ENERGETICS OF ALLOSTERIC REGULATION IN MUSCLE PYRUVATE-KINASE

The regulatory mechanism of rabbit muscle pyruvate kinase has been studied as a function of temperature in conjunction with phenylalanine, the allosteric inhibitor. The inhibitory effect of phenylalanine is modulated by temperature. At low temperatures, the presence of phenylalanine is almost inconsequential, but as the temperature increases so does the phenylalanine-dependent inhibition of the kinetic activity. In addition, the presence of phenylalanine induces cooperativity in the relation between velocity and substrate concentration. This effect is especially pronounced at elevated temperature. The kinetic data were analyzed using an equation that describes the steady-state kinetic velocity data as a function of five equilibrium constants and two rate constants. Van't Hoff analysis of the temperature dependence of the equilibrium constants determined by nonlinear curve fitting revealed that the interaction of pyruvate kinase with its substrate, phosphoenolpyruate, is an enthalpy-driven process. This is consistent with an interaction that involves electrostatic forces, and indeed, phosphoenolpyruvate is a negatively charged substrate. In contrast, the interaction of pyruvate kinase with phenylalanine is strongly entropy driven. These results imply that the binding of phenylalanine involves hydrophobic interaction and are consistent with the basic concepts of strengthening of the hydrophobic effect with an increase in temperature. The effect of phenylalanine at high temperatures is the net consequence of weakening of substrate-enzyme interaction and significant strengthening of inhibitor binding to the inactive state of pyruvate kinase. The effects of salts were also studied. The results show that salts also exert a differential effect on the binding of substrate and inhibitor to the enzyme. When the results of temperature, pH, and salt perturbations on the regulatory mechanism of muscle pyruvate kinase are combined, it is apparent that the regulatory behavior of pyruvate kinase observed in different solution environments is the net result of differential perturbation of substrate and inhibitor binding to the two conformational states of the enzyme.