Involvement of Ser-451 and Ser-452 in the catalysis of human γ-glutamyl transpeptidase

Abstract

The serine residue required for catalysis of γ-glutamyl transpeptidase was identified by site-specific mutagenesis of the conserved serine residues on the basis of sequence alignment of the light subunit of human, rat, pig and two bacterial enzymes. Recombinant human γ-glutamyl transpeptidases with replacements of these serine residues by Ala were expressed using a baculovirus-insect cell system. Substitutions of Ala at Ser-385, -413 or - 425 yielded almost fully active enzymes. However, substitutions of Ala at Ser-451 or -452 yielded enzymes that were only about 1% as active as the wild-type enzyme. Further, their double mutant is only 0.002% as active as the wild type. Kinetic analysis of transpeptidation using glycylglycine as acceptor indicates that the V(max) values of Ser-451 and -452 mutants are substantially decreased (to about 3% of the wild type); however, their K(m) values for L-γ-glutamyl-p-nitroanilide as donor were only increased about 5 fold compared to that of the wild type. The double mutation of Ser-451 and - 452 further decreased the V(max) value to only about 0.005% of the wild type, while this mutation produced only a minor effect (2-fold increase) on the K(m) value for the donor. The kinetic values for the hydrolysis reaction of L-γ-glutamyl-p-nitroanilide in the mutants followed similar trends to those for transpeptidation. The rates of inactivation of Ser-451, -452 and their double mutant enzymes by acivicin, a potent inhibitor, were less than 1% that of the wild-type enzyme. The K(i) value of the double mutant for L-serine as a competitive inhibitor of the γ-glutamyl group is only 9 fold increased over that of the wild type, whereas the K(i) for the serine-borate complex, which acts as an inhibitory transition-state analog, was more than 1,000 times higher than for the wild-type enzyme. These results suggest that both Ser-451 and -452 are located at the position able to interact with the γ- glutamyl group and participate in catalysis, probably as nucleophiles or through stabilization of the transition state.