RNA-Dependent RNA Polymerase From SARS-CoV-2. Mechanism Of Reaction And Inhibition By Remdesivir.

We combine sequence analysis, molecular dynamics and hybrid quantum mechanics/molecular mechanics simulations to obtain the first description of the mechanism of reaction of SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) and of the inhibition of the enzyme by Remdesivir. Despite its evolutionary youth, the enzyme is highly optimized to have good fidelity in nucleotide incorporation and a good catalytic efficiency. Our simulations strongly suggest that Remdesivir triphosphate (the active form of drug) is incorporated into the nascent RNA replacing ATP, leading to a duplex RNA which is structurally very similar to an unmodified one. We did not detect any reason to explain the inhibitory activity of Remdesivir at the active site. Displacement of the nascent Remdesivir-containing RNA duplex along the exit channel of the enzyme can occur without evident steric clashes which would justify delayed inhibition. However, after the incorporation of three more nucleotides we found a hydrated Serine which is placed in a perfect arrangement to react through a Pinners reaction with the nitrile group of Remdesivir. Kinetic barriers for crosslinking and polymerization are similar suggesting a competition between polymerization and inhibition. Analysis of SARS-CoV-2 mutational landscape and structural analysis of polymerases across different species support the proposed mechanism and suggest that virus has not explored yet resistance to Remdesivir inhibition.