The pH dependence of the reductive carboxylation of pyruvate by malic enzyme.
The maximum velocity of the malic enzyme (L-malate: NADP+ oxidoreductase (oxaloacetate-decarboxylating), EC 1.1.1.40) reductive carboxylation of pyruvate and V/KCO2 are pH-independent from pH 5.5 to pH 8.5. V/K for pyruvate exhibits pK values values of 6.50 +/- 0.25 and 7.25 +/- 0.25. These data suggest that the binding of pyruvate locks the protonation state of enzyme. In addition, the pK values are within experimental error identical for the pH dependence of V/Kmalate and V/Kpyruvate. Thus, the catalytic groups appear to have reverse protonation states in the two reaction directions. The ratio of (V/Kmalate)/(V/Kpyruvate) is 100, suggesting that the protonation state of enzyme is optimum in the malate oxidative decarboxylation direction. Thus, the group with a pK of about 6 is unprotonated and the group with a pK of 7.5 is protonated for malate decarboxylation, and the opposite is true for pyruvate reductive carboxylation.
D000818:Animals; D002645:Chickens; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008291:Malate Dehydrogenase / Q000378:metabolism*; D010084:Oxidation-Reduction; D011773:Pyruvates; D019289:Pyruvic Acid
D011773:Pyruvates; D019289:Pyruvic Acid; D008291:Malate Dehydrogenase