Studies on the Hydrolytic Properties of (Serine) Carboxypeptidase Y

The activity of serine carboxypeptidases is dependent on a catalytic triad, an oxyanion hole, and a binding site equivalent to those found in the serine endopeptidases. The action of carboxypeptidase Y on substrates containing amino acids, alcohols, and amines as leaving groups is described. It is demonstrated that the features common to serine endopeptidases and carboxypeptidases are sufficient for hydrolysis of ester bonds. However, rapid hydrolysis of amide bonds is dependent on interactions between the C-terminal carboxylate group of the substrate and the C-terminal recognition site of the enzyme. Furthermore, on the basis of the pH dependencies of wild-type and mutant enzyme, combined with the ability of the enzyme to utilize binding energy to promote catalysis, alternative models for the high activity of carboxypeptidase Y at low pH are discussed. They describe how the catalytically essential histidine is maintained in its active deprotonated state through perturbation of its pKa value in the enzyme-substrate complex.