The rate of deacylation of acetyl-α-chymotrypsin is accelerated by the addition of an ionizable nucleophile, monoisonitrosoacetone (pKa 8.3) in a linear, first-order fashion at all pH's. The pH dependence of the rate enhancement is a bell-shaped curve with true pKa's at 7.3 and 8.3, corresponding to ionization of an enzyme active site group and of nucleophile, respectively. Extrakinetic evidence and model ester reactions suggest that the most likely mechanism involves neutral imidazole acting as a general base toward protonated monoisonitrosoacetone, rather than protonated imidazole acting as a general acid to assist the attack of monoisonitrosoacetone anion.
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