Some properties of glutamine synthetase and glutamate synthase from Chlorobium vibrioforme f. thiosulfatophilum

The phototrophic green sulphur bacterium Chlorobium vibrioforme f. thiosulfatophilum assimilated ammonia via glutamine synthetase and glutamate synthase when grown with ammonia up to 30 mM, but above this level glutamate dehydrogenase was the key enzyme. Glutamine synthetase purified 42-fold was found to be adenylylated. The -glutamyltransferase activity of the enzyme was markedly inhibited by alanine, glycine, serine and lysine, and these amino acids in various combinations showed cumulative inhibition. Adenine nucleotides also inhibited enzyme activity, especially ATP. Glutamate synthase purified 222-fold had a maximum absorption at 440 nm which was reduced by sodium dithionite, and the enzyme was inhibited by atebrin indicating the presence of a flavin component. The enzyme had specific requirements for NADH, -ketoglutarate and l-glutamine, the K_m values for these were 13.5, 270 and 769 M respectively. Glutamate synthase was sensitive to feedback inhibition by amino acids, adenine nucleotides and other metabolites and the combined effects of these inhibitors was cumulative.Abbreviations GS glutamine synthetase - GOGAT glutamate synthase - GDH glutamic dehydrogenase