γ‐Glutamyl Transpeptidase: Catalytic Mechanism and Gene Expression

Abstract

B y NAOYUKI TANIGUCHI and YOSHITAKA IKEDA, The D e p a r t m e n t of B i o c h e m i s t r y , O s a k a U n i v e r s i t y M e d i c a l School, S u i t a , O s a k a 5 6 5 , J a p a n

Novel Insights into Eukaryotic γ-Glutamyltranspeptidase 1 from the Crystal Structure of the Glutamate-bound Human Enzyme

“…In the transpeptidation reaction, the ␥-glutamyl group bound to Thr-381 is transferred to the ␣-nitrogen of a dipeptide acceptor, thereby forming a new ␥-glutamyl compound. The hydrolysis and transpeptidation reactions proceed by a modified pingpong mechanism (25,26).…”

Crystal structures of γ-glutamyltranspeptidase from Escherichia coli , a key enzyme in glutathione metabolism, and its reaction intermediate

“…First, the precursor protein of E. coli GGT has a signal peptide at its N-terminal region (13) directing export to the periplasmic space (14), but mammalian GGTs have anchor domains in their N-terminal regions leading to membrane association (15). Additionally, mammalian GGTs are heterologously glycosylated, whereas E. coli GGT is nonglycosylated (2). The ability to highly purify large quantities of soluble enzyme makes E. coli GGT an ideal enzyme to study the general reaction mechanisms of this class of enzyme.…”

“…GGT is evolutionarily conserved and found in organisms ranging from bacteria to mammals; the enzyme has a variety of physiological roles. Glutathione͞xenobiotic conjugates are cleaved by GGT at the ␥-glutamyl linkage, and these products are metabolized via the mercapturic acid pathway leading to the excretion of mercapturic acids into the bile and urine (2). GGT also cleaves exogenous glutathione for use as a cysteine and nitrogen source in Escherichia coli, yeast, and mammalian cells (3)(4)(5).…”

Cloning and molecular analysis of radish (Raphanus sativus L.) cDNAs encoding heterodimeric .GAMMA.-glutamyltransferases

“…Among them are one arginine and two aspartic acid residues (R107, D422 and D423 in human GGT), which are believed to be of critical importance for binding to the g-glutamyl donor substrate, and two serine residues, which are necessary for GGT catalytic activity (S451 and S452) (Taniguchi and Ikeda 1998) (Figure 1). Radish GGTs presented also a putative protease cleavage site that includes the conserved threonine residue (T381) (Hashimoto et al 1995) (Figure 1).…”

“…Structural analysis of the deduced radish GGT amino acid sequences (Figure 1) revealed that they contain amino acid residues necessary for mammalian heterodimeric GGT function (Taniguchi and Ikeda 1998). Among them are one arginine and two aspartic acid residues (R107, D422 and D423 in human GGT), which are believed to be of critical importance for binding to the g-glutamyl donor substrate, and two serine residues, which are necessary for GGT catalytic activity (S451 and S452) (Taniguchi and Ikeda 1998) (Figure 1).…”