Common acylcarboxypeptidase A intermediates for ester substrates containing different cleaving alcohols.

In the carboxypeptidase A-catalyzed ester hydrolysis of the trans-alpha-(benzoylamino)cinnamoyl derivatives of both L-mandelate and L-beta-phenyllactate, kcat stands for the breakdown of an additional enzyme-substrate complex (ES'). The pH dependence of kcat indicates that ES' is the anhydride acylcarboxypeptidase A formed by the nucleophilic attack of Glu-270 for both of the ester substrates. Furthermore, the very similar kcat values for the two ester substrates which share a common acyl moiety strongly suggest that common acylenzyme intermediates are involved, whose breakdown process is little affected by the cleaved alcohol portions. This provides the most direct evidence that has been reported for the nucleophilic mechanism of the carboxypeptidase A action.