Glutamine-binding subunit of glutamate synthase and partial reactions catalyzed by this glutamine amidotransferase

Abstract

In the course of studies on glutamine-dependent carbamyl phosphate synthetase from Aerobacter aerogenes, we purified another protein which was found to be glutamate synthase (EC 2.6.1.53). The enzyme, obtained in apparently homogeneous form (monomer molecular weight about 227,000; s(20,omega) = 17.6 S), was found to be a typical glutamine amidotransferase in that it exhibits glutaminase activity and can utilize ammonia in place of glutamine as a nitrogen donor. The enzyme also catalyzes at low rates the oxidative deamination of glutamate in the presence of TPN, and it exhibits TPNH oxidase activity. The enzyme is similar to the glutamate synthase found in Escherichia coli in that it is an iron-sulfide flavoprotein. Treatment of the enzyme with sodium dodecyl sulfate or potassium thiocyanate dissociates it into nonidentical subunits exhibiting molecular weights of about 175,000 and 51,500. The glutamine-dependent activity of the enzyme is inhibited by L-2-amino-4-oxo-5-chloropentanoic acid, but this chloroketone analog of glutamine does not affect the ammonia-dependent glutamate synthase activity. Studies with [(14)C]chloroketone show that the reagent binds to the heavy subunit only. Inhibition by the chloroketone and its binding to the heavy subunit are markedly reduced in the presence of L-glutamine. Sedimentation velocity studies carried out in potassium thiocyanate indicate that iron-sulfide and flavin sites are also located on the heavy subunit. While these studies show that glutamate synthase resembles other glutamine amidotransferases in certain of its catalytic properties, the findings indicate that the light subunit of this enzyme, in contrast to that of several other glutamine amidotransferases, does not function to bind glutamine. It is of interest that the enzyme exhibits an unusually high affinity for ammonia as compared to a number of other glutamine amidotransferases. Glutamate synthase is inhibited (competitively with respect to glutamine) by low concentrations of methionine sulfone, methionine sulfoximine, and methionine sulfoxide.