Chicken liver phosphofructokinase. III. Kinetics and allosteric properties

Some kinetic and allosteric properties of crystalline chicken liver phosphofructokinase were investigated. Lineweaver Burk plots with fructose 6 phosphate and ATP as substrates yield a series of apparently parallel lines. The liver enzyme also catalyzes the phosphorylation of fructose 1 P, and the double reciprocal plot with fructose 1 P as the substrate yields intersecting lines. Phosphorylation of both fructose 6 P and fructose 1 P are lost at 4° at equal rate. Of the many sugar phosphates examined, only glucose 6 P, 6 P gluconate, and ribulose 5 P relieve ATP inhibition of liver phosphofructokinase. These sugar Ps are competitive inhibitors of fructose 6 P at noninhibitory concentrations of ATP. Isocitrate and citrate inhibit phosphofructokinase, while NADH does not. Arrhenius plots of enzyme activity of both ATP inhibited and uninhibited activities show a sharp break at approximately 15°. The K(m) value for ATP decreases from 0.041 mM to 0.012 mM below the transition temperature, while the K(m) for fructose 6 P is not affected by different temperature. These results suggest the existence of two different conformational states of liver phosphofructokinase. Fructose 1,6 diphosphatase at high concentration enhances ATP inhibition of phosphofructokinase, but at lower concentration it relieves the inhibition. The maximum inhibition by FDPase is usually obtained at a molar ratio (fructose diphosphatase to phosphofructokinase) of approximately 150. Moreover, fructose diphosphatase protects phosphofructokinase against cold inactivation. Physiological significance of these observations and a possible mechanism for the regulation of phosphofructokinase by fructose diphosphatase are discussed.