title: Kinetic characterization and identification of the acylation and glycosylation sites of recombinant human gamma-glutamyltranspeptidase.
abstract: Gamma-glutamyltranspeptidase (GGT) is a heterodimeric enzyme important for glutathione homeostasis control. It has also been implicated in many physiological disorders, including Parkinson's disease, apoptosis inhibition, and diabetes. In the first step of its ping-pong mechanism it binds glutathione, its in vivo substrate, and releases cysteinylglycine upon formation of an acyl-enzyme intermediate. This intermediate can then react with water to release glutamate as a hydrolysis product or with an amino acid or dipeptide to form a transpeptidation product. Further detailed study of the mechanism underlying these reactions is hindered at least for some GGTs by the low quantities of protein available after a multistep purification from tissue. In the present work the gene for human GGT was cloned into the pPICZalphaA vector and transformed into Pichia pastoris to express as a 68 kDa His-tagged protein. The optimized expression and secretion of this enzyme in 1 L of culture and subsequent purification by immobilized metal affinity chromatography yielded 1.6 mg of purified enzyme having a specific activity of 237 U/mg. Kinetic parameters for the transpeptidation reaction between glutathione and glycylglycine were determined by mass spectrometry, giving a kcat of 13.4 x 10(3) min-1 and apparent KM values of 1.11 mM for glutathione and 8.1 mM for glycylglycine. The GGT-mediated hydrolysis of glutathione was also studied, providing a kcat of 53 min-1 and a KM value of 7.3 microM for glutathione. Incubation of the enzyme with a mechanism-based inhibitor, enzymatic digest, and mass spectrometric analysis provided the first unambiguous identification of Thr381 as the active site nucleophile of human gamma-glutamyltranspeptidase, and confirmed four of the seven N-linked glycosylation sites. These structural and kinetic data are discussed with respect to a homology model generated to facilitate visualization.
authors.affiliation: Département de chimie, Université de Montréal, Montréal, Québec, Canada H3C 3J7.
authors.forename: Roselyne
authors.initials: R
authors.lastname: Castonguay
authors.forename: Dany
authors.initials: D
authors.lastname: Halim
authors.forename: Mylène
authors.initials: M
authors.lastname: Morin
authors.forename: Alexandra
authors.initials: A
authors.lastname: Furtos
authors.forename: Christian
authors.initials: C
authors.lastname: Lherbet
authors.forename: Eric
authors.initials: E
authors.lastname: Bonneil
authors.forename: Pierre
authors.initials: P
authors.lastname: Thibault
authors.forename: Jeffrey W
authors.initials: JW
authors.lastname: Keillor
mesh_terms: D000215:Acylation; D000595:Amino Acid Sequence; D001483:Base Sequence; D002853:Chromatography, Liquid; D017931:DNA Primers; D006031:Glycosylation; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D010843:Pichia / Q000235:genetics; D016133:Polymerase Chain Reaction; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D053719:Tandem Mass Spectrometry; D005723:gamma-Glutamyltransferase / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D011994:Recombinant Proteins; D005723:gamma-Glutamyltransferase