Mechanism of carboxypeptidase-Y-catalysed peptide semisynthesis

Synthesis of hydroxymethylglutathione from glutathione and L-serine catalyzed by carboxypeptidase Y

Hydroxymethylglutathione (gamma-L-glutamyl-L-cysteinyl-L-serine; hmGSH) occurs in many species belonging to the family Gramineae, but the biosynthetic pathway for hmGSH has not been identified. We found that carboxypeptidase Y (CPY), but not carboxypeptidase A, catalyzed hmGSH synthesis from glutathione and L-serine in vitro at acidic pH. CPY also catalyzed methylglutathione synthesis from glutathione and L-alanine. These findings suggested that a carboxypeptidase-like enzyme may be involved in hmGSH synthesis in vivo.

gamma-Glutamyltranspeptidase-catalysed acyl-transfer to the added acceptor does not proceed via the ping-pong mechanism

Acyl-transfer catalysed by gamma-glutamyltranspeptidase from bovine kidney was studied using gamma-L- and gamma-D-Glu-p-nitroanilide as the donor and GlyGly as the acceptor. The transfer of the gamma-Glu group to GlyGly was shown to be accompanied by transfer of the gamma-Glu group to water (hydrolysis). The results were compared with acyl-transfer catalysed by the representative serine protease, alpha-chymotrypsin. The main difference between the kinetic mechanism of the acyl-transfer reactions catalysed by these enzymes, which contain an active-site serine and form an acyl-enzyme intermediate but belong to different enzyme classes, was found to consist in the role of the enzyme-donor-acceptor complex. This complex is not formed at any acceptor concentrations in the acyl-transfer reactions catalysed by the serine proteases. In contrast, in the gamma-glutamyltranspeptidase-catalysed acyl-transfer the pathway going through the ternary enzyme-donor-acceptor complex formed from the enzyme-acceptor complex becomes the main pathway of the transfer reaction even at moderate acceptor concentrations. As a result, gamma-glutamyltranspeptidase catalysis follows a sequential mechanism with random equilibrium addition of the substrates and ordered release of the products. The second distinction concerns the inhibitory effect of the acceptor. In the case of alpha-chymotrypsin this was the result of true inhibition, i.e. a dead-end formation of the enzyme-acceptor complex. A salt effect caused by the acceptor was the rationale of a similar effect observed in acyl-transfer catalysed by gamma-glutamyltranspeptidase.

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.

The second nucleophile molecule binds to the acyl-enzyme-nucleophile complex in alpha-chymotrypsin catalysis. Kinetic evidence for the interaction

alpha-Chymotrypsin-catalyzed acyl transfer was studied using three acyl-group donors (Mal-L-Ala-L-Ala-L-PheOMe, Bz-L-TyrOEt and Ac-L-TrpOEt; Mal, maleyl; Bz, benzoyl; OMe, methyl ester; OEt, ethyl ester) and a series of amino-acid amides. Most of the reactions studied can be described by the simplest kinetic model without the nucleophile binding to the acyl-enzyme. The alpha-chymotrypsin-catalyzed transfer of the Mal-L-Ala-L-Ala-L-Phe group to the amides of L-Phe and L-Tyr showed a linear dependence of the partition constant, p, on the nucleophile concentration which can be interpreted by the hydrolysis of the acyl-enzyme-nucleophile complex. The alpha-chymotrypsin-catalyzed transfer of the Bz-L-Tyr and Ac-L-Trp groups to several amino-acid amides showed unusual behavior which can be interpreted by the kinetic model involving formation of a complex of the acyl-enzyme with two nucleophile molecules. These observations can explain the conflicting conclusions concerning the kinetics of alpha-chymotrypsin-catalyzed acyl transfer evident in previous studies.

Serine carboxypeptidases: a new and versatile family of enzymes

Several structural and biochemical studies in the past year have revealed the potential application of a new family of serine proteinase, the serine carboxypeptidases, in peptide synthesis, carboxy-terminal peptide sequencing and the production of biologically active carboxy-terminal peptide amides. The recent determination of the high-resolution crystal structures for two members of the family, CPDW-II and CPDY, should permit protein engineering to further increase their utility.