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.
Gololobov MYu
Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg 39406-5043.
Bateman
R C
RC
D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D002417:Cattle; D002918:Chymotrypsin / Q000378:metabolism; D005973:Glutamine / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D006868:Hydrolysis; D007700:Kinetics; D008954:Models, Biological; D009842:Oligopeptides / Q000378:metabolism / Q000494:pharmacology; D012694:Serine / Q000378:metabolism; D013237:Stereoisomerism; D005723:gamma-Glutamyltransferase / Q000378:metabolism*
D009842:Oligopeptides; D005973:Glutamine; D012694:Serine; C001300:gamma-glutamine-4-nitroanilide; D005723:gamma-Glutamyltransferase; D002918:Chymotrypsin