Enhancement of the glutaminase activity of carbamyl phosphate synthetase by alterations in the interaction between the heavy and light subunits

Abstract

Glutamine-dependent carbamyl phosphate synthetase (from Escherichia coli) was previously shown to be composed of a light subunit (molecular weight similar to 42,000) which has the binding site for glutamine and a heavy subunit (molecular weight similar to 130,000) which has binding sites for the other reactants and allosteric effectors. The subunits may be separated with retention of catalytic activities; only the separated light subunit exhibits glutaminase activity. The previous finding that storage of the native enzyme at pH 9 at 0 degrees increased its glutaminase activity by about 25-fold was further investigated; such storage markedly decreased the glutamine- and ammonia-dependent synthetase activities of the enzyme. Treatment of the enzyme with p-hydroxymercuribenzoate led to transient increase of glutaminase activity followed by inhibition. When the enzyme was treated with N-ethylmaleimide or with 5,5'-dithiobis-(2-nitrobenzoate), the glutaminase activity was increased by about 250-fold with concomitant loss of synthetase activities. The enhancement of glutaminase produced by storage of the enzyme at pH 9 was associated with intermolecular disulfide bond formation and aggregation of the enzyme. Aggregation also was observed after extensive treatment of the enzyme with 5,5'-dithiobis-(2-nitrobenzoate) or N-ethylmaleimide. However, a moderate increase of glutaminase activity (about 30-fold) was observed without aggregation under conditions in which one sulfhydryl group on the light subunit reacted with either reagent. The findings suggest that the increased glutaminase activities observed here are associated with structural changes in the enzyme in which the intersubunit relationship is altered so as to uncouple the catalytic functions of the enzyme and to facilitate access of water to the glutamine binding site on the light subunit.