Determination of the Mechanism of Human Malic Enzyme with Natural and Alternate Dinucleotides by Isotope Effects

Human malic enzyme was studied by steady state kinetics, deuterium isotope effects, and 13C isotope effects with both the physiological dinucleotide cofactor and several alternate cofactors. The log V vs pH profile with NAD revealed two pKa values too close to be separately determined, but with an average value of 7.33. The log V/K vs pH profile with NAD revealed two pKa values at 7.4 and 5.6. Deuterium and 13C isotope effects indicate that the mechanism of human malic enzyme is stepwise with both NAD and ϵNAD, but that hyperconjugation in the transition state for hydride transfer is detectable only with the former. With thioNAD and APAD, the isotope effects do not clearly indicate whether the mechanism is stepwise or concerted. The intrinsic 13C isotope effect for decarboxylation was calculated to be 1.0485 by measurement of the partition ratio of oxaloacetate in the presence of NADH and human malic enzyme (decarboxylation to pyruvate/reduction to malate = 2.33). The isotope effect and partitioning data suggest that the energy barrier for decarboxylation of oxaloacetate is not as high relative to the barrier for reduction of oxaloacetate as with the chicken liver enzyme.