SARS coronavirus main protease (SARS-CoV Mpro) is essential for the replication of the virus and regarded as a major antiviral drug target. The enzyme is a cysteine protease, with a catalytic dyad (Cys-145/His-41 ) in the active site. Aldehyde inhibitors can bind reversibly to the active-site sulfhydryl of SARS-CoV Mpro. Previous studies using peptidic substrates and inhibitors showed that the substrate specificity of SARS-CoV Mpro requires glutamine in the P1 position and a large hydrophobic residue in the P2 position. We determined four crystal structures of SARS-CoV M^pro in complex with pentapeptide aldehydes (Ac-EST-LQ-H, Ac-NSFSQ-H, Ac-DSFDQ-H, and Ac-NSTSQ-H). Kinetic data showed that all of these aldehydes exhibit inhibitory activity towards SARS-CoV M^pro, with K_i values in the μM range. Surprisingly, the X-ray structures revealed that the hydrophobic S2 pocket of the enzyme can accommodate serine and even aspartic-acid side-chains in the P2 positions of the inhibitors. Consequently, we reassessed the substrate specificity of the enzyme by testing the cleavage of 20 different tetradecapeptide substrates with varying amino-acid residues in the P2 position. The cleavage efficiency for the substrate with serine in the P2 position was 160-times lower than that for the original substrate (P2 = Leu); furthermore, the substrate with aspartic acid in the P2 position was not cleaved at all. We also determined a crystal structure of SARS-CoV M^pro in complex with aldehyde Cm-FF-H, which has its P1-phenylalanine residue bound to the relatively hydrophilic S1 pocket of the enzyme and yet exhibits a high inhibitory activity against SARS-CoV M^pro, with K_{i =} 2.24 ± 0.58 μM. These results show that the stringent substrate specificity of the SARS-CoV M^pro with respect to the P1 and P2 positions can be overruled by the highly electrophilic character of the aldehyde warhead, thereby constituting a deviation from the dogma that peptidic inhibitors need to correspond to the observed cleavage specificity of the target protease.

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{{Explor lien
   |wiki=    Sante
   |area=    SrasV1
   |flux=    Main
   |étape=   Exploration
   |type=    RBID
   |clé=     Pascal:12-0002101
   |texte=   Peptide aldehyde inhibitors challenge the substrate specificity of the SARS-coronavirus main protease
}}