Zn(II)-induced cooperativity of Escherichia coli ornithine transcarbamoylase

The steady-state reaction of ornithine transcarbamoylase (ornithine carbamoyltransferase, carbamoyl phosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3) purified from the argI gene product of Escherichia coli strain K-12 exhibits Michaelis-Menten kinetics over an extended range of concentration for both L-ornithine and carbamoyl phosphate. In the presence of Zn2+, however, the saturation curve of L-ornithine becomes sigmoidal, revealing positive cooperativity for this anabolic enzyme. The kinetic data give a limiting Hill coefficient of 2.7 for this substrate at 0.3 mM Zn2+. The allosteric effect of Zn2+ on the enzyme is not altered by the concentration of carbamoyl phosphate, and the saturation curve of carbamoyl phosphate remains hyperbolic in the presence of the metal ion. At fixed substrate concentrations, initial velocity data obtained at 0.-0.3 mM Zn2+ indicate cooperative binding of the metal ion to ornithine transcarbamoylase; a Hill coefficient of 1.7 +/- 0.1 is found that is independent of the level of L-ornithine. These results suggest competitive and exclusive binding to the enzyme between L-ornithine and Zn2+ with conformational changes induced in the subunits of the enzyme only by the metal ligand. Neither Co2+ nor Cu2+ exerts an effect on the kinetic behavior of the enzyme. This finding reveals not only specific allosteric control of ornithine transcarbamoylase by Zn2+ but also the possibility of an interlocking metabolic regulation between the urea cycle and the pathway for pyrimidine biosynthesis.