From Structure to Dynamics: Activation Mechanism of the G Protein-Coupled Bile Acid Receptor 1-Gs Complex

Abstract The G protein-coupled bile acid receptor 1 (GPBAR1, also known as TGR5) is a key mediator of bile acid signaling, exerting its physiological effects through coupling with the stimulatory G protein (Gs). This interaction is essential for stabilizing the receptor’s active conformation and triggering downstream signaling. Among endogenous ligands, lithocholic acid (LCA) is the most potent natural agonist. However, the dynamic features underlying its binding and activation mechanisms remain poorly defined. In this study, we investigated the molecular basis of the interaction between LCA and GPBAR1, as well as the functional consequences of this interaction on receptor activation by integrating homology modelling, molecular docking, and molecular dynamics (MD) simulations. Our calculations reveal that LCA binding stabilizes the active state of GPBAR1, biasing the conformational ensemble of TM5 and TM6, as well as the main microswitches. These ligand-induced rearrangements enhance the coupling interface with the α5 helix of Gαs and facilitate allosteric communication between the orthosteric and intracellular sites. Overall, our findings provide dynamic insight into how LCA modulates GPBAR1 activation and G protein engagement, highlighting its role as a molecular effector in bile acid signaling, and furnishing molecular detail relevant to ongoing efforts in GPBAR1-targeted compound development. Competing Interest Statement The authors have declared no competing interest. Footnotes Abbreviations - GPBAR1 - G protein-coupled bile acid receptor 1 - TGR5 - Takeda G protein-coupled receptor 5 - AHD - α-helical domain - LCA - Lithocholic acid - SAR - Structure–activity relationship - TM - Transmembrane helix - ECL - Extracellular loop - MD - Molecular Dynamics - PCA - Principal Component Analysis - RMSD - Root mean square deviation - RMSF - Root mean square fluctuation - Rg - Radius of gyration - NMI - Normalized mutual information - SIFt - Structural interaction fingerprint - GUI - Graphical user interface - PRCG - Polak–Ribiere conjugate gradient - OPLS - Optimized potentials for liquid simulations - CGenFF - CHARMM General Force Field - DOPE - Discrete optimized protein energy - cryo-EM - Cryo-electron microscopy - PDB - Protein Data Bank - PBC - Periodic boundary conditions - POPC - 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine - CHL - Cholesterol - GlcNAc - N-acetylglucosamine - cAMP - Cyclic adenosine monophosphate - PKA - Protein kinase A - GLP-1 - Glucagon-like peptide-1 - IBD - Inflammatory bowel disease.