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 investigated. The apparent equilibrium constant, K', for the biochemical reaction P-enolpyruvate + ADP = ATP + Pyr was defined as K' = [ATP][Pyr]/[ADP][P-enolpyruvatel] 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 X 10(4) (n = 8) at 25 degreesC. The standard apparent enthalpy (DeltaH'degrees) for the biochemical reaction was -4.31 kJmol(-1) in the direction of ATP formation. The corresponding standard apparent entropy (DeltaS'degrees) was +73.4 J K-1 mol(-1). The DeltaHdegrees and AS' values for the reference reaction, P-enolpyruvate(3-) + ADp(3-) + H+ = ATp(4-) + Pyr(1-), were -6.43 kJmol(-1) and + 180 J K-1 mol(-1), respectively (5 to 38 degreesC). We examined further the mass action ratio in rat heart and skeletal muscle at rest and found that the pyruvate kinase reaction in vivo was close to equilibrium i.e. within a factor of about 3 to 6 of K' in the direction of ATP at the same pH, free [Mg2+], and T. We conclude that the pyruvate kinase reaction may be reversed under some conditions in vivo, a finding that challenges the long held dogma that the reaction is displaced far from equilibrium.