Unified picture of mechanisms of catalysis by carboxypeptidase A.

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Abstract
We have reported evidence that an anhydride intermediate is not involved in the hydrolysis of typical peptide substrates by carboxypeptidase A (peptidyl-L-amino-acid hydrolase, EC 3.4.12.2), and we describe further evidence here. Recently an anhydride intermediate has been detected in the hydrolysis of an ester substrate by this enzyme. Other evidence also suggests that esters and peptides may not be cleaved by the same type of mechanism. A possible explanation is that the substrate carbonyl and a water molecule are always aligned between glutamate-270 and the zinc atom of the enzyme, but not always in the same sequence. With peptides the carbonyl is coordinated to zinc, and the water is delivered by glutamate acting as a general base. Esters are weaker ligands, and in some cases the ester carbonyl may not displace water from zinc. This would lead to a nucleophilic mechanism, with glutamate-270 forming an anhydride while zinc-aquo serves as a Brönsted acid. This picture is consistent with other evidence on ester cleavage, and resolves the otherwise baffling discrepant data on peptide as compared to ester substrates.