The donor specificity and kinetics of the hydrolysis reaction of gamma-glutamyltransferase.

The donor specificity of the hydrolytic reaction of gamma-glutamyltransferase [5-glutamyl)-peptide:amino-acid 5-glutamyltransferase, EC 2.3.2.2) has been studied by the use of specifically synthesised gamma-glutamyl substrates. It was found that a wide variety of gamma-glutamylated adducts were hydrolysed by the enzyme. The structure of the adduct was relatively unimportant for donor specificity and the enzyme appears to 'recognise' the gamma-glutamyl portion of the donor molecule. In particular the alpha-amino group and the free proton of the gamma-peptide bond appear to be essential for donor activity. The Vmax of hydrolysis increased proportionally to the electron-withdrawing capacity of the adduct moiety. The rate of formation of gamma-glutamyl-enzyme intermediate was therefore dependent upon the structure of the adduct of the gamma-glutamyl donor. The results suggest that the enzyme shows little specificity beyond that for gamma-glutamyl amides and there is therefore no reason to postulate the presence of a specific glutathione-binding site.