A Novel Y-Shaped, S-O-N-O-S-Bridged Crosslink between Three Residues C22, C44, and K61 Is Frequently Observed in the SARS-CoV-2 Main Protease

As the COVID-19 pathogen, SARS-CoV-2 relies on its main protease (M Pro ) for pathogenesis and replication. During the crystallographic analyses of M Pro crystals that were exposed to the air, a uniquely Y-shaped, S-O-N-O-S-bridged posttranslational crosslink that connects three residues C22, C44, and K61 at their side chains was frequently observed. As a novel covalent modification, this crosslink serves potentially as a redox switch to regulate the catalytic activity of M Pro , a demonstrated drug target of COVID-19. The formation of this linkage leads to a much more opened active site that can be potentially targeted for the development of novel SARS-CoV-2 antivirals. The structural rearrangement of M Pro by this crosslink indicates that small molecules that lock M Pro in the crosslinked form can be potentially used with other active site-targeting molecules such as paxlovid for synergistic effects in inhibiting the SARS-CoV-2 viral replication.