The effect of pH on the transpeptidation and hydrolytic reactions of rat kidney gamma-glutamyltransferase.
The effect of pH upon the transpeptidation and hydrolytic reactions of gamma-glutamyltransferase [5-glutamyl)-peptide:amino-acid 5-glutamyltransferase, EC 2.3.2.2) have been investigated. It was found that the enzyme was irreversibly inactivated below pH 7.5 or above pH 9.4. Transpeptidation was markedly pH-dependent, while hydrolysis was pH-independent. The pH optimum for transpeptidation was found to vary for different acceptors. The ascending limb of the pH-optimum curve is attributed to the pK of the alpha-amino group of the acceptor, while the descending limb of the pH-optimum curve is attributed to an ionisable group in the active site of the enzyme. These observations provide much information about the interaction of the enzyme with the acceptor: (1) the true acceptor for gamma-glutamyltransferase is the deprotonated form of the amino acid; (2) glycylglycine has a similar acceptor activity to methionine, its apparent higher activity being due to the low pK of the alpha-amino group; (3) the enzyme is reversibly inactivated at higher pH by the deprotonation of a group in the active site which is involved in both binding of acceptor and catalysis of transpeptidation (this group is not involved in the hydrolysis reaction); (4) at pH 8.5, the normal pH for assay, only 47% of the enzyme is active, while at pH 7.4 gamma-glutamyltransferase is 93% in the active form.
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D000818:Animals; D055598:Chemical Phenomena; D002621:Chemistry; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007668:Kidney / Q000201:enzymology*; D007700:Kinetics; D051381:Rats; D005723:gamma-Glutamyltransferase
D005723:gamma-Glutamyltransferase