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J. Biol. Chem., Vol. 277, Issue 30, 27176-27182, July 26, 2002

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

Geoffrey P. Dobson^§, Sam Hitchins, and Walter E. Teague Jr.^¶

From the  Division of Physiology and Pharmacology, School of Biomedical and Molecular Sciences, James Cook University, Townsville, Queensland 4811, Australia and the ^¶ Section of Nuclear Magnetic Resonance Studies, Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcohol Abuse and Alcoholism, Rockville, Maryland 20852

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-enolpyruvate], 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 was 4.31 kJmol¹ in the direction of ATP formation. The corresponding standard apparent entropy (S'°) was +73.4 J K¹ mol¹. The H° and S° values for the reference reaction, P-enolpyruvate³ + ADP³ + H⁺ = ATP⁴ + Pyr¹, were 6.43 kJmol¹ and +180 J K¹ mol¹, 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.

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