Structure adaptation in Omicron SARS-CoV-2/hACE2:Biophysical origins of evolutionary driving forces
Since its emergence, the Covid19 pandemic has been sustained by a series of transmission waves initiated by new variants of the SARS-CoV-2 virus. Some of these arise with higher transmissivity and/or increased disease severity. Here we use molecular dynamics simulations to examine the modulation of the fundamental interactions between the receptor binding domain (RBD) of the spike glycoprotein and the host cell receptor (human angiotensin-converting enzyme 2: hACE2) arising from Omicron variant mutations (BA.1 and BA.2) relative to the original wild type strain. We find significant structural differences in the complexes which overall bring the spike protein and its receptor into closer proximity. These are consistent with and attributed to the higher positive charge on the RBD conferred by BA.1 and BA.2 mutations relative to the wild type. However, further differences between sub-variants BA.1 and BA.2 (which have equivalent RBD charges) are also evident: Mutations affect interdomain interactions between the up-chain and its clockwise neighbor chain, resulting in enhanced flexibility for BA.2. Consequently, additional close contacts arise in BA.2 which include binding to hACE2 by a second spike protein monomer, in addition to the up-chain - a motif not found in BA.1. Finally, the mechanism by which the glycans stabilize the up state of the Spike protein differs for the wild type and the Omicrons. We also found the glycan on N90 of hACE2 turns from inhibiting, to facilitating the binding to Omicron spike protein. These structural and electrostatic differences offer further insight into the mechanisms by which viral mutations modulate host cell binding and provide a biophysical basis for evolutionary driving forces.