Regulation of glutamine synthetase. VI. Interactions of inhibitors for Bacillus licheniformis glutamine synthetase

The relationships of five feedback inhibitors for the Bacillus licheniformis glutamine synthetase were investigated. The inhibitors were distinguishable by differences in their competitive relationship for the substrates of the enzyme. Mixtures of l-glutamine and adenosine-5'-monophosphate (AMP) or histidine and AMP caused synergistic inhibition of glutamine synthesis. Histidine, alanine, and glycine acted antagonistically toward the l-glutamine inhibition. Alanine acted antagonistically toward the glycine and histidine inhibitions. Independence of inhibitory action was observed with the other pairs of effectors. Possible mechanisms by which the inhibitors may interact to control glutamine synthesis are discussed. The low rate of catalysis of the glutamyl transfer reaction by the B. licheniformis glutamine synthetase can be attributed to the fact that l-glutamine serves both as a substrate and an inhibitor for the enzyme. Effectors which act antagonistically toward the l-glutamine inhibition stimulated glutamotransferase activity. The stimulation was not observed when d-glutamine was used as substrate for the glutamyl transfer reaction.