Thermodynamics of the pyruvate kinase reaction and the reversal of glycolysis in heart and skeletal muscle

Abstract

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-enolpyrurate], 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 (ΔH′°) for the biochemical reaction was -4.31 kJmol-1 in the direction of ATP formation. The corresponding standard apparent entropy (ΔS′°) was +73.4 J K-1 mol-1. The ΔH° and ΔS° values for the reference reaction, P-enolpyruvate3- + ADP3- + H+ = ATP4- + Pyr1-, were -6.43 kJmol-1 and + 180 J K-1 mol-1, respectively (5 to 38°C). 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.