Critical role of the cysteine 323 residue in the catalytic activity of human glutamate dehydrogenase isozymes

The role of residue C323 in catalysis by human glutamate dehydrogenase isozymes (hGDH1 and hGDH2) was examined by substituting Arg, Gly, Leu, Met, or Tyr at C323 by cassette mutagenesis using synthetic human GDH isozyme genes. As a result, the Km of the enzyme for NADH and a-ketoglutarate increased up to 1.6-fold and 1.1-fold, respectively. It seems likely that C323 is not responsible for substrate-binding or coenzyme-binding. The efficiency (k(cat)/K-m) of the mutant enzymes was only 11-14% of that of the wild-type isozymes, mainly due to a decrease in k(cat) values. There was a linear relationship between incorporation of [C-14]p-chloromercuribenzoic acid and loss of enzyme activity that extrapolated to a stoichiometry of one mol of [C-14] incorporated per mol of monomer for wild type hGDHs. No incorporation of [C-14]p-chloromercuribenzoic acid was observed with the C323 mutants. ADP and GTP had no effect on the binding of p-chloromercuribenzoic acid, suggesting that C323 is not directly involved in allosteric regulation. There were no differences between the two hGDH isozymes in sensitivities to mutagenesis at C323. Our results suggest that C323 plays an important role in catalysis by human GDH isozymes.