Glutamate synthase. Properties of the glutamine-dependent activity.

Properties of glutamine-dependent glutamate synthase have been investigated using homogeneous enzyme from Escherichia coli K-12. In contrast to results with enzyme from E. coli strain B (Miller, R. E., and Stadtman, E. R. (1972) J. Biol. Chem. 247, 7407-7419), this enzyme catalyzes NH3-dependent glutamate synthase activity. Selective inactivation of glutamine-dependent activity was obtained by treatment with the glutamine analog. L-2-amino-4-oxo-5-chloropentanoic acid (chloroketone). Inactivation by chloroketone exhibited saturation kinetics; glutamine reduced the rate of inactivation and exhibited competitive kinetics. Iodoacetamide, other alpha-halocarbonyl compounds, and sulfhydryl reagents gave similar selective inactivation of glutamine-dependent activity. Saturation kinetics were not obtained for inactivation by iodoacetamide but protection by glutamine exhibited competitive kinetics. The stoichiometry for alkylation by chloroketone and iodoacetamide was approximately 1 residue per protomer of molecular weight approximately 188,000. The single residue alkylated with iodo [1-14C]acetamide was identified as cysteine by isolation of S-carboxymethylcysteine. This active site cysteine is in the large subunit of molecular weight approximately 153,000. The active site cysteine was sensitive to oxidation by H2O2 generated by autooxidation of reduced flavin and resulted in selective inactivation of glutamine-dependent enzyme activity. Similar to other glutamine amidotransferases, glutamate synthase exhibits glutaminase activity. Glutaminase activity is dependent upon the functional integrity of the active site cysteine but is not wholly dependent upon the flavin and non-heme iron. Collectively, these results demonstrate that glutamate synthase is similar to other glutamine amidotransferases with respect to distinct sites for glutamine and NH3 utilization and in the obligatory function of an active site cysteine residue for glutamine utilization.