square Deviation (RMSD) trajectories over 250-nanosecond molecular dynamics (MD) simulations for both protein and ligand components of C. difficile toxin A (TcdA) glucosyltransferase in complex with various substrates or inhibitors. Each subplot corresponds to a distinct PDB ID, reflecting different binding scenarios. RMSD values, plotted against simulation time (ns), offer insights into the structural stability and conformational dynamics of the protein and associated ligands. The protein RMSD, represented by a blue line, reflects backbone (Cα) atomic deviations from the initial conformation and serves as an indicator of global structural fluctuations. The ligand RMSD, shown as a red line, captures atomic positional changes of the ligand relative to its initial pose, providing a measure of binding stability. All simulations were run for 250 ns to ensure sufficient sampling of conformational space. For PDB ID 5UQL (U2F), the protein RMSD increased gradually from approximately 1.0 Å to 3.6 Å, indicating notable conformational shifts. The ligand U2F remained stably bound, as evidenced by consistently low RMSD values. In PDB ID 3SZA (UPG), the protein RMSD followed a similar trend, rising to around 3.6 Å, while the UPG ligand also demonstrated minimal fluctuation, signifying tight binding. For PDB ID 4DMW (UDP), a more pronounced initial deviation was observed in the protein RMSD, which rose from about 0.3 Å to 3.6 Å, suggesting higher flexibility. The UDP ligand exhibited slightly increased but still stable RMSD, indicating modest mobility within the binding site. In the case of PDB ID 7U2P (UPG + TcdA + RhoA), the presence of the host protein RhoA appeared to enhance protein flexibility, as shown by the protein RMSD rising from 0.8 Å to 3.6 Å. Despite these structural changes, the UPG ligand remained firmly bound. Similarly, for PDB ID 7UBY (UPG + TcdA + AH3), the protein exhibited increasing RMSD values reaching approximately 3.6 Å, consistent with conformational adaptation in the presence of the inhibitor AH3. The UPG ligand remained stable, indicating persistent binding throughout the simulation. Overall, all five complexes demonstrated increasing protein RMSD over time, reflecting substantial conformational flexibility of the TcdA glucosyltransferase domain. In contrast, ligand RMSD values remained relatively low and stable across all simulations, indicating that the ligands (U2F, UPG, UDP) maintained strong binding affinity regardless of the structural variations observed in the protein. The presence of additional interacting partners, such as RhoA or AH3, influenced protein dynamics but did not compromise ligand stability. Figure 4 illustrates the temporal profiles of protein–ligand contacts during molecular dynamics (MD) simulations of the glucosyltransferase domain of C. difficile toxin A (TcdA) in complex with various ligands. Each panel corresponds to a distinct PDB entry, representing different structural complexes of TcdA with substrates, cofactors, or inhibitors. Contact analysis of the AA involved in the various complexes Each plot spans a 250-nanosecond simulation window, with the top panel in each subplot displaying the total number of atomic contacts between TcdA and its ligand over time. The corresponding heatmaps detail residue-level interactions, where the intensity of colouration reflects the frequency of contact for each residue: darker shades signify more persistent or repeated interactions (Fig. 4 ). In the TcdA–U2F complex (PDB ID 5UQL), the number of protein–ligand contacts fluctuated around 15–20 throughout the simulation, indicating relatively stable binding. Residues such as VAL_100, TRP_101, ILE_102, and GLY_104 maintained consistent interactions, while ARG_272, ASP_285, and LEU_518 exhibited particularly strong and sustained contacts. The TcdA–UPG complex (PDB ID 3SZA) demonstrated a higher number of contacts, ranging from 25 to 35, suggesting more extensive ligand engagement. In addition to residues observed in 5UQL, ASN_383 and GLN_384 also showed notable interaction frequencies, indicating broader interaction surfaces with UPG. In the TcdA–UDP complex (PDB ID 4DMW), the contact profile resembled that of 5UQL, with total contacts fluctuating between 15 and 20. Interactions were again dominated by VAL_100, TRP_101, and ILE_102, although fewer additional residues contributed significantly to ligand binding, implying a more restricted engagement compared to the UPG complex. The inclusion of RhoA in the TcdA–UPG–RhoA complex (PDB ID 7U2P) resulted in enhanced ligand interactions, with contact counts ranging from 25 to 35. Strong and stable interactions were observed for VAL_100, TRP_101, ILE_102, ASN_383, and GLN_384. In contrast, the presence of the inhibitor AH3 in the TcdA–UPG–AH3 complex (PDB ID 7UBY) led to reduced contact frequencies, with totals ranging between 15 and 25. While residues such as VAL_100, TRP_101, and ILE_102 retained limited interaction, the overall number and strength of contacts were diminished, indicating that AH3 likely disrupts UPG binding through competitive inhibition or allosteric effects. Interactions fractions showing the various residues and their bonds Figure 5 presents a series of bar plots illustrating the interaction fractions between specific residues in C. difficile toxin A (TcdA) glucosyltransferase and various ligands. These interactions are categorized into four types: hydrogen bonds, hydrophobic interactions, ionic interactions, and water bridges. Each bar plot corresponds to a unique protein-ligand complex represented by distinct PDB IDs, including 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY. The x-axis of each plot denotes the residue names involved in the interactions, while the y-axis indicates the fraction of each type of interaction contributed by individual residues. Interaction types are colour-coded: green for hydrogen bonds, purple for hydrophobic interactions, pink for ionic interactions, and blue for water bridges. In the 5UQL complex (TcdA bound to U2F), residues such as VAL_100, TRP_101, and ILE_102 show substantial hydrogen bonding activity. Hydrophobic interactions are also prominent, particularly in GLY_104, SER_107, and LEU_264. Ionic interactions are relatively sparse, observed mainly in ARG_272 and ASP_285. Several residues, including VAL_100 and TRP_101, also engage in water-bridged contacts. A similar interaction pattern is observed in the 3SZA complex (TcdA with UPG), where the same set of residues predominates in hydrogen bonding and hydrophobic interactions. Again, ionic contacts are limited, and water bridges are common among the key residues. The 4DMW complex (TcdA with UDP) continues this trend, with VAL_100, TRP_101, and ILE_102 demonstrating consistent hydrogen bonding, and GLY_104, SER_107, and LEU_264 contributing to hydrophobic interactions. ARG_272 and ASP_285 remain the principal residues involved in ionic bonding. Water bridges also occur frequently in this complex. The 7U2P structure, which includes TcdA, UPG, and RhoA, shows that the presence of RhoA does not significantly alter the core interaction pattern. Hydrogen bonds and hydrophobic interactions are still dominant, and the same residues are involved, although subtle shifts in interaction strength may be influenced by RhoA binding. Similarly, the 7UBY complex (TcdA with UPG and AH3 inhibitor) mirrors the interaction landscape of the other complexes. Hydrogen bonding remains prevalent among VAL_100, TRP_101, and ILE_102, with hydrophobic interactions involving GLY_104, SER_107, and LEU_264. Ionic interactions remain minimal and are restricted to ARG_272 and ASP_285, while water bridges continue to be a common mode of interaction. Comprehensive analysis of dynamic properties of the formed complexes Figure 6 presents a comprehensive analysis of the dynamic properties of C. difficile toxin A (TcdA) glucosyltransferase in various ligand-bound complexes, as observed through 250-nanosecond molecular dynamics simulations. Each panel of the figure corresponds to a specific PDB ID and ligand combination and illustrates changes in Root Mean Square Deviation (RMSD), Radius of Gyration (Rg), Molecular Surface Area (MolSA), Solvent-Accessible Surface Area (SASA), and Protein Surface Area (PSA) over time. These properties reflect key aspects of protein behaviour: RMSD quantifies deviation from the initial structure, Rg indicates compactness, MolSA represents the total molecular surface, SASA captures the area accessible to solvent, and PSA reflects the protein's overall surface exposure. For the 5UQL (U2F) complex, the RMSD remains stable around 1.5 Å, and the Rg fluctuates slightly around 4.3 Å, indicating a consistent and compact conformation. MolSA, SASA, and PSA values also show minimal fluctuations, stabilizing around 410 Ų, 160 Ų, and 500 Ų, respectively, which collectively reflect a structurally stable and solvent-protected state. In contrast, the 3SZA (UPG) complex displays increased structural deviations with RMSD values ranging from 1.5 to 2.0 Å and higher Rg values around 4.5 Å, suggesting a less compact structure. Correspondingly, MolSA and SASA fluctuate more noticeably around 420 Ų and 180 Ų, while PSA stabilizes around 520 Ų. The 4DMW (UDP) complex mirrors the structural stability of 5UQL, maintaining RMSD values near 1.5 Å and a slightly lower Rg around 4.2 Å. MolSA and PSA remain stable around 400 Ų and 490 Ų, while SASA drops to approximately 150 Ų, suggesting decreased solvent exposure. In contrast, the 7U2P (UPG + TcdA + RhoA) complex shows significant structural flexibility with RMSD values up to 2.5 Å and a higher Rg of approximately 4.8 Å. This is accompanied by substantial fluctuations in MolSA (~ 450 Ų), SASA (~ 200 Ų), and PSA (~ 550 Ų), indicating a more dynamic and solvent-exposed conformation. Interestingly, the 7UBY (UPG + TcdA + AH3) complex exhibits structural behaviour comparable to the more stable complexes. The RMSD hovers around 1.5 Å, and Rg is slightly lower at 4.1 Å, suggesting a compact and stable structure. MolSA, SASA, and PSA are consistently lower, at 390 Ų, 140 Ų, and 480 Ų respectively, implying effective stabilization and reduced solvent exposure in the presence of the AH3 inhibitor. Overall, the analysis reveals that TcdA glucosyltransferase exhibits varied dynamic behaviours depending on the bound ligand and associated molecular partners. Complexes with U2F (5UQL), UDP (4DMW), and AH3 (7UBY) tend to stabilize the protein, maintaining compact structures and low solvent accessibility. Conversely, UPG-containing complexes, particularly with RhoA (7U2P), induce greater structural deviation and surface exposure, indicating increased flexibility. These findings enhance our understanding of the structural dynamics of TcdA and may inform strategies for targeting its activity in therapeutic contexts. Principal component analysis of the simulation process Figure 7 presents a Principal Component Analysis (PCA) of the molecular dynamics simulations for various complexes of C. difficile toxin A (TcdA) glucosyltransferase. PCA is employed to reduce the dimensionality of the simulation data and to identify the predominant modes of motion within the protein structures. Each panel in the figure corresponds to a different PDB ID and ligand combination, illustrating scatter plots of the first three principal components (PC1, PC2, and PC3) alongside their respective eigenvalue distributions. The scatter plots depict the distribution of atomic fluctuations along the first three principal components, while the line plots represent the proportion of variance explained by each component, with eigenvalue rank on the x-axis and the proportion of variance on the y-axis. For the complex with PDB ID 5UQL (U2F), the scatter plots of PC1 versus PC2 display a broad dispersion of data points, indicating significant conformational motion along these axes. PC1 versus PC3 shows a narrower spread, suggesting lower variance captured by PC3, while PC2 versus PC3 displays even more clustered points, further confirming the dominance of PC1. The eigenvalue distribution reveals that PC1 explains 25.45% of the total variance, followed by PC2 and PC3, which account for 8.90% and 8.30%, respectively. In the case of PDB ID 3SZA (UPG), a similar pattern is observed. The PC1 versus PC2 scatter plot shows wide dispersion, indicating notable conformational mobility. The PC1 versus PC3 and PC2 versus PC3 plots are more clustered, consistent with lower contributions from these components. The eigenvalue analysis indicates that PC1 accounts for 26.14% of the variance, while PC2 and PC3 explain 7.47% and 8.46%, respectively. For PDB ID 4DMW (UDP), the scatter of data points along PC1 versus PC2 is slightly less pronounced than in the previous complexes, indicating relatively moderate motion. The PC1 versus PC3 and PC2 versus PC3 plots remain tightly clustered. PC1 explains 21.35% of the variance, with PC2 and PC3 contributing 17.90% and 7.78%, respectively, suggesting that PC2 may be relatively more significant in this complex. The complex represented by PDB ID 7U2P (UPG + TcdA + RhoA) also demonstrates considerable motion along PC1 and PC2, as evidenced by the wide dispersion in the scatter plots. The variance explained by PC1 is the highest among the complexes at 34.10%, with PC2 and PC3 contributing 8.37% and 7.25%, respectively. These results suggest a stronger dominant mode of motion in this protein-ligand configuration. Similarly, for PDB ID 7UBY (UPG + TcdA + AH3), the scatter plots show moderate dispersion along PC1 and PC2, while PC1 versus PC3 and PC2 versus PC3 remain more clustered. PC1 accounts for 26.53% of the variance, followed by PC2 (15.79%) and PC3 (6.04%). Overall, the PCA results indicate that PC1 consistently captures the most significant dynamic mode across all TcdA complexes, explaining a substantial portion of the total variance. The inclusion of additional interacting partners, such as RhoA or inhibitors like AH3, does not drastically alter the overall motion patterns but may influence the degree of contribution from each principal component. The greater dispersion along PC1 and PC2, as opposed to PC3, suggests these components encapsulate the key conformational transitions of TcdA. These findings enhance our understanding of the structural dynamics of TcdA and may inform the development of targeted therapeutic interventions. Amino acid residue cross-correlation maps for various complexes Figure 8 presents residue cross-correlation maps for various complexes of C. difficile toxin A (TcdA) glucosyltransferase. These maps illustrate the correlated motions between different residues within the protein structure during molecular dynamics simulations. Each panel corresponds to a specific PDB ID and ligand combination, displaying correlation coefficients between residue pairs. The colour scale spans from − 1.0, indicating fully anti-correlated motion, to 1.0, denoting complete positive correlation, while a value of 0.0 reflects no correlation. The diagonal line, where each residue is correlated with itself, always registers a value of 1.0. Both axes represent the residue numbers, facilitating the identification of residue pairs exhibiting correlated or anti-correlated motion. For the TcdA–U2F complex (PDB ID: 5UQL), strong positive correlations are evident in regions such as residues 100–200 and 300–400, suggesting coordinated motions. Conversely, residues within 200–300 and 400–500 demonstrate prominent anti-correlated dynamics. These observations imply that U2F binding triggers complex dynamic rearrangements within the TcdA structure. The TcdA–UPG complex (PDB ID: 3SZA) exhibits a similar dynamic pattern. Regions around residues 100–200 and 300–400 show strong positive correlations, while residues between 200–300 and 400–500 demonstrate strong anti-correlations. Although the overall pattern is reminiscent of that in 5UQL, differences in the specific residue interactions suggest that UPG binding modulates dynamics in distinct ways. In the TcdA–UDP complex (PDB ID: 4DMW), the cross-correlation map continues to reveal both positively and negatively correlated motions, notably around the same residue ranges observed in prior complexes. This pattern reinforces the notion of conserved dynamic regions within TcdA, though ligand-specific variations are also apparent. For the complex including UPG, TcdA, and RhoA (PDB ID: 7U2P), the introduction of RhoA adds further complexity to the correlation map. While strong positive and negative correlations persist in previously noted residue ranges, the presence of RhoA introduces new patterns of interaction and motion, highlighting its potential role in modulating TcdA structural dynamics. Similarly, in the TcdA complex containing UPG and the inhibitor AH3 (PDB ID: 7UBY), the cross-correlation map reveals distinct features. While many of the same regions exhibit positive and negative correlations, the influence of AH3 appears to induce unique shifts in the motion of specific residue pairs, suggesting a differential mode of modulation compared to other ligands or cofactors. Overall, these cross-correlation analyses highlight the dynamic sensitivity of TcdA to ligand binding and interaction with additional proteins or inhibitors. Each complex displays a unique yet partially conserved pattern of residue motion, emphasizing the importance of dynamic behaviour in TcdA function and regulation. Secondary structure composition of the five protein structures Table 3 presents the secondary structure composition of five protein structures identified by their PDB IDs: 5uql, 3SRZ, 4dmw, 7u2p, and 7uby. The secondary structure is categorized into helices, strands, and total secondary structure elements (SSE). A comparative analysis of these components reveals key structural differences and similarities among the proteins. The helical content is highest in 4dmw (52.83%), followed closely by 5uql and 3SRZ, which both exhibit identical helical proportions of 51.08%. This suggests a strong similarity in the helical architecture of 5uql and 3SRZ. In contrast, 7u2p and 7uby show significantly lower helical content at 44.71% and 41.47%, respectively, indicating fewer regions adopting a α-helical conformation. Regarding strand content, 5uql and 3SRZ again show near-identical values of 5.41% and 5.45%, respectively, implying comparable β-strand structure. The strand content of 4dmw is slightly lower at 4.92%, while 7u2p and 7uby exhibit markedly higher strand content at 9.79% and 10.73%, respectively. This suggests that these two proteins incorporate more β-sheet structures relative to the others. In terms of total SSE content, 5uql and 3SRZ remain closely aligned, with 56.48% and 56.53%, respectively. 4dmw shows a slightly higher total SSE of 57.75%, indicating a marginally more defined secondary structure overall. Conversely, 7u2p and 7uby display reduced total SSE values of 54.50% and 52.20%, respectively, pointing to a higher proportion of unstructured or disordered regions. The structure of 4dmw is slightly more enriched in helices and total SSEs but remains broadly similar. In contrast, 7u2p and 7uby deviate significantly, with less helical content and more β-strand elements, resulting in lower overall SSE proportions. These distinctions in secondary structure composition may reflect functional divergences among the proteins and warrant further investigation in the context of their biological roles. Table 3 Protein Secondary Structure 5uql 3SRZ 4dmw 7u2p 7uby % Helix 51.08 51.08 52.83 44.71 41.47 % Strand 5.41 5.45 4.92 9.79 10.73 % Total SSE 56.48 56.53 57.75 54.50 52.20 The RMSD values for different molecular dynamics simulation trajectories of protein structures Figure 9 presents the root mean square deviation (RMSD) values for different molecular dynamics simulation trajectories of protein structures with PDB IDs 5UQL, 3SRZ, 4DMW, 7U2P, and 7UBY. RMSD quantifies the average distance between atoms of superimposed proteins, serving as a metric for structural deviation over time. The values are provided at three distinct time points: 0 nanoseconds (ns), 125 ns, and 250 ns, capturing both short- and long-term conformational changes. For the protein structure 5UQL, the RMSD between 0 ns and 125 ns was 2.283 Å, suggesting a moderate deviation from its initial conformation. However, at 250 ns, the RMSD decreased to 1.931 Å, indicating some degree of structural stabilization or a return toward the original configuration. The RMSD between 125 ns and 250 ns was 1.985 Å, showing minimal structural changes over this interval. In the case of 3SRZ, the RMSD rose from 2.307 Å at 125 ns to 2.612 Å at 250 ns, implying continued structural deviation. The RMSD between 125 ns and 250 ns was 2.263 Å, reflecting a moderate shift during this phase. The protein structure 4DMW exhibited a higher RMSD of 2.470 Å between 0 ns and 125 ns. Interestingly, this value dropped to 2.286 Å at 250 ns, with a substantial reduction to 1.508 Å between 125 ns and 250 ns, indicating notable stabilization within this period. In contrast, 7U2P showed a relatively lower RMSD of 2.056 Å at 125 ns, which increased to 2.478 Å at 250 ns, pointing to progressive deviation from the initial structure. The RMSD between 125 ns and 250 ns was 1.992 Å, indicating a moderate structural shift. For 7UBY, the RMSD between 0 ns and 125 ns was 2.153 Å, followed by a slight decrease to 2.118 Å at 250 ns, suggesting limited but observable stabilization. The RMSD between 125 ns and 250 ns was notably reduced to 1.333 Å, indicating significant structural stabilization during this time frame. Collectively, the RMSD analyses provide insight into the dynamic behaviour and stability of the protein structures under investigation. All proteins demonstrated some degree of deviation from their initial conformations. Notably, 4DMW and 7UBY exhibited pronounced stabilization between 125 ns and 250 ns, whereas structures such as 3SRZ continued to diverge from their original states. These findings enhance our understanding of the structural flexibility and potential functional dynamics of the analysed proteins. Discussion The structural findings presented in this study proffer critical insights into the molecular architecture and functional mechanisms of the glucosyltransferase domain (GTD) of C. difficile toxin A (TcdA), particularly in relation to substrate recognition, catalysis, and inhibition. Across multiple crystal structures, a conserved network of interactions involving key residues, metal ion coordination, and hydrogen bonding was observed. The coordination of Mn²⁺ by acidic residues such as Asp285, Asp287, and Glu514 appears to be a recurring and essential feature that stabilises substrate binding and may be integral to catalytic function. This observation is consistent with previous studies that have highlighted the requirement of divalent metal ions for glucosyltransferase activity in C. difficile toxins 28 , 29 . The PDB structure 5UQL, representing the isolated glucosyltransferase domain of C. difficile toxin A (TcdA), provides crucial insight into the active site architecture and potential substrate-binding residues in its unbound form, serving as a foundational model for understanding TcdA’s catalytic function 23 . Within the TcdA–U2F complex (5UQL), specific residues such as Trp101, Asn138, and Trp519 have been identified as key players in substrate stabilization through hydrogen bonding, suggesting that aromatic and polar interactions play a pivotal role in properly orienting the substrate within the active site. This mechanism appears conserved in related complexes, such as 3SZA (TcdA–UPG), where substrate specificity for glucosyl donors is further refined through additional interactions involving Asn383 and Tyr283. These structural insights are consistent with findings from other glycosyltransferases, which similarly rely on aromatic stacking and hydrogen bonding to ensure sugar donor recognition and catalytic efficiency 30 , 31 . The crystal structure of TcdA in complex with Mn²⁺ and UDP (PDB: 4DMW) reveals key insights into the metal-ion and co-substrate coordination essential for its UDP-glucose-dependent glucosylation activity 22 . This binary complex captures an important stage of the catalytic cycle, highlighting the enzyme’s configuration when bound to its sugar donor and cofactor. In contrast, the ternary complex (PDB: 7U2P), which includes UDP-glucose and the protein substrate RhoA, offers a more comprehensive view of TcdA's dynamic glucosyltransferase domain (GTD) during catalysis. The simultaneous presence of both the sugar donor and the protein target in 7U2P provides a valuable model of dual-substrate engagement. Critical residues such as Lys428, Glu381, and Gln29 are directly involved in RhoA binding, underscoring the extensive interaction interface required for precise substrate recognition. This interface corresponds with previous observations in Rho-glucosylating toxins, where comparable residue contacts have been implicated in substrate specificity 32 . Together, these structures underscore the conformational flexibility and coordinated substrate recognition that characterize the enzymatic function of TcdA. The structural analysis of C. difficile toxin A (TcdA) across different PDB entries reveals key insights into its functional mechanisms and potential therapeutic targets. The structure captured in PDB 7U2P illustrates the TcdA–RhoA complex, providing a direct view of the molecular interface between the toxin and its GTPase substrate. This conformation is particularly valuable for identifying substrate recognition motifs and glucosylation sites on the host protein, thereby offering critical insight into the substrate specificity of TcdA 1 . In contrast, PDB 7UBY represents TcdA in complex with a neutralizing camelid antibody (VHH), furnishing a structural basis for understanding immune-mediated inhibition of the toxin. The 7UBY structure reveals an alternative conformation of the glucosyltransferase domain–UDP-glucose (GTD–UPG) complex, suggestive of a putative intermediate state. Subtle differences in residue interactions and conserved Mn²⁺ coordination in this state imply conformational flexibility within the active site, an attribute proposed to be essential for catalytic turnover in related glucosyltransferases 33 . Additionally, the inhibitor-bound structure PDB 7UBZ demonstrates how small molecules such as AH3 engage the catalytic cleft to block substrate access. Interactions involving key residues, Tyr189, Gln248, and Asp203, highlight a pharmacophore that may inform rational drug design efforts aimed at neutralizing TcdA’s toxic effects 1 . Together, these structures collectively advance our understanding of TcdA’s mechanism of action and facilitate the development of targeted therapeutics. The structural analysis offers significant insights into the molecular mechanisms of substrate recognition and inhibition of the glucosyltransferase domain of C. difficile toxin A (TcdA). The ternary complex in Panel A (PDB ID: 7U2P) reveals the precise coordination between TcdA, its natural substrate UDP-glucose (UPG), and the glycosyl acceptor RhoA, with Mn²⁺ stabilizing the transition state at the active site. This arrangement aligns with previously reported mechanisms of glycosyltransferase activity in bacterial toxins, where divalent metal ions such as Mn²⁺ play critical roles in catalysis by stabilizing the enzyme-substrate complex and facilitating glycosidic bond formation 32 , 34 . The competitive inhibition observed in Panel B (PDB ID: 7UBY), where the small-molecule inhibitor AH3 occupies the UPG-binding site, provides mechanistic validation of substrate mimicry as an effective strategy for neutralizing TcdA activity. The overlap of AH3 with UPG in the binding pocket underscores its potential to block access of the natural substrate, thereby inhibiting the glucosylation of host GTPases. Similar strategies have been employed in the design of UDP-sugar analogs and glycomimetic inhibitors against other bacterial toxins, including TcdB, which shares functional similarity with TcdA 35 . Panel C further extends this understanding by illustrating a proteinaceous inhibitor interacting with the TcdA surface, likely disrupting conformational rearrangements or protein-protein interactions necessary for enzymatic function. This mode of inhibition is particularly noteworthy because it targets regions beyond the active site, representing an allosteric or steric hindrance approach. The co-presence of a small-molecule inhibitor suggests that combinatorial targeting of active and regulatory domains could yield synergistic effects, a concept gaining traction in the development of next-generation antitoxin therapeutics 28 . The observed pattern of amino acid conservation within the glucosyltransferase domain of C. difficile TcdA provides critical insights into the structural and functional architecture of this virulence factor. The consistent presence of residues such as Trp101, Ile102, Arg272, Val286, Leu518, and Trp519 across all five analysed structures strongly suggests that these residues are integral to the enzyme’s catalytic core. Their conservation implies functional indispensability, likely contributing to substrate binding, catalytic turnover, and maintenance of structural stability. Similar findings have been reported in studies of other glucosyltransferases, where conserved aromatic and hydrophobic residues play key roles in stabilizing substrate interactions and transition state configurations 36 . The partial conservation of residues such as Asn138 and Tyr283, identified in four and three of the structures respectively, highlights potential context-dependent roles. These residues may not be essential under all conformational states but could influence substrate specificity or contribute to the enzyme’s allosteric regulation. Similar modulatory functions of semi-conserved residues have been reported in related toxin families, including TcdB and large clostridial glucosylating toxins, where subtle variations enable adaptability to diverse host environments or immune evasion. Residues uniquely present in individual structures, such as Gln384 and Pro470 in 7UBY, may reflect conformational intermediates or states relevant to catalytic cycling or regulatory switching. Their exclusive occurrence supports the notion of structural plasticity within the glucosyltransferase domain, which may be critical for accommodating different substrates or interacting with cofactors. Previous work has shown that such transient or state-specific residues can play pivotal roles in dynamic enzyme functions 37 . The identification of unique residues in the 7U2P and 7UBY structures of the TcdA glucosyltransferase domain has significant implications for understanding the enzyme’s conformational dynamics and functional versatility. The exclusive presence of residues such as Lys172, Gly379, Val381, Lys428, His431, Asp432, Lys448, Arg462, Thr491, and Ser520 in the 7U2P structure suggests that this conformation may be tailored for specific catalytic or substrate-binding roles. The strategic location of these residues near the putative active site supports their likely involvement in substrate stabilization, catalysis, or maintaining the structural integrity required for enzymatic activity. This observation aligns with previous studies emphasizing the importance of conformation-specific residues in the active site regions of glucosyltransferases 33 . In contrast, the distinct set of residues identified exclusively in the 7UBY structure, Ala98, Arg100, Ser110, Tyr112, Gly113, Lys114, and Asp115, suggests an alternative conformational state of the enzyme. These residues, not observed in 7U2P, may represent components of a regulatory or inhibitor-binding interface, potentially involved in modulating enzymatic function or enabling allosteric regulation. Their unique presence may also reflect a transition state or an inhibited configuration of the enzyme, as structural shifts have been shown to expose or conceal functional motifs in toxin-related glucosyltransferases 18 . The complete lack of overlapping residues between 7U2P and 7UBY further supports the hypothesis that these structures correspond to functionally distinct conformational states. Such structural heterogeneity is a hallmark of dynamic enzymes that undergo significant rearrangements during their catalytic cycle. In the case of TcdA, this may represent different stages such as pre-catalysis substrate recognition, the active catalytic conformation, or a post-catalysis/inhibited state. This notion is consistent with findings from structural studies on other large clostridial glucosylating toxins, where domain flexibility and allosteric regulation are critical for function 35 . The observed RMSD trends across all five molecular dynamics simulations provide critical insight into the conformational flexibility and ligand-binding stability of the TcdA glucosyltransferase domain in different structural contexts. In all protein structures 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY the RMSD values increased to approximately 3.6 Å over the 250 ns simulation period, indicating a consistent and significant degree of conformational change. This uniform pattern of backbone fluctuation suggests that the TcdA protein is inherently flexible, a characteristic often associated with functional plasticity in large, multi-domain toxins 38 . Notably, the inclusion of host interaction partners appears to influence the degree of structural flexibility. For instance, in the 7U2P complex containing both UPG and the host protein RhoA, and the 7UBY complex containing the inhibitor AH3, the protein RMSD increased to a comparable extent as in ligand-only structures, yet these complexes may represent biologically distinct states. The presence of RhoA in 7U2P may simulate a catalytically active conformation or a substrate engagement state, as increased protein flexibility can facilitate allosteric activation or substrate accommodation 39 . The stable contact profile observed in the TcdA–U2F (5UQL) and TcdA–UDP (4DMW) complexes, with contact frequencies maintained between 15–20, indicates moderate but consistent ligand engagement mediated primarily by core residues such as VAL_100, TRP_101, ILE_102, and GLY_104. These residues appear to form a conserved binding motif essential for basic ligand interaction, serving as a foundation for substrate positioning. The TcdA–UPG complex (3SZA) displayed a broader interaction surface, with contact counts reaching up to 35. This suggests an enhanced binding affinity possibly attributable to additional interactions involving ASN_383 and GLN_384. These residues likely contribute to a more extensive recognition interface, facilitating stronger and possibly more specific substrate accommodation. Such extended contact profiles are often associated with improved catalytic efficiency or substrate specificity in enzymatic systems 40 . Interestingly, the inclusion of RhoA in the TcdA–UPG–RhoA complex (7U2P) further increased contact frequency and interaction stability. The consistent involvement of residues such as ASN_383 and GLN_384 alongside the core interacting residues indicates that RhoA plays a stabilizing role, likely optimizing the enzyme's conformation for substrate engagement or catalysis. Conversely, the introduction of the inhibitor AH3 in the TcdA–UPG–AH3 complex (7UBY) resulted in a notable reduction in contact frequency, with fewer strong interactions observed. This diminished interaction profile implies that AH3 either competitively occupies the binding pocket or induces allosteric changes that disrupt favourable ligand contacts. Such inhibitory behaviour is characteristic of molecules designed to target catalytic residues or key stabilizing interactions, offering a mechanistic explanation for the disruption of enzymatic activity. The principal component analysis (PCA) of the protein-ligand complexes reveals distinct patterns of conformational dynamics across the different PDB structures, with implications for understanding the flexibility and functional motions of the TcdA glucosyltransferase domain in various binding environments. Notably, the complex 7U2P (UPG + TcdA + RhoA) exhibits the highest proportion of variance along PC1 (34.10%), indicating a dominant global motion likely induced by ligand binding or protein-protein interaction. This elevated dynamic signature may correspond to significant structural rearrangements essential for catalytic activation or substrate accommodation 41 . The 5UQL and 3SZA complexes also demonstrate substantial dispersion in their PC1 vs PC2 plots, with PC1 accounting for 25.45% and 26.14% of the variance, respectively. These findings suggest robust global motions that may reflect conformational readiness for ligand binding or catalytic turnover. Conversely, the 4DMW complex (UDP) shows a relatively moderate dispersion along PC1 vs PC2, with PC1 contributing 21.35% of the variance but a comparatively high PC2 contribution (17.90%). This indicates that the second mode of motion plays a more substantial role in the structural variability of this complex, potentially reflecting a different allosteric modulation pathway or conformational restraint 42 . Limitations The present study provides structural in-sight that has formed the baseline for future studies and potential therapeutics development. However there are some limitations that we would like to highlight. These include the exclusive reliance on in silico crystal structures, the absence of kinetic or mutational validation to confirm the functional roles of key residues, and the lack of integration with in vivo or clinical data to contextualize the findings further. Declarations Data availability All the data are within the manuscript or its supplementary files. Funding Ongoing Research Funding Program (ORF-2025-543) of King Saud University. Consent for publication Consent was obtained from all the authors for the publication of the study. 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Awadalla","email":"","orcid":"","institution":"King Fahad Medical City","correspondingAuthor":false,"prefix":"","firstName":"Maaweya","middleName":"E.","lastName":"Awadalla","suffix":""},{"id":472187618,"identity":"06ccb2f6-9d67-413a-a68e-3b13646efbd8","order_by":2,"name":"Uwem Okon Edet","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4ElEQVRIiWNgGAWjYNACNgYGfvnHB4AsCRmitUhINqQlgLTwEK/F4ECOAYhJWIvBjeSjGz6U2dQxHDjz+dWNGgseBvbDRzfg15KWdnPGuTQJxsbebdY5x4AO40lLu4FXy+0cs9u8bYclmJl5txnnsAG1SPCYEdCS/+3237b/EmxsPM+Mc/4RpSWH7TZj2wEJHh4e5se5bURokbz/zOxmz7lkyRkSbGbMuX0SPGyE/MJ35vCzGz/K7PjtbzA//pzzrU6On/3wMbxakAGbBJgkVjkIMH8gRfUoGAWjYBSMHAAAqX9I7dCBmpkAAAAASUVORK5CYII=","orcid":"","institution":"National Veterinary Research Institute (NVRI)","correspondingAuthor":true,"prefix":"","firstName":"Uwem","middleName":"Okon","lastName":"Edet","suffix":""},{"id":472187619,"identity":"ea2f3fc4-d896-4995-ab96-e843f918251e","order_by":3,"name":"Reham M Alahmadi","email":"","orcid":"","institution":"King Saud University","correspondingAuthor":false,"prefix":"","firstName":"Reham","middleName":"M","lastName":"Alahmadi","suffix":""},{"id":472187620,"identity":"003192f4-83ec-4362-9224-15405bb0239e","order_by":4,"name":"Edema Enogiomwan Imalele","email":"","orcid":"","institution":"University of Calabar","correspondingAuthor":false,"prefix":"","firstName":"Edema","middleName":"Enogiomwan","lastName":"Imalele","suffix":""},{"id":472187621,"identity":"514096ab-cecf-4483-854a-35a92c1910ab","order_by":5,"name":"Halah Z. Al-Rawi","email":"","orcid":"","institution":"King Fahad Medical City","correspondingAuthor":false,"prefix":"","firstName":"Halah","middleName":"Z.","lastName":"Al-Rawi","suffix":""},{"id":472187622,"identity":"38ed93b3-1152-4748-acec-297cabda4b23","order_by":6,"name":"Nizar H. Saeedi","email":"","orcid":"","institution":"University of Tabuk","correspondingAuthor":false,"prefix":"","firstName":"Nizar","middleName":"H.","lastName":"Saeedi","suffix":""},{"id":472187623,"identity":"5246b0dc-6ff1-4faf-833a-5f3639c3d9ba","order_by":7,"name":"Aniekan-Augusta Okon Eyo","email":"","orcid":"","institution":"University of Calabar","correspondingAuthor":false,"prefix":"","firstName":"Aniekan-Augusta","middleName":"Okon","lastName":"Eyo","suffix":""},{"id":472187624,"identity":"f5793ba3-89be-4d5e-ba66-11815db55a11","order_by":8,"name":"Abdullah F. Shater","email":"","orcid":"","institution":"University of Tabuk, Kingdom of Saudi Arabia","correspondingAuthor":false,"prefix":"","firstName":"Abdullah","middleName":"F.","lastName":"Shater","suffix":""},{"id":472187625,"identity":"a9e2f228-a368-46d1-a9a7-05700074ad9b","order_by":9,"name":"Sozan M. Abdelkhalig","email":"","orcid":"","institution":"AlMaarefa University","correspondingAuthor":false,"prefix":"","firstName":"Sozan","middleName":"M.","lastName":"Abdelkhalig","suffix":""},{"id":472187626,"identity":"3ea01f16-b0ff-4ece-8d2c-a3821fdf7204","order_by":10,"name":"Wafa Ali Eltayb","email":"","orcid":"","institution":"Shendi University","correspondingAuthor":false,"prefix":"","firstName":"Wafa","middleName":"Ali","lastName":"Eltayb","suffix":""}],"badges":[],"createdAt":"2025-06-11 10:38:33","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6870900/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6870900/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":84881810,"identity":"18a8d65a-49b4-4aab-9f2d-d350385dbb05","added_by":"auto","created_at":"2025-06-18 11:02:11","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":847105,"visible":true,"origin":"","legend":"\u003cp\u003eStructural insights into substrate and inhibitor interactions with glucosyltransferase domain of \u003cem\u003eC. difficile \u003c/em\u003etoxin A (TcdA)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/2de36f3075dad8fe7b7c3602.png"},{"id":84881809,"identity":"d4b5fbc3-237b-4a5f-83f9-b15526a478b4","added_by":"auto","created_at":"2025-06-18 11:02:11","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":403356,"visible":true,"origin":"","legend":"\u003cp\u003eInteraction of \u003cem\u003eC. difficile\u003c/em\u003etoxin A (TcdA) glucosyltransferase with various substrates and inhibitors\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/0004d811ff43811c6651cd4e.png"},{"id":84881811,"identity":"b956dcb0-2ac8-4c3d-9ebd-37f38265869e","added_by":"auto","created_at":"2025-06-18 11:02:11","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":590162,"visible":true,"origin":"","legend":"\u003cp\u003eRMSD plots of the various ligands and target protein (TcdA).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/0cc0900b099c7712c29a9320.png"},{"id":84882789,"identity":"384b2305-9ff6-4b83-af59-6e06361f19f6","added_by":"auto","created_at":"2025-06-18 11:10:11","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1073550,"visible":true,"origin":"","legend":"\u003cp\u003eProtein–ligand contact dynamics in \u003cem\u003eC. difficile \u003c/em\u003etoxin A complexes\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/6913167d527f16a61850589e.png"},{"id":84881812,"identity":"25bd33e1-50f4-40e3-a512-5eb533fac24c","added_by":"auto","created_at":"2025-06-18 11:02:11","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":441562,"visible":true,"origin":"","legend":"\u003cp\u003eInteraction fractions between specific residues in \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase and their ligands\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/e695d3f574045dd7a97516f1.png"},{"id":84881824,"identity":"db375a33-76a2-43e5-8e19-c6e8021b2971","added_by":"auto","created_at":"2025-06-18 11:02:11","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":819553,"visible":true,"origin":"","legend":"\u003cp\u003eDynamic properties of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase in different complexes, observed through molecular dynamics simulations\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/b7a67942e377a06c51c42380.png"},{"id":84882791,"identity":"4b282cf4-44f4-4c2c-bcb6-30c7b963c3ed","added_by":"auto","created_at":"2025-06-18 11:10:11","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":530193,"visible":true,"origin":"","legend":"\u003cp\u003ePrincipal Component Analysis (PCA) of the molecular dynamics simulations for various complexes of \u003cem\u003eC. difficile \u003c/em\u003etoxin A (TcdA) glucosyltransferase\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/72bd6172bfe20210a6eb42cc.png"},{"id":84881822,"identity":"a0d71208-b6ec-444a-acce-c6156a57b123","added_by":"auto","created_at":"2025-06-18 11:02:11","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1034004,"visible":true,"origin":"","legend":"\u003cp\u003eResidue cross-correlation maps for various complexes of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/9b2ce3f0cb2a68f19a0c2b17.png"},{"id":84881817,"identity":"f959564e-dbd9-444f-9724-a2874741b2d1","added_by":"auto","created_at":"2025-06-18 11:02:11","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":99879,"visible":true,"origin":"","legend":"\u003cp\u003eRoot mean square deviation (RMSD) values for different molecular dynamics simulation trajectories of protein structures\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/bb9c7678f652f513d6f085fe.png"},{"id":91040573,"identity":"ea40991e-a426-4726-a7cd-4fda09d940c8","added_by":"auto","created_at":"2025-09-11 03:46:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6698318,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6870900/v1/0c175c76-472a-4160-a24f-f3210b5e59a6.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Investigating the Interaction Mechanisms of Clostridium difficile Toxins with Host GTPases: A Bioinformatic Approach","fulltext":[{"header":"Introduction","content":"\u003cp\u003e \u003cem\u003eClostridium difficile\u003c/em\u003e, now known as \u003cem\u003eClostridioides difficile\u003c/em\u003e, is a Gram-positive, spore-forming bacterium. The bacillus is an obligate anaerobe that inhabits the gastrointestinal tract of humans\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. The bacterium is the leading cause of diarrhoea following the administration of antibiotics in either community or hospital settings in developed nations, and its infection presents with high morbidity and mortality, as well as a significant societal and financial burden \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. In addition, it is also implicated in colitis globally. \u003cem\u003eC. difficile\u003c/em\u003e is well known for its ability to secrete two large exotoxins, toxin A (TcdA) and toxin B (TcdB), which are considered its virulence or pathogenic factors\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Both toxins are glycosyltransferases capable of inactivating small GTPases within host cells via monoglucosylation, which leads to the functional inactivation of Rho GTPases and causes disruption of the actin cytoskeleton, leading to cytoskeletal breakdown, cell rounding, and eventual cell death\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eRecently, surveillance, prevalence and epidemiological studies collectively indicate an increase in cases of \u003cem\u003eC. difficile\u003c/em\u003e infection (CDI) across the globe \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. The 21st century has witnessed a marked increase in CDI cases across the world with cases now reported in African countries where it has been shown to drive mortality in patients diagnosed with tuberculosis \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. The changing epidemiological landscape of the bacterium is characterised by increased incidence and severity, occurring at a disproportionately higher frequency in older patients \u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Although its infection was previously considered to be nosocomial, cases are now reported in community settings that were previously labelled as low risk. There has been an increase in the prevalence of CDI \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. The increased prevalence and severity associated with CDI correlates with the emergence and rapid spread of a previously rare strain, ribotype 027 or NAP1, which shows high resistance to fluoroquinolone antibiotics \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Raising public health concerns. Several antibiotic classes, including the first and second generations of fluoroquinolones, have been shown to induce CDI. The current treatment options include the administration of antibiotics such as metronidazole, vancomycin, and fidaxomicin. However, even for these first-line treatment antibiotics, resistance has been reported\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. This underscores the pressing need for new therapeutic strategies and vaccines to effectively manage the pathogen\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. To achieve the development of newer and safer therapeutics, studies have aimed at understanding the structure and functional mechanisms of TcdA and TcdB \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. The TcdA and TcdB toxins are structurally composed of four domains, namely the N-terminal glucosyltransferase domain (GTD), a cysteine protease domain (CPD), a delivery and receptor binding domain (DRBD), and combined repetitive oligopeptides (CROP) \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. The glucosyltransferase domain of the protein is responsible for the catalytic inactivation of small GTPases\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. The GTD component of the toxins drives the glucosylation of host GTPases, which impairs critical signalling pathways that regulate cellular functions\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Structural studies have revealed that the GTDs of TcdA and TcdB share a similar overall architecture, including a conserved glycosyltransferase fold\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFurthermore, significant differences in their surface properties and substrate-binding regions suggest divergence in their enzymatic activities. For instance, while both toxins glucosylate Rho family GTPases, TcdA has also been shown to modify Rap family GTPases, which is not the case for TcdB. These differences in target specificity may underlie the varying clinical manifestations associated with infections caused by different strains of \u003cem\u003eC. difficile\u003c/em\u003e \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. The necessity for effective therapeutic interventions against \u003cem\u003eC. difficile\u003c/em\u003e infections has spurred interest in the development of small-molecule inhibitors targeting the GTD. Inhibitors can potentially block the enzymatic activity of these toxins, thereby preventing the glucosylation of host proteins and mitigating the toxic effects of the bacterium. The identification of such inhibitors requires a thorough understanding of the structural dynamics and interaction mechanisms of the GTD with its substrates \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Recent advances in structural biology have facilitated the elucidation of the co-crystal structures of TcdA and TcdB GTDs in complex with their respective substrates\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. These studies have provided insights into the conformational changes that occur upon substrate binding and have highlighted the role of various residues in substrate recognition and catalysis. For instance, the identification of critical residues involved in interaction with GTPases has paved the way for targeted mutagenesis studies aimed at further understanding the functional implications of these interactions \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eMoreover, understanding the structural and functional differences between TcdA and TcdB can provide insights into their distinct pathogenic mechanisms, specifically their substrate specificity and activity levels. For instance, TcdA has been shown to glycosylate a broader range of GTPases than TcdB, which may have implications for its role in disease severity and the development of potential therapeutic interventions. The studies also highlight the importance of accessory factors such as the cofactor Mn\u0026sup2;⁺ and the substrate UDP-glucose in modulating the activity and stability of these toxins. To assess the potential differences in structure and functional reactions that can arise when TcdA interacts with diverse substrates and proteins, we examined five structural complexes. These were 5UQL \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, 3SZA \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, 7UBY \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, PDB 7U2P \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, and 4DMW \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Specifically, we assessed the interactions between human ALDH3A1 (aldehyde dehydrogenase) and the TcdA glucosyltransferase domain (3SZA), the interactions between the TcdA domain bound to Mn\u0026sup2;⁺ and UDP (4DMW), TcdA in complex with human RhoA (7U2P), a host or human GTPase, and TcdA bound to a VHH antibody (7UBY).\u003c/p\u003e"},{"header":"Methodology","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eRetrieval of Study Protein and Ligands\u003c/h2\u003e \u003cp\u003eThe proteins 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY were retrieved from the RCSB (Research Collaboratory for Structural Bioinformatics) database hosted at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.rcsb.org/\u003c/span\u003e\u003cspan address=\"https://www.rcsb.org/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. Specifically, the proteins were: 5UQL, the TcdA glucosyltransferase domain (\u003cem\u003eC. difficile\u003c/em\u003e); 4DMW, the TcdA domain bound to Mn\u0026sup2;⁺ and UDP; 7U2P, TcdA in complex with human RhoA; 7UBY, TcdA in complex with a VHH antibody; and 3SZA, human aldehyde dehydrogenase ALDH3A1. The selection of the various proteins was done to allow for the exploration of diverse reactions of the TcdA with different substrates.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eMolecular Simulation (MS)\u003c/h3\u003e\n\u003cp\u003eThe retrieved proteins were subjected to a 250 ns molecular simulation using the Desmond tool, with parameters set to default as previously reported \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. The Desmond suite was chosen as it supports long molecular simulation runs between proteins. To achieve solvation and neutrality in the system, the TIP3P water model and a sodium chloride (NaCl) concentration of 0.15 M were set, respectively. To attain equilibrium, the temperature and pressure were set at 310 K and 1.013 bar. The boundary of the simulation was defined by an orthorhombic box, while the force field utilised was the Optimised Potential for Liquid Simulations (OPLS). The results of the simulations were visualised and analysed using the Desmond Virtual Molecular Operating Environment (DVMOE) and in PyMOL \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eStructural interactions of TcdA and various substrates, cofactors, and inhibitors\u003c/h2\u003e \u003cp\u003eThe molecular interactions between the glucosyltransferase domain (GTD) of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) and various substrates, cofactors, and inhibitors revealed diverse interactions (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) revealed by molecular docking. The various panels represent different high-resolution crystal structures obtained from the Protein Data Bank (PDB), with key amino acid residues, substrate analogues, and coordination of manganese ions (Mn\u0026sup2;⁺) highlighted to illustrate the enzymatic mechanism and potential sites for therapeutic targeting. As shown in Panel A (PDB ID: 5UQL), the GTD of TcdA bound to the uridine-based substrate analogue U2F. Key interacting amino acid (AA) residues include Ile102, Trp101, Val286, Asn138, Trp519, Asp285, and Arg272. Mn\u0026sup2;⁺ is coordinated by Asp285 and Glu514, while substrate stabilisation is facilitated by hydrogen bonding with Asn138 and Trp519. Panel B (PDB ID: 3SZA) shows the GTD bound to uridine diphosphate glucose (UPG), a natural sugar donor. Key interacting AA residues were Asn383, Asp287, Glu514, Val286, Tyr283, Arg272, Trp101, Leu518, and Asn138. The manganese ion (Mn\u0026sup2;⁺) is coordinated primarily by Asp287 and Glu514. There is also the presence of Hydrogen bonding with Asn383 and Asn138.\u003c/p\u003e \u003cp\u003ePanel C (PDB ID: 4DMW) displays the glucosyltransferase domain (GTD) in association with uridine diphosphate (UDP). Critical AA residues implicated in this interaction include Asp287, Val286, Tyr283, Arg272, Trp101, and Ile102. The Mn\u0026sup2;⁺ ion is coordinated primarily by Asp287, facilitating stabilization of the UDP molecule through surrounding residue interactions, especially with Tyr283 and Arg272. Panel D (PDB ID: 7U2P) illustrates the GTD bound simultaneously to UPG and its target protein RhoA. The interaction involves a broad network of AA residues that includes Ile102, Asn138, Trp101, Leu518, Trp519, Ser520, Asp462, Lys432, His431, Lys428, Asp65, Glu40, Glu381, Val381, Phe39, Thr491, Asn41, Gly379, Gln29, and Lys27. Coordination of Mn\u0026sup2;⁺ is mediated by Glu514 and Asp285. Panel E (PDB ID: 7UBY) captures an alternative conformation of the GTD\u0026ndash;UPG complex, shedding light on structural dynamics during the catalytic cycle. The key interacting AA residues include Val286, Leu518, Ile102, Trp101, Trp519, Tyr283, Arg272, Gln384, and Pro470. As in other complexes, Mn\u0026sup2;⁺ coordination by Asp285 and Glu514 is preserved. Also noted was the presence of hydrogen bonding between UPG and residues such as Asn138 and Trp519. Also, panel E (PDB ID: 7UBY) shows the GTD interacting with AH3, a small-molecule inhibitor. The inhibitor binds within a pocket comprising AA residues Tyr189, Tyr112, Ser110, Gly113, Lys114, Gln248, Asn194, Asn198, Val193, Ala98, Pro196, Lys195, Asp115, Arg100, and Asp203.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eStructural interactions revealed by x-ray crystallography\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the structural interactions between the glucosyltransferase domain of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) and various ligands, including its natural substrate, a small-molecule inhibitor, and a protein inhibitor. The panels display crystallographic structures obtained from the Protein Data Bank (PDB), highlighting the molecular mechanisms underlying substrate recognition and inhibition. Panel A (PDB ID: 7U2P) illustrates the ternary complex of TcdA glucosyltransferase (green) bound to uridine diphosphate glucose (UDP-glucose, UPG; magenta) and its substrate RhoA (pink), with a catalytic manganese ion (Mn\u0026sup2;⁺) positioned at the active site. The UPG molecule serves as the glycosyl donor, while RhoA functions as the glycosyl acceptor. In Panel B (PDB ID: 7UBY) the structure shows TcdA glucosyltransferase (green) in complex with a small-molecule inhibitor, AH3 (cyan), which occupies the UPG-binding pocket. The positioning of AH3 overlaps with that of UPG (magenta), indicating competitive inhibition. The Mn\u0026sup2;⁺ ion is also present in the active site. Panel C (Protein Inhibitor Complex) represents TcdA glucosyltransferase (green) bound to a proteinaceous inhibitor, along with a chemical inhibitor, likely similar to AH3.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSummary of the AA residues involved in the various interactions\u003c/h2\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the presence or absence of selected amino acid residues in five crystallographic structures of the \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase domain, as indicated by their respective Protein Data Bank (PDB) entries: 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY. Residues marked with an asterisk (*) denote its presence in a given structure, implying potential involvement in structural stability or enzymatic function. Conserved residues Trp101, Ile102, Arg272, Val286, Leu518, and Trp519 were consistently identified in all five structures. Their invariant presence shows their potential role as components of the catalytic core. Partially conserved residue Asn138 was observed in four of the five structures (5UQL, 3SZA, 7U2P, and 7UBY). Similarly, Tyr283 appeared in three structures (3SZA, 4DMW, and 7UBY), potentially indicating its involvement in non-essential but relevant interactions. Residues Gln384 and Pro470 were found only in the 7UBY structure. Less conserved residues Asp258, Asp287, Asn383, and Glu514 were detected in one or two structures. For example, Asp258 was exclusive to 5UQL, whereas Asp287 was found in 3SZA and 4DMW. Partially conserved residues included Asn138 and Tyr283, while Gln384 and Pro470 was present in a single structural state.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents a comparative assessment of specific amino acid residues identified in two crystal structures of the \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase domain, corresponding to Protein Data Bank (PDB) entries 7U2P and 7UBY. The comparison focuses on residues presumed to be functionally or structurally significant within each conformation. Residues marked with an asterisk (*) indicate its presence in the respective crystal structure. The following residues were observed exclusively in the 7U2P structure: Lys172, Gly379, Val381, Lys428, His431, Asp432, Lys448, Arg462, Thr491, and Ser520 close to or within its active site. In contrast, residues Ala98, Arg100, Ser110, Tyr112, Gly113, Lys114, and Asp115 were observed solely in the 7UBY structure. There was an absence of overlapping residues between 7U2P and 7UBY implying distinct conformational states of the TcdA glucosyltransferase domain.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of the presence or absence of specific amino acid residues in various crystal structures of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5UQL\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3SZA\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4DMW\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7U2P\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e7UBY\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTrp101\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIle102\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsn138\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eArg272\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsp258\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTyr283\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVal286\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsp287\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsn383\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGln384\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePro470\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGlu514\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeu518\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTrp519\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of the presence of specific amino acid residues in two different crystal structures of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase, represented by PDB IDs 7U2P and 7UBY.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7U2P\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7UBY\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAla98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eArg100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSer110\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTyr112\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGly113\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLys114\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsp115\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLys172\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGly379\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVal381\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLys428\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHis431\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsp432\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLys448\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eArg462\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eThr491\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSer520\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eMolecular simulation dynamics evaluations of the various interactions\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents Root Mean Square Deviation (RMSD) trajectories over 250-nanosecond molecular dynamics (MD) simulations for both protein and ligand components of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase in complex with various substrates or inhibitors. Each subplot corresponds to a distinct PDB ID, reflecting different binding scenarios. RMSD values, plotted against simulation time (ns), offer insights into the structural stability and conformational dynamics of the protein and associated ligands. The protein RMSD, represented by a blue line, reflects backbone (Cα) atomic deviations from the initial conformation and serves as an indicator of global structural fluctuations. The ligand RMSD, shown as a red line, captures atomic positional changes of the ligand relative to its initial pose, providing a measure of binding stability. All simulations were run for 250 ns to ensure sufficient sampling of conformational space.\u003c/p\u003e \u003cp\u003eFor PDB ID 5UQL (U2F), the protein RMSD increased gradually from approximately 1.0 \u0026Aring; to 3.6 \u0026Aring;, indicating notable conformational shifts. The ligand U2F remained stably bound, as evidenced by consistently low RMSD values. In PDB ID 3SZA (UPG), the protein RMSD followed a similar trend, rising to around 3.6 \u0026Aring;, while the UPG ligand also demonstrated minimal fluctuation, signifying tight binding. For PDB ID 4DMW (UDP), a more pronounced initial deviation was observed in the protein RMSD, which rose from about 0.3 \u0026Aring; to 3.6 \u0026Aring;, suggesting higher flexibility. The UDP ligand exhibited slightly increased but still stable RMSD, indicating modest mobility within the binding site. In the case of PDB ID 7U2P (UPG\u0026thinsp;+\u0026thinsp;TcdA\u0026thinsp;+\u0026thinsp;RhoA), the presence of the host protein RhoA appeared to enhance protein flexibility, as shown by the protein RMSD rising from 0.8 \u0026Aring; to 3.6 \u0026Aring;. Despite these structural changes, the UPG ligand remained firmly bound. Similarly, for PDB ID 7UBY (UPG\u0026thinsp;+\u0026thinsp;TcdA\u0026thinsp;+\u0026thinsp;AH3), the protein exhibited increasing RMSD values reaching approximately 3.6 \u0026Aring;, consistent with conformational adaptation in the presence of the inhibitor AH3. The UPG ligand remained stable, indicating persistent binding throughout the simulation.\u003c/p\u003e \u003cp\u003eOverall, all five complexes demonstrated increasing protein RMSD over time, reflecting substantial conformational flexibility of the TcdA glucosyltransferase domain. In contrast, ligand RMSD values remained relatively low and stable across all simulations, indicating that the ligands (U2F, UPG, UDP) maintained strong binding affinity regardless of the structural variations observed in the protein. The presence of additional interacting partners, such as RhoA or AH3, influenced protein dynamics but did not compromise ligand stability.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the temporal profiles of protein\u0026ndash;ligand contacts during molecular dynamics (MD) simulations of the glucosyltransferase domain of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) in complex with various ligands. Each panel corresponds to a distinct PDB entry, representing different structural complexes of TcdA with substrates, cofactors, or inhibitors.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eContact analysis of the AA involved in the various complexes\u003c/h3\u003e\n\u003cp\u003eEach plot spans a 250-nanosecond simulation window, with the top panel in each subplot displaying the total number of atomic contacts between TcdA and its ligand over time. The corresponding heatmaps detail residue-level interactions, where the intensity of colouration reflects the frequency of contact for each residue: darker shades signify more persistent or repeated interactions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In the TcdA\u0026ndash;U2F complex (PDB ID 5UQL), the number of protein\u0026ndash;ligand contacts fluctuated around 15\u0026ndash;20 throughout the simulation, indicating relatively stable binding. Residues such as VAL_100, TRP_101, ILE_102, and GLY_104 maintained consistent interactions, while ARG_272, ASP_285, and LEU_518 exhibited particularly strong and sustained contacts. The TcdA\u0026ndash;UPG complex (PDB ID 3SZA) demonstrated a higher number of contacts, ranging from 25 to 35, suggesting more extensive ligand engagement. In addition to residues observed in 5UQL, ASN_383 and GLN_384 also showed notable interaction frequencies, indicating broader interaction surfaces with UPG. In the TcdA\u0026ndash;UDP complex (PDB ID 4DMW), the contact profile resembled that of 5UQL, with total contacts fluctuating between 15 and 20. Interactions were again dominated by VAL_100, TRP_101, and ILE_102, although fewer additional residues contributed significantly to ligand binding, implying a more restricted engagement compared to the UPG complex.\u003c/p\u003e \u003cp\u003eThe inclusion of RhoA in the TcdA\u0026ndash;UPG\u0026ndash;RhoA complex (PDB ID 7U2P) resulted in enhanced ligand interactions, with contact counts ranging from 25 to 35. Strong and stable interactions were observed for VAL_100, TRP_101, ILE_102, ASN_383, and GLN_384. In contrast, the presence of the inhibitor AH3 in the TcdA\u0026ndash;UPG\u0026ndash;AH3 complex (PDB ID 7UBY) led to reduced contact frequencies, with totals ranging between 15 and 25. While residues such as VAL_100, TRP_101, and ILE_102 retained limited interaction, the overall number and strength of contacts were diminished, indicating that AH3 likely disrupts UPG binding through competitive inhibition or allosteric effects.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eInteractions fractions showing the various residues and their bonds\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents a series of bar plots illustrating the interaction fractions between specific residues in \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase and various ligands. These interactions are categorized into four types: hydrogen bonds, hydrophobic interactions, ionic interactions, and water bridges. Each bar plot corresponds to a unique protein-ligand complex represented by distinct PDB IDs, including 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY. The x-axis of each plot denotes the residue names involved in the interactions, while the y-axis indicates the fraction of each type of interaction contributed by individual residues. Interaction types are colour-coded: green for hydrogen bonds, purple for hydrophobic interactions, pink for ionic interactions, and blue for water bridges.\u003c/p\u003e \u003cp\u003eIn the 5UQL complex (TcdA bound to U2F), residues such as VAL_100, TRP_101, and ILE_102 show substantial hydrogen bonding activity. Hydrophobic interactions are also prominent, particularly in GLY_104, SER_107, and LEU_264. Ionic interactions are relatively sparse, observed mainly in ARG_272 and ASP_285. Several residues, including VAL_100 and TRP_101, also engage in water-bridged contacts. A similar interaction pattern is observed in the 3SZA complex (TcdA with UPG), where the same set of residues predominates in hydrogen bonding and hydrophobic interactions. Again, ionic contacts are limited, and water bridges are common among the key residues. The 4DMW complex (TcdA with UDP) continues this trend, with VAL_100, TRP_101, and ILE_102 demonstrating consistent hydrogen bonding, and GLY_104, SER_107, and LEU_264 contributing to hydrophobic interactions. ARG_272 and ASP_285 remain the principal residues involved in ionic bonding. Water bridges also occur frequently in this complex. The 7U2P structure, which includes TcdA, UPG, and RhoA, shows that the presence of RhoA does not significantly alter the core interaction pattern. Hydrogen bonds and hydrophobic interactions are still dominant, and the same residues are involved, although subtle shifts in interaction strength may be influenced by RhoA binding. Similarly, the 7UBY complex (TcdA with UPG and AH3 inhibitor) mirrors the interaction landscape of the other complexes. Hydrogen bonding remains prevalent among VAL_100, TRP_101, and ILE_102, with hydrophobic interactions involving GLY_104, SER_107, and LEU_264. Ionic interactions remain minimal and are restricted to ARG_272 and ASP_285, while water bridges continue to be a common mode of interaction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eComprehensive analysis of dynamic properties of the formed complexes\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents a comprehensive analysis of the dynamic properties of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase in various ligand-bound complexes, as observed through 250-nanosecond molecular dynamics simulations. Each panel of the figure corresponds to a specific PDB ID and ligand combination and illustrates changes in Root Mean Square Deviation (RMSD), Radius of Gyration (Rg), Molecular Surface Area (MolSA), Solvent-Accessible Surface Area (SASA), and Protein Surface Area (PSA) over time. These properties reflect key aspects of protein behaviour: RMSD quantifies deviation from the initial structure, Rg indicates compactness, MolSA represents the total molecular surface, SASA captures the area accessible to solvent, and PSA reflects the protein's overall surface exposure.\u003c/p\u003e \u003cp\u003eFor the 5UQL (U2F) complex, the RMSD remains stable around 1.5 \u0026Aring;, and the Rg fluctuates slightly around 4.3 \u0026Aring;, indicating a consistent and compact conformation. MolSA, SASA, and PSA values also show minimal fluctuations, stabilizing around 410 \u0026Aring;\u0026sup2;, 160 \u0026Aring;\u0026sup2;, and 500 \u0026Aring;\u0026sup2;, respectively, which collectively reflect a structurally stable and solvent-protected state. In contrast, the 3SZA (UPG) complex displays increased structural deviations with RMSD values ranging from 1.5 to 2.0 \u0026Aring; and higher Rg values around 4.5 \u0026Aring;, suggesting a less compact structure. Correspondingly, MolSA and SASA fluctuate more noticeably around 420 \u0026Aring;\u0026sup2; and 180 \u0026Aring;\u0026sup2;, while PSA stabilizes around 520 \u0026Aring;\u0026sup2;. The 4DMW (UDP) complex mirrors the structural stability of 5UQL, maintaining RMSD values near 1.5 \u0026Aring; and a slightly lower Rg around 4.2 \u0026Aring;. MolSA and PSA remain stable around 400 \u0026Aring;\u0026sup2; and 490 \u0026Aring;\u0026sup2;, while SASA drops to approximately 150 \u0026Aring;\u0026sup2;, suggesting decreased solvent exposure. In contrast, the 7U2P (UPG\u0026thinsp;+\u0026thinsp;TcdA\u0026thinsp;+\u0026thinsp;RhoA) complex shows significant structural flexibility with RMSD values up to 2.5 \u0026Aring; and a higher Rg of approximately 4.8 \u0026Aring;. This is accompanied by substantial fluctuations in MolSA (~\u0026thinsp;450 \u0026Aring;\u0026sup2;), SASA (~\u0026thinsp;200 \u0026Aring;\u0026sup2;), and PSA (~\u0026thinsp;550 \u0026Aring;\u0026sup2;), indicating a more dynamic and solvent-exposed conformation. Interestingly, the 7UBY (UPG\u0026thinsp;+\u0026thinsp;TcdA\u0026thinsp;+\u0026thinsp;AH3) complex exhibits structural behaviour comparable to the more stable complexes. The RMSD hovers around 1.5 \u0026Aring;, and Rg is slightly lower at 4.1 \u0026Aring;, suggesting a compact and stable structure. MolSA, SASA, and PSA are consistently lower, at 390 \u0026Aring;\u0026sup2;, 140 \u0026Aring;\u0026sup2;, and 480 \u0026Aring;\u0026sup2; respectively, implying effective stabilization and reduced solvent exposure in the presence of the AH3 inhibitor.\u003c/p\u003e \u003cp\u003eOverall, the analysis reveals that TcdA glucosyltransferase exhibits varied dynamic behaviours depending on the bound ligand and associated molecular partners. Complexes with U2F (5UQL), UDP (4DMW), and AH3 (7UBY) tend to stabilize the protein, maintaining compact structures and low solvent accessibility. Conversely, UPG-containing complexes, particularly with RhoA (7U2P), induce greater structural deviation and surface exposure, indicating increased flexibility. These findings enhance our understanding of the structural dynamics of TcdA and may inform strategies for targeting its activity in therapeutic contexts.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003ePrincipal component analysis of the simulation process\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e presents a Principal Component Analysis (PCA) of the molecular dynamics simulations for various complexes of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase. PCA is employed to reduce the dimensionality of the simulation data and to identify the predominant modes of motion within the protein structures. Each panel in the figure corresponds to a different PDB ID and ligand combination, illustrating scatter plots of the first three principal components (PC1, PC2, and PC3) alongside their respective eigenvalue distributions. The scatter plots depict the distribution of atomic fluctuations along the first three principal components, while the line plots represent the proportion of variance explained by each component, with eigenvalue rank on the x-axis and the proportion of variance on the y-axis.\u003c/p\u003e \u003cp\u003eFor the complex with PDB ID 5UQL (U2F), the scatter plots of PC1 versus PC2 display a broad dispersion of data points, indicating significant conformational motion along these axes. PC1 versus PC3 shows a narrower spread, suggesting lower variance captured by PC3, while PC2 versus PC3 displays even more clustered points, further confirming the dominance of PC1. The eigenvalue distribution reveals that PC1 explains 25.45% of the total variance, followed by PC2 and PC3, which account for 8.90% and 8.30%, respectively. In the case of PDB ID 3SZA (UPG), a similar pattern is observed. The PC1 versus PC2 scatter plot shows wide dispersion, indicating notable conformational mobility. The PC1 versus PC3 and PC2 versus PC3 plots are more clustered, consistent with lower contributions from these components. The eigenvalue analysis indicates that PC1 accounts for 26.14% of the variance, while PC2 and PC3 explain 7.47% and 8.46%, respectively.\u003c/p\u003e \u003cp\u003eFor PDB ID 4DMW (UDP), the scatter of data points along PC1 versus PC2 is slightly less pronounced than in the previous complexes, indicating relatively moderate motion. The PC1 versus PC3 and PC2 versus PC3 plots remain tightly clustered. PC1 explains 21.35% of the variance, with PC2 and PC3 contributing 17.90% and 7.78%, respectively, suggesting that PC2 may be relatively more significant in this complex. The complex represented by PDB ID 7U2P (UPG\u0026thinsp;+\u0026thinsp;TcdA\u0026thinsp;+\u0026thinsp;RhoA) also demonstrates considerable motion along PC1 and PC2, as evidenced by the wide dispersion in the scatter plots. The variance explained by PC1 is the highest among the complexes at 34.10%, with PC2 and PC3 contributing 8.37% and 7.25%, respectively. These results suggest a stronger dominant mode of motion in this protein-ligand configuration. Similarly, for PDB ID 7UBY (UPG\u0026thinsp;+\u0026thinsp;TcdA\u0026thinsp;+\u0026thinsp;AH3), the scatter plots show moderate dispersion along PC1 and PC2, while PC1 versus PC3 and PC2 versus PC3 remain more clustered. PC1 accounts for 26.53% of the variance, followed by PC2 (15.79%) and PC3 (6.04%).\u003c/p\u003e \u003cp\u003eOverall, the PCA results indicate that PC1 consistently captures the most significant dynamic mode across all TcdA complexes, explaining a substantial portion of the total variance. The inclusion of additional interacting partners, such as RhoA or inhibitors like AH3, does not drastically alter the overall motion patterns but may influence the degree of contribution from each principal component. The greater dispersion along PC1 and PC2, as opposed to PC3, suggests these components encapsulate the key conformational transitions of TcdA. These findings enhance our understanding of the structural dynamics of TcdA and may inform the development of targeted therapeutic interventions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eAmino acid residue cross-correlation maps for various complexes\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents residue cross-correlation maps for various complexes of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) glucosyltransferase. These maps illustrate the correlated motions between different residues within the protein structure during molecular dynamics simulations. Each panel corresponds to a specific PDB ID and ligand combination, displaying correlation coefficients between residue pairs. The colour scale spans from \u0026minus;\u0026thinsp;1.0, indicating fully anti-correlated motion, to 1.0, denoting complete positive correlation, while a value of 0.0 reflects no correlation. The diagonal line, where each residue is correlated with itself, always registers a value of 1.0. Both axes represent the residue numbers, facilitating the identification of residue pairs exhibiting correlated or anti-correlated motion.\u003c/p\u003e \u003cp\u003eFor the TcdA\u0026ndash;U2F complex (PDB ID: 5UQL), strong positive correlations are evident in regions such as residues 100\u0026ndash;200 and 300\u0026ndash;400, suggesting coordinated motions. Conversely, residues within 200\u0026ndash;300 and 400\u0026ndash;500 demonstrate prominent anti-correlated dynamics. These observations imply that U2F binding triggers complex dynamic rearrangements within the TcdA structure. The TcdA\u0026ndash;UPG complex (PDB ID: 3SZA) exhibits a similar dynamic pattern. Regions around residues 100\u0026ndash;200 and 300\u0026ndash;400 show strong positive correlations, while residues between 200\u0026ndash;300 and 400\u0026ndash;500 demonstrate strong anti-correlations. Although the overall pattern is reminiscent of that in 5UQL, differences in the specific residue interactions suggest that UPG binding modulates dynamics in distinct ways. In the TcdA\u0026ndash;UDP complex (PDB ID: 4DMW), the cross-correlation map continues to reveal both positively and negatively correlated motions, notably around the same residue ranges observed in prior complexes. This pattern reinforces the notion of conserved dynamic regions within TcdA, though ligand-specific variations are also apparent.\u003c/p\u003e \u003cp\u003eFor the complex including UPG, TcdA, and RhoA (PDB ID: 7U2P), the introduction of RhoA adds further complexity to the correlation map. While strong positive and negative correlations persist in previously noted residue ranges, the presence of RhoA introduces new patterns of interaction and motion, highlighting its potential role in modulating TcdA structural dynamics. Similarly, in the TcdA complex containing UPG and the inhibitor AH3 (PDB ID: 7UBY), the cross-correlation map reveals distinct features. While many of the same regions exhibit positive and negative correlations, the influence of AH3 appears to induce unique shifts in the motion of specific residue pairs, suggesting a differential mode of modulation compared to other ligands or cofactors. Overall, these cross-correlation analyses highlight the dynamic sensitivity of TcdA to ligand binding and interaction with additional proteins or inhibitors. Each complex displays a unique yet partially conserved pattern of residue motion, emphasizing the importance of dynamic behaviour in TcdA function and regulation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eSecondary structure composition of the five protein structures\u003c/h2\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the secondary structure composition of five protein structures identified by their PDB IDs: 5uql, 3SRZ, 4dmw, 7u2p, and 7uby. The secondary structure is categorized into helices, strands, and total secondary structure elements (SSE). A comparative analysis of these components reveals key structural differences and similarities among the proteins. The helical content is highest in 4dmw (52.83%), followed closely by 5uql and 3SRZ, which both exhibit identical helical proportions of 51.08%. This suggests a strong similarity in the helical architecture of 5uql and 3SRZ. In contrast, 7u2p and 7uby show significantly lower helical content at 44.71% and 41.47%, respectively, indicating fewer regions adopting a α-helical conformation.\u003c/p\u003e \u003cp\u003eRegarding strand content, 5uql and 3SRZ again show near-identical values of 5.41% and 5.45%, respectively, implying comparable β-strand structure. The strand content of 4dmw is slightly lower at 4.92%, while 7u2p and 7uby exhibit markedly higher strand content at 9.79% and 10.73%, respectively. This suggests that these two proteins incorporate more β-sheet structures relative to the others. In terms of total SSE content, 5uql and 3SRZ remain closely aligned, with 56.48% and 56.53%, respectively. 4dmw shows a slightly higher total SSE of 57.75%, indicating a marginally more defined secondary structure overall. Conversely, 7u2p and 7uby display reduced total SSE values of 54.50% and 52.20%, respectively, pointing to a higher proportion of unstructured or disordered regions. The structure of 4dmw is slightly more enriched in helices and total SSEs but remains broadly similar. In contrast, 7u2p and 7uby deviate significantly, with less helical content and more β-strand elements, resulting in lower overall SSE proportions. These distinctions in secondary structure composition may reflect functional divergences among the proteins and warrant further investigation in the context of their biological roles.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eProtein Secondary Structure\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5uql\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3SRZ\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4dmw\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7u2p\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e7uby\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e% Helix\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e51.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e51.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e52.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e44.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e41.47\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e% Strand\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e9.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e10.73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e% Total SSE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e56.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e56.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e57.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e54.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e52.20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eThe RMSD values for different molecular dynamics simulation trajectories of protein structures\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e presents the root mean square deviation (RMSD) values for different molecular dynamics simulation trajectories of protein structures with PDB IDs 5UQL, 3SRZ, 4DMW, 7U2P, and 7UBY. RMSD quantifies the average distance between atoms of superimposed proteins, serving as a metric for structural deviation over time. The values are provided at three distinct time points: 0 nanoseconds (ns), 125 ns, and 250 ns, capturing both short- and long-term conformational changes. For the protein structure 5UQL, the RMSD between 0 ns and 125 ns was 2.283 \u0026Aring;, suggesting a moderate deviation from its initial conformation. However, at 250 ns, the RMSD decreased to 1.931 \u0026Aring;, indicating some degree of structural stabilization or a return toward the original configuration. The RMSD between 125 ns and 250 ns was 1.985 \u0026Aring;, showing minimal structural changes over this interval. In the case of 3SRZ, the RMSD rose from 2.307 \u0026Aring; at 125 ns to 2.612 \u0026Aring; at 250 ns, implying continued structural deviation. The RMSD between 125 ns and 250 ns was 2.263 \u0026Aring;, reflecting a moderate shift during this phase. The protein structure 4DMW exhibited a higher RMSD of 2.470 \u0026Aring; between 0 ns and 125 ns. Interestingly, this value dropped to 2.286 \u0026Aring; at 250 ns, with a substantial reduction to 1.508 \u0026Aring; between 125 ns and 250 ns, indicating notable stabilization within this period. In contrast, 7U2P showed a relatively lower RMSD of 2.056 \u0026Aring; at 125 ns, which increased to 2.478 \u0026Aring; at 250 ns, pointing to progressive deviation from the initial structure. The RMSD between 125 ns and 250 ns was 1.992 \u0026Aring;, indicating a moderate structural shift. For 7UBY, the RMSD between 0 ns and 125 ns was 2.153 \u0026Aring;, followed by a slight decrease to 2.118 \u0026Aring; at 250 ns, suggesting limited but observable stabilization. The RMSD between 125 ns and 250 ns was notably reduced to 1.333 \u0026Aring;, indicating significant structural stabilization during this time frame.\u003c/p\u003e \u003cp\u003eCollectively, the RMSD analyses provide insight into the dynamic behaviour and stability of the protein structures under investigation. All proteins demonstrated some degree of deviation from their initial conformations. Notably, 4DMW and 7UBY exhibited pronounced stabilization between 125 ns and 250 ns, whereas structures such as 3SRZ continued to diverge from their original states. These findings enhance our understanding of the structural flexibility and potential functional dynamics of the analysed proteins.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe structural findings presented in this study proffer critical insights into the molecular architecture and functional mechanisms of the glucosyltransferase domain (GTD) of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA), particularly in relation to substrate recognition, catalysis, and inhibition. Across multiple crystal structures, a conserved network of interactions involving key residues, metal ion coordination, and hydrogen bonding was observed. The coordination of Mn\u0026sup2;⁺ by acidic residues such as Asp285, Asp287, and Glu514 appears to be a recurring and essential feature that stabilises substrate binding and may be integral to catalytic function. This observation is consistent with previous studies that have highlighted the requirement of divalent metal ions for glucosyltransferase activity in \u003cem\u003eC. difficile\u003c/em\u003e toxins\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe PDB structure 5UQL, representing the isolated glucosyltransferase domain of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA), provides crucial insight into the active site architecture and potential substrate-binding residues in its unbound form, serving as a foundational model for understanding TcdA\u0026rsquo;s catalytic function\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Within the TcdA\u0026ndash;U2F complex (5UQL), specific residues such as Trp101, Asn138, and Trp519 have been identified as key players in substrate stabilization through hydrogen bonding, suggesting that aromatic and polar interactions play a pivotal role in properly orienting the substrate within the active site. This mechanism appears conserved in related complexes, such as 3SZA (TcdA\u0026ndash;UPG), where substrate specificity for glucosyl donors is further refined through additional interactions involving Asn383 and Tyr283. These structural insights are consistent with findings from other glycosyltransferases, which similarly rely on aromatic stacking and hydrogen bonding to ensure sugar donor recognition and catalytic efficiency\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe crystal structure of TcdA in complex with Mn\u0026sup2;⁺ and UDP (PDB: 4DMW) reveals key insights into the metal-ion and co-substrate coordination essential for its UDP-glucose-dependent glucosylation activity \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. This binary complex captures an important stage of the catalytic cycle, highlighting the enzyme\u0026rsquo;s configuration when bound to its sugar donor and cofactor. In contrast, the ternary complex (PDB: 7U2P), which includes UDP-glucose and the protein substrate RhoA, offers a more comprehensive view of TcdA's dynamic glucosyltransferase domain (GTD) during catalysis. The simultaneous presence of both the sugar donor and the protein target in 7U2P provides a valuable model of dual-substrate engagement. Critical residues such as Lys428, Glu381, and Gln29 are directly involved in RhoA binding, underscoring the extensive interaction interface required for precise substrate recognition. This interface corresponds with previous observations in Rho-glucosylating toxins, where comparable residue contacts have been implicated in substrate specificity\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Together, these structures underscore the conformational flexibility and coordinated substrate recognition that characterize the enzymatic function of TcdA.\u003c/p\u003e \u003cp\u003eThe structural analysis of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA) across different PDB entries reveals key insights into its functional mechanisms and potential therapeutic targets. The structure captured in PDB 7U2P illustrates the TcdA\u0026ndash;RhoA complex, providing a direct view of the molecular interface between the toxin and its GTPase substrate. This conformation is particularly valuable for identifying substrate recognition motifs and glucosylation sites on the host protein, thereby offering critical insight into the substrate specificity of TcdA\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. In contrast, PDB 7UBY represents TcdA in complex with a neutralizing camelid antibody (VHH), furnishing a structural basis for understanding immune-mediated inhibition of the toxin. The 7UBY structure reveals an alternative conformation of the glucosyltransferase domain\u0026ndash;UDP-glucose (GTD\u0026ndash;UPG) complex, suggestive of a putative intermediate state. Subtle differences in residue interactions and conserved Mn\u0026sup2;⁺ coordination in this state imply conformational flexibility within the active site, an attribute proposed to be essential for catalytic turnover in related glucosyltransferases\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Additionally, the inhibitor-bound structure PDB 7UBZ demonstrates how small molecules such as AH3 engage the catalytic cleft to block substrate access. Interactions involving key residues, Tyr189, Gln248, and Asp203, highlight a pharmacophore that may inform rational drug design efforts aimed at neutralizing TcdA\u0026rsquo;s toxic effects\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Together, these structures collectively advance our understanding of TcdA\u0026rsquo;s mechanism of action and facilitate the development of targeted therapeutics.\u003c/p\u003e \u003cp\u003eThe structural analysis offers significant insights into the molecular mechanisms of substrate recognition and inhibition of the glucosyltransferase domain of \u003cem\u003eC. difficile\u003c/em\u003e toxin A (TcdA). The ternary complex in Panel A (PDB ID: 7U2P) reveals the precise coordination between TcdA, its natural substrate UDP-glucose (UPG), and the glycosyl acceptor RhoA, with Mn\u0026sup2;⁺ stabilizing the transition state at the active site. This arrangement aligns with previously reported mechanisms of glycosyltransferase activity in bacterial toxins, where divalent metal ions such as Mn\u0026sup2;⁺ play critical roles in catalysis by stabilizing the enzyme-substrate complex and facilitating glycosidic bond formation\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. The competitive inhibition observed in Panel B (PDB ID: 7UBY), where the small-molecule inhibitor AH3 occupies the UPG-binding site, provides mechanistic validation of substrate mimicry as an effective strategy for neutralizing TcdA activity. The overlap of AH3 with UPG in the binding pocket underscores its potential to block access of the natural substrate, thereby inhibiting the glucosylation of host GTPases. Similar strategies have been employed in the design of UDP-sugar analogs and glycomimetic inhibitors against other bacterial toxins, including TcdB, which shares functional similarity with TcdA\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Panel C further extends this understanding by illustrating a proteinaceous inhibitor interacting with the TcdA surface, likely disrupting conformational rearrangements or protein-protein interactions necessary for enzymatic function. This mode of inhibition is particularly noteworthy because it targets regions beyond the active site, representing an allosteric or steric hindrance approach. The co-presence of a small-molecule inhibitor suggests that combinatorial targeting of active and regulatory domains could yield synergistic effects, a concept gaining traction in the development of next-generation antitoxin therapeutics\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe observed pattern of amino acid conservation within the glucosyltransferase domain of \u003cem\u003eC. difficile\u003c/em\u003e TcdA provides critical insights into the structural and functional architecture of this virulence factor. The consistent presence of residues such as Trp101, Ile102, Arg272, Val286, Leu518, and Trp519 across all five analysed structures strongly suggests that these residues are integral to the enzyme\u0026rsquo;s catalytic core. Their conservation implies functional indispensability, likely contributing to substrate binding, catalytic turnover, and maintenance of structural stability. Similar findings have been reported in studies of other glucosyltransferases, where conserved aromatic and hydrophobic residues play key roles in stabilizing substrate interactions and transition state configurations\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. The partial conservation of residues such as Asn138 and Tyr283, identified in four and three of the structures respectively, highlights potential context-dependent roles. These residues may not be essential under all conformational states but could influence substrate specificity or contribute to the enzyme\u0026rsquo;s allosteric regulation. Similar modulatory functions of semi-conserved residues have been reported in related toxin families, including TcdB and large clostridial glucosylating toxins, where subtle variations enable adaptability to diverse host environments or immune evasion. Residues uniquely present in individual structures, such as Gln384 and Pro470 in 7UBY, may reflect conformational intermediates or states relevant to catalytic cycling or regulatory switching. Their exclusive occurrence supports the notion of structural plasticity within the glucosyltransferase domain, which may be critical for accommodating different substrates or interacting with cofactors. Previous work has shown that such transient or state-specific residues can play pivotal roles in dynamic enzyme functions\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe identification of unique residues in the 7U2P and 7UBY structures of the TcdA glucosyltransferase domain has significant implications for understanding the enzyme\u0026rsquo;s conformational dynamics and functional versatility. The exclusive presence of residues such as Lys172, Gly379, Val381, Lys428, His431, Asp432, Lys448, Arg462, Thr491, and Ser520 in the 7U2P structure suggests that this conformation may be tailored for specific catalytic or substrate-binding roles. The strategic location of these residues near the putative active site supports their likely involvement in substrate stabilization, catalysis, or maintaining the structural integrity required for enzymatic activity. This observation aligns with previous studies emphasizing the importance of conformation-specific residues in the active site regions of glucosyltransferases \u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. In contrast, the distinct set of residues identified exclusively in the 7UBY structure, Ala98, Arg100, Ser110, Tyr112, Gly113, Lys114, and Asp115, suggests an alternative conformational state of the enzyme. These residues, not observed in 7U2P, may represent components of a regulatory or inhibitor-binding interface, potentially involved in modulating enzymatic function or enabling allosteric regulation. Their unique presence may also reflect a transition state or an inhibited configuration of the enzyme, as structural shifts have been shown to expose or conceal functional motifs in toxin-related glucosyltransferases\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. The complete lack of overlapping residues between 7U2P and 7UBY further supports the hypothesis that these structures correspond to functionally distinct conformational states. Such structural heterogeneity is a hallmark of dynamic enzymes that undergo significant rearrangements during their catalytic cycle. In the case of TcdA, this may represent different stages such as pre-catalysis substrate recognition, the active catalytic conformation, or a post-catalysis/inhibited state. This notion is consistent with findings from structural studies on other large clostridial glucosylating toxins, where domain flexibility and allosteric regulation are critical for function\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe observed RMSD trends across all five molecular dynamics simulations provide critical insight into the conformational flexibility and ligand-binding stability of the TcdA glucosyltransferase domain in different structural contexts. In all protein structures 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY the RMSD values increased to approximately 3.6 \u0026Aring; over the 250 ns simulation period, indicating a consistent and significant degree of conformational change. This uniform pattern of backbone fluctuation suggests that the TcdA protein is inherently flexible, a characteristic often associated with functional plasticity in large, multi-domain toxins\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Notably, the inclusion of host interaction partners appears to influence the degree of structural flexibility. For instance, in the 7U2P complex containing both UPG and the host protein RhoA, and the 7UBY complex containing the inhibitor AH3, the protein RMSD increased to a comparable extent as in ligand-only structures, yet these complexes may represent biologically distinct states. The presence of RhoA in 7U2P may simulate a catalytically active conformation or a substrate engagement state, as increased protein flexibility can facilitate allosteric activation or substrate accommodation\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. The stable contact profile observed in the TcdA\u0026ndash;U2F (5UQL) and TcdA\u0026ndash;UDP (4DMW) complexes, with contact frequencies maintained between 15\u0026ndash;20, indicates moderate but consistent ligand engagement mediated primarily by core residues such as VAL_100, TRP_101, ILE_102, and GLY_104. These residues appear to form a conserved binding motif essential for basic ligand interaction, serving as a foundation for substrate positioning.\u003c/p\u003e \u003cp\u003eThe TcdA\u0026ndash;UPG complex (3SZA) displayed a broader interaction surface, with contact counts reaching up to 35. This suggests an enhanced binding affinity possibly attributable to additional interactions involving ASN_383 and GLN_384. These residues likely contribute to a more extensive recognition interface, facilitating stronger and possibly more specific substrate accommodation. Such extended contact profiles are often associated with improved catalytic efficiency or substrate specificity in enzymatic systems\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Interestingly, the inclusion of RhoA in the TcdA\u0026ndash;UPG\u0026ndash;RhoA complex (7U2P) further increased contact frequency and interaction stability. The consistent involvement of residues such as ASN_383 and GLN_384 alongside the core interacting residues indicates that RhoA plays a stabilizing role, likely optimizing the enzyme's conformation for substrate engagement or catalysis. Conversely, the introduction of the inhibitor AH3 in the TcdA\u0026ndash;UPG\u0026ndash;AH3 complex (7UBY) resulted in a notable reduction in contact frequency, with fewer strong interactions observed. This diminished interaction profile implies that AH3 either competitively occupies the binding pocket or induces allosteric changes that disrupt favourable ligand contacts. Such inhibitory behaviour is characteristic of molecules designed to target catalytic residues or key stabilizing interactions, offering a mechanistic explanation for the disruption of enzymatic activity.\u003c/p\u003e \u003cp\u003eThe principal component analysis (PCA) of the protein-ligand complexes reveals distinct patterns of conformational dynamics across the different PDB structures, with implications for understanding the flexibility and functional motions of the TcdA glucosyltransferase domain in various binding environments. Notably, the complex 7U2P (UPG\u0026thinsp;+\u0026thinsp;TcdA\u0026thinsp;+\u0026thinsp;RhoA) exhibits the highest proportion of variance along PC1 (34.10%), indicating a dominant global motion likely induced by ligand binding or protein-protein interaction. This elevated dynamic signature may correspond to significant structural rearrangements essential for catalytic activation or substrate accommodation\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. The 5UQL and 3SZA complexes also demonstrate substantial dispersion in their PC1 vs PC2 plots, with PC1 accounting for 25.45% and 26.14% of the variance, respectively. These findings suggest robust global motions that may reflect conformational readiness for ligand binding or catalytic turnover. Conversely, the 4DMW complex (UDP) shows a relatively moderate dispersion along PC1 vs PC2, with PC1 contributing 21.35% of the variance but a comparatively high PC2 contribution (17.90%). This indicates that the second mode of motion plays a more substantial role in the structural variability of this complex, potentially reflecting a different allosteric modulation pathway or conformational restraint\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003eLimitations\u003c/h2\u003e \u003cp\u003eThe present study provides structural in-sight that has formed the baseline for future studies and potential therapeutics development. However there are some limitations that we would like to highlight. These include the exclusive reliance on in silico crystal structures, the absence of kinetic or mutational validation to confirm the functional roles of key residues, and the lack of integration with in vivo or clinical data to contextualize the findings further.\u003c/p\u003e \u003c/div\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the data are within the manuscript or its supplementary files.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOngoing Research Funding Program (ORF-2025-543) of King Saud University.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConsent was obtained from all the authors for the publication of the study.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere is none to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors’ contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualisation: \u0026nbsp;MA, MEA, and UOE. \u003cstrong\u003eMethods, software, and validation:\u003c/strong\u003e MA, MEA, RMA, HZA, NHS, AFS, SMA and WAE. Funding: RMA, MA and MEA \u003cstrong\u003eSupervision\u003c/strong\u003e: MA and UOE. \u003cstrong\u003eWriting of the original draft approval\u003c/strong\u003e: MA, UOE, EEI and AOE. \u003cstrong\u003eReading and approval:\u003c/strong\u003e all the authors.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eChen, B., Liu, Z., Perry, K. \u0026amp; Jin, R. Structure of the glucosyltransferase domain of TcdA in complex with RhoA provides insights into substrate recognition. \u003cem\u003eScientific reports\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 9028, doi:10.1038/s41598-022-12909-8 (2022).\u003c/li\u003e\n\u003cli\u003eMada, P. K. \u0026amp; Alam, M. 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Applying Structure-Based Drug Design Approaches to Allosteric Modulators of GPCRs. \u003cem\u003eTrends in pharmacological sciences\u003c/em\u003e\u003cstrong\u003e38\u003c/strong\u003e, 837-847, doi:10.1016/j.tips.2017.05.010 (2017).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Clostridioides difficile, Toxins, Bioinformatics, Simulation, Therapeutics, Conserved amino acids","lastPublishedDoi":"10.21203/rs.3.rs-6870900/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6870900/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e \u003cp\u003e \u003cem\u003eClostridioides difficile\u003c/em\u003e is frequently implicated in colitis and antibiotics-induced diarrhoea in both community and hospital settings around the world, and there reports of resistance to the antibiotics of choice used in the management of its infections. Yet, there is limited information on the structural dynamics of its toxins TcdA and TcdB that could guide potential therapeutic candidates.\u003c/p\u003e\u003ch2\u003eAim\u003c/h2\u003e \u003cp\u003eWe undertook a structural insights study into the glucosyltransferase domain (GTD) of \u003cem\u003eClostridioides difficile\u003c/em\u003e toxin A (TcdA).\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eStructural analyses and molecular dynamics simulation (250 ns) were carried out for the various TcdA glucosyltransferase domain of 5UQL, 3SZA, 4DMW, 7U2P, and 7UBY.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eour findings highlights conserved catalytic residues (e.g., Trp101, Glu514), Mn\u0026sup2;⁺ coordination, and substrate-binding motifs. Key residues (Lys428, Glu381) were shown to mediate RhoA engagement, while small-molecule and antibody inhibitors targeted both active and allosteric sites. Molecular dynamics revealed RMSD increases to ~\u0026thinsp;3.6 \u0026Aring; across five TcdA structures over 250 ns, indicating intrinsic conformational flexibility. Complexes with RhoA or inhibitors showed altered contact profiles and dynamic behaviour, supporting functional plasticity. Principal component analysis (PCA) revealed that the ternary complex 7U2P exhibited the highest global motion, suggestive of catalytically relevant conformational changes. Contact frequency analysis confirmed stable ligand engagement in active complexes and disruption in inhibitor-bound states.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eThese findings underscore the structural adaptability of TcdA\u0026rsquo;s GTD and reveal potential therapeutic targets through inhibition of conserved residues or conformational states essential for substrate recognition and catalysis.\u003c/p\u003e","manuscriptTitle":"Investigating the Interaction Mechanisms of Clostridium difficile Toxins with Host GTPases: A Bioinformatic Approach","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-18 11:02:06","doi":"10.21203/rs.3.rs-6870900/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a779f800-0bb8-4a60-b546-1a3117787bac","owner":[],"postedDate":"June 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":50145789,"name":"Biological sciences/Cell biology"},{"id":50145790,"name":"Biological sciences/Computational biology and bioinformatics"},{"id":50145791,"name":"Biological sciences/Drug discovery"}],"tags":[],"updatedAt":"2025-09-11T03:38:40+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-18 11:02:06","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6870900","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6870900","identity":"rs-6870900","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}