[51] γ-Glutamyl transpeptidase: Kinetics and mechanism

Abstract

This chapter focuses on the kinetics and mechanism of γ-glutamyl transpeptidase. Three catalytic activities are associated with γ-glutamyl transpeptidase. All of these activities involve the transfer of the γ-glutamyl moiety of glutathione (or of other γ-glutamyl compounds such as glutamine, glutathione disulfide, glutathione S-conjugates, and model substrates such as γ-glutamyl-p-nitroanilide) to an acceptor substrate. Transpeptidation occurs when the acceptor is an amino acid, dipeptide, or tripeptide, leading to formation of the corresponding γ-glutamyl amino acid or γ-glutamyl dior tripeptide. If an amino acid acceptor contains two stereochemically distinct α-aminogroups from which two separate γ-glutamyl products are formed, the experimentally determined kinetic parameters are composite values reflecting both activities. L-Cystathionine is such a substrate for γ-glutamyl transpeptidase. At L-γ-glutamyl donor concentrations less than 30 μM, L-cystathionine exhibits a relative reactivity (Vb/Kb) comparable to that of L-cystine, the most active acceptor of the common amino acids. © 1985, Elsevier Inc. All rights reserved.