Flap structure within receptor binding domain of SARS-CoV-2 spike periodically obstructs hACE2 Binding subdomain bearing similarities to HIV-1 protease flap

The SARS-CoV-2 prefusion spike protein is characterized by a high degree of flexibility and temporal transformations associated with its multifunctional behavior. In this study, we have examined the dynamics of the Receptor Binding Domain (RBD) of the SARS-CoV-2 spike protein in detail. Its primary, binding subdomain with human Angiotensin Covering Enzyme II includes a highly conspicuous flap or loop that is part of a beta hairpin loop structural motif. Dynamic details of the RBD obtained through RMSF and Order Parameter calculations are consistent with structural details including the stability of "glue" points or dominant interaction energy residues of the RBD in the Up and Down states with its neighboring N-terminal domain (NTD) protomer. The RBD flap in the Up state protomer periodically obstructs the binding site on an approximate 70 ns time interval and is reminiscent of an HIV-1 protease polypeptide flap that opens and closes to modulate that enzymes activity. No claim is made here regarding the possible modulating role of the flap; however, the flap may be a potential site for therapeutic targeting aimed at keeping it in the closed state, as previously demonstrated in the inhibition of the HIV-1 protease polypeptide. The RBD primary binding subdomain is further shown to have not only similar dynamics but, also, an approximate 30% sequence similarity to the HIV-1 protease polypeptide.