nsp5

nsp5, or non-structural protein 5, is a cysteine protease encoded by coronaviruses as part of the replication-transcription complex. It is produced from the ORF1a/1ab polyprotein and functions as the main protease (Mpro or 3CLpro), cleaving the polyprotein at multiple sites to yield mature non-structural proteins essential for replication and transcription.

Substrate specificity and cleavage: Nsp5 recognizes canonical cleavage sequences predominantly at Leu-Gln↓(Ser,Ala,Gly) and cleaves at about 11 distinct sites in SARS-CoV-2–like polyproteins.

Structure and mechanism: It is a cysteine protease with a catalytic dyad comprising Cys145 and His41 (SARS-CoV-2 numbering) within a chymotrypsin-like fold. The enzyme functions as a homodimer; dimerization is required for activity, and the N-finger of one protomer helps shape the substrate-binding site of the other.

Role and importance: By processing the polyprotein, nsp5 triggers formation of the replication–transcription complex and is essential for viral replication. Disruption of its activity halts replication.

Inhibitors and research: Nsp5 is a major antiviral target. Numerous inhibitors have been developed, including covalent and non-covalent compounds. Nirmatrelvir, the component of Paxlovid, inhibits Mpro and has clinical use against SARS-CoV-2. Ongoing structural and medicinal chemistry efforts aim to broaden efficacy and monitor resistance.

Conservation and resources: Nsp5 is conserved among coronaviruses, with variation across genera. Its study is aided by extensive structural data and biochemical assays, supporting ongoing research in drug design and viral biology.