Unraveling the Enzymatic Mechanism of the SARS-CoV-2 RNA-Dependent-RNA-Polymerase. An Unusual Active Site Leading to High Replication Rates.

Viral infection relies on the hijacking of cellular machineries to enforce the reproduction of the infecting virus and its subsequent diffusion. In this context the replication of the viral genome is a key step performed by specific enzymes, i.e. polymerases. The replication of SARS-CoV-2, the causative agent of the COVID-19 pandemics, is based on the duplication of its RNA genome, an action performed by the viral RNA-dependent-RNA polymerase. In this contribution, for the first time and by using two-dimensional enhanced sampling quantum mechanics/ molecular mechanics, we have determined the chemical mechanisms leading to the inclusion of a nucleotide in the nascent viral RNA strand. We prove the high efficiency of the polymerase, which lowers the activation free energy to less than 10 kcal/mol. Furthermore, the SARS-CoV-2 polymerase active site is slightly different from those found usually found in other similar enzymes, and particularly it lacks the possibility to enforce a proton shuttle via a nearby histidine. Our simulations show that this absence is partially compensate by lysine, whose proton assist the reaction opening up an alternative, but highly efficient, reactive channel. Our results present the first mechanistic resolution of SARS-CoV-2 genome replication and shed light on unusual enzymatic reactivity paving the way for future rational design of antivirals targeting emerging RNA viruses.