Kinetic characterization and identification of the acylation and glycosylation sites of recombinant human γ-glutamyltranspeptidase

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 QGTs 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 pPICZ alpha A vector and transformed into Pichia pastoris to express as a 68 kDa His-tagged protein. The optimized expression and secretion of this enzyme in I 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 k(cat) 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 k(cat) of 53 min(-1) and a Km value of 7.3 mu M 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.