Glutamine synthetase of pea leaves: divalent cation effects, substrate specificity, and other properties

Glutamine Synthesis and Its Relation to Photophosphorylation in Pisum Chloroplasts: Effects of 3-(3,4-Dichlorophenyl)-1,1-dimethylurea and Antimycin A

Illuminated pea (Pisum sativum) chloroplasts can convert glutamate to glutamine using ATP generated by photophosphorylation to drive the glutamine-synthetase reaction. Light-dependent glutamine synthesis is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), but only at concentrations higher than are necessary to suppress photoreduction of ferricyanide or phosphoglycerate. Conversely, glutamine synthesis is far more sensitive to antimycin A than is photoconversion of phosphoglycerate to triosephosphate. When 3.8 mm phosphoglycerate is supplied, glutamine synthesis is stimulated in both the presence and absence of antimycin A.These data seem to be consistent with the operation of an endogenous, DCMU-sensitive, phosphorylation process-possibly cyclic-which can support glutamine synthesis in white light under aerobic conditions. The stimulatory effect of phosphoglycerate suggests that noncyclic phosphorylation is initiated or accelerated when this substrate is supplied. This noncyclic process evidently provides ATP over and above the amount required for phosphoglycerate photoreduction, i.e. the ATP/e(2) ratio exceeds 1.0. The additional ATP produced under these conditions is available for glutamine synthesis and lessens its dependence on cyclically (or pseudocyclically) generated ATP.

The purification and properties of the glutamine synthetase from the cytosol of Soya-bean root nodules

The major portion of glutamine synthetase activity in root nodules of soya-bean plants is associated with the cytosol rather than with Rhizobium japonicum bacteroids. Glutamine synthetase accounts for about 2% of the total soluble protein in nodule cytosol. Glutamine synthetase from nodule cytosol has been purified by a procedure involving fractionation with protamine sulphate, ammonium sulphate and polypropylene glycol, chromatography on DEAE-Bio-Gel A and Bio-Gel A-5m and affinity chromatography on glutamate-agarose columns. The purified preparation appeared to be homogeneous in the analytical ultracentrifuge. From sedimentation-equilibrium experiments a mol. wt. of about 376000 was determined for the native enzyme and 47300 for the enzyme in guanidinium chloride. From these data and measurements of electron micrographs, we have concluded that glutamine synthetase from nodule cytosol consists of eight subunits arranged in two sets of planar tetramers which form a cubical configuration with dimensions of about 10 nm (100 A) across each side. Glutamine synthetase from nodule cytosol has a higher glycine and proline content and a lower content of phenylalanine than the glutamine synthetase that has been prepared from pea seed. The cytosol enzyme contains four half-cystine molecules per subunit, which is in contrast with two reported for the enzyme from pea seed. Enzyme activity is striking influenced by the relative proportion of Mg2+ and Mn2+ in the assay medium. Activity is inhibited by feedback inhibitors and is influenced by energy charge.

Pea leaf glutamine synthetase: regulatory properties

Of a variety of purine and pyrimidine nucleotides tested, only ADP and 5'AMP significantly inhibited the Mg(2+)-dependent activity of pea leaf glutamine synthetase. They were less effective inhibitors where Mn(2+) replaced Mg(2+). They were competitive inhibitors with respect to ATP, with inhibition constant (Ki) values of 1.2 and 1.8 mm, respectively. The energy charge significantly affects the activity of glutamine synthetase, especially with Mg(2+). Of a variety of amino acids tested, l-histidine and l-ornithine were the most inhibitory, but significant inhibition was seen only where Mn(2+) was present. Both amino acids appeared to compete with l-glutamate, and the Ki values were 1.9 mm for l-histidine (pH 6.2) and 7.8 mm for l-ornithine (pH 6.2). l-Alanine, glycine, and l-serine caused slight inhibition (Mn(2+)-dependent activity) and were not competitive with ATP or l-glutamate.Carbamyl phosphate was an effective inhibitor only when Mn(2+) was present, and did not compete with substrates. Inorganic phosphate and pyrophosphate caused significant inhibition of the Mg(2+)-dependent activity.