Regulation of Glutamine Synthetase Activity

Abstract

The role ofglutamine as a donor of amino group and as a source ofnitrogen for a variety of important precursors of macromolecules is discussed. The regulation of glutamine synthetase activity in Escherichia coli occurs by repression, cumulative feedback inhibition, by divalent catir.ns, and by adenylylation and deadenylylation ofthe enzymes. The cascade consisting of the interaction, of modifying enzymes and effectors, a novel method of regulation, was discovered using this enzyme system. The regulation in the case of Gram negative organisms is achieved by synergistic inhibition by end products and by simple product inhibition.In view-of the large concentration of glutamine present in the body fluids of several mammalian organisms, the regulation of glutamine synthetase does not appear to be of major  significance.Glutamine synthetase from plant sources has not been subjected to the same degree of intensive investigation. The enzymes frommung beaq (Phaseolus aureus) was purified and shown to catalyze the 6-glutamyl transferase reaction by a ping-pong mechanism. Glycine, alanine and histidine as well as adenine nucleotides inhibited in a cumulative manner. Multiple inhibition analysis revealed non exclusive binding sites for alanine and glycine and partial non-competitive and mixed type inhibition for these amino acids. Similar non-exclusivity in the binding sites for histidine and glycine were observed. ADP and AMP had mutually exclusive binding sites and were competitive inhibitors. The enzyme-antibody reaction was used to show that conformational changes occurred on the binding of substrates.A comparison of the properties of the glutamine synthetases from bacteria), animal and plant sources revealed several common features and a few significant differences emphasizing the multitude functions of glutamine in the metabolism of the cell.