Protonation states in SARS-CoV-2 main protease mapped by neutron crystallography

The main protease (3CL M^pro) from SARS-CoV-2, the etiological agent of COVID-19, is an essential enzyme for viral replication, possessing an unusual catalytic dyad composed of His41 and Cys145. A long-standing question in the field has been what the protonation states of the ionizable residues in the substrate-binding active site cavity are. Here, we present the room-temperature neutron structure of 3CL M^profrom SARS-CoV-2, which allows direct determination of hydrogen atom positions and, hence, protonation states. The catalytic site natively adopts a zwitterionic reactive state where His41 is doubly protonated and positively charged, and Cys145 is in the negatively charged thiolate state. The neutron structure also identified the protonation states of other amino acid residues, mapping electrical charges and intricate hydrogen bonding networks in the SARS-CoV-2 3CL M^proactive site cavity and dimer interface. This structure highlights the ability of neutron protein crystallography for experimentally determining protonation states at near-physiological temperature – the critical information for structure-assisted and computational drug design.