alphacoronavirus non-structural protein 5, also called Main protease (Mpro)
This subfamily contains the coronavirus (CoV) non-structural protein 5 (Nsp5) also called the Main protease (Mpro), or 3C-like protease (3CLpro), found in alphacoronaviruses. CoVs utilize a multi-subunit replication/transcription machinery. A set of non-structural proteins (Nsps) generated as cleavage products of the ORF1a and ORF1ab viral polyproteins assemble to facilitate viral replication and transcription. Mpro/Nsp5 is a key enzyme in this process, making it a high value target for the development of anti-coronavirus therapeutics. These enzymes belong to the MEROPS peptidase C30 family, where the active site residues His and Cys form a catalytic dyad. The structures of Mpro/Nsp5 consist of three domains with the first two containing anti-parallel beta barrels and the third consisting of an arrangement of alpha-helices. The catalytic residues are found in a cleft between the first two domains. Mpro/Nsp5 requires a Gln residue in the P1 position of the substrate and space for only small amino-acid residues such as Gly, Ala, or Ser in the P1' position; since there is no known human protease with a specificity for Gln at the cleavage site of the substrate, these viral proteases are suitable targets for the development of antiviral drugs.
Jiyao W et al.(2023). "The conserved domain database in 2023.",