Effects of pH on carboxypeptidase-Y-catalyzed hydrolysis and aminolysis reactions.

The pH dependencies of serine carboxypeptidase-Y-catalysed hydrolysis and aminolysis reactions using L-amino acids and L-amino acid amides as nucleophiles, have been studied and analyzed. The results reveal two catalytically important ionizing groups of the enzyme with rather similar pK values (5-6), the active site His397 and a possibly Glu residue, which is not only important in interactions with carboxylic groups of substrates and nucleophiles [Liao, D.-I., Breddam, K., Sweet, R. M., Bullock, T. & Remmington, S. J. (1992) Biochemistry 31, 9796-9812], but also indirectly play a role in catalysis. This explains the pH behaviour of hydrolysis of both peptide and ester substrates and further, that L-amino acid amides are better nucleophiles in aminolysis reactions than L-amino acids.