Thermodynamics of the Pyruvate Kinase Reaction and the Reversal of Glycolysis in Heart and Skeletal Muscle*

The effect of temperature, pH, and free [Mg2] on the apparent equilibrium constant of pyruvate kinase (phosphoenol transphosphorylase) (EC 2.7.1.40) was in-vestigated. The apparent equilibrium constant, K, for the biochemical reaction P-enolpyruvate ADP ATP Pyr was defined as K [ATP][Pyr]/[ADP][P-enolpyru-vate], where each reactant represents the sum of all the ionic and metal complexed species in M. The K at pH 7.0, 1.0 mM free Mg2 and I of 0.25 M was 3.89 104 (n 8) at 25 °C. The standard apparent enthalpy (H°) for the biochemical reaction was4.31 kJmol1 in the direction of ATP formation. The corresponding standard appar-ent entropy (S°) was 73.4 J K1 mol1. The H ° and S ° values for the reference reaction, P-enolpyruvate3 ADP3 H ATP4 Pyr1, were6.43 kJmol1 an