Kinetic Characterization and Identification of the Acylation and Glycosylation Sites of Recombinant Human γ-Glutamyltranspeptidase
2007; American Chemical Society; Volume: 46; Issue: 43 Linguagem: Inglês
10.1021/bi700956c
ISSN1943-295X
AutoresRoselyne Castonguay, Dany Halim, Mylène Morin, Alexandra Fürtös, Christian Lherbet, Éric Bonneil, Pierre Thibault, Jeffrey W. Keillor,
Tópico(s)Peptidase Inhibition and Analysis
Resumoγ-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 pPICZαA 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 × 103 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 μM 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 γ-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.
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