Implications of Spike Protein Interactions with Zn-bound form of ACE2: A Computational Structural Study

The COVID-19 pandemic has generated a major interest in designing inhibitors to prevent SARS-CoV-2 binding on host cells to protect against infection. One promising approach to such research utilizes molecular dynamics (MD) to identify potential inhibitors that can prevent the interaction between spike (S) protein on the virus and angiotensin converting enzyme 2 (ACE2) receptor on the host cells. In these studies, many groups have chosen to exclude a zinc (Zn) ion bound to the ACE2 molecule which is critical for enzymatic activity. While the relatively distant location of Zn ion from the S protein binding site (S1 domain), combined with the difficulties in modeling this ion have motivated the decision of exclusion, Zn can potentially contribute to the structural stability of the entire protein, and thus, may have implications on spike protein interaction. In this study, we explored the effects of excluding Zn on the structural stability and binding free energy of the ACE2-S1 protein complex. We generated two versions of an experimentally-derived structure of the ACE2-S1 protein complex: one with Zn and one without. Examining the differences between these two complexes during MD simulation, we found that the Zn-bound complex exhibited greater instability at nearly all residues except for the interacting residues, which were more stable in the Zn-bound complex. Additionally, the Zn-bound complex had a stronger binding free energy at all internal dielectric constants greater than one. Since binding free energy is often used to score inhibitors' performances, excluding Zn could potentially have implications on inhibitor selection and performance, both in the ACE2-S1 protein system and other protein complexes that include the Zn ion.