Figurę 15-23 Catalytic mechanism of the senne proteases. I*hc rcaction involves (1) the nucleophilic attack of the active site Ser on the carbonyl carbon atom of the scissilc pcptide bond to form the tctrahedral intcrmediatc;(2) the decomposition of the tetrahedral intermediatc to the acyl-enzyme intcrmediate through generał acid catalysis by the activc site Asp-polarized His. followed by loss of the aminę product and its replacement by a water molccule; (3) the reversal of Step 2 to form a second tctrahedral intcrmediate; and (4) the rcvcrsal of Step 1 to yicld the rcaction’s carboxyl product and the active enzyme.