Alternative substrates for malic enzyme: oxidative decarboxylation of L-aspartate

The NAD-malic enzyme from Ascaris suum will utilize L-aspartate, (2S,3R)-tartrate, and meso-tartrate as substrates with V/K values 10(-4)-10(-5) with respect to malate. There is a strict requirement for the 2S stereochemistry for all of these reactants. Since aspartate is unique as an amino acid reactant for malic enzyme, it was informative to determine the details of its mechanism of oxidative decarboxylation. The initial rate of NADH appearance is directly proportional to the concentration of aspartate, and saturation is difficult to achieve. The pH dependence of V/K(aspartate)E(t) shows a decrease at low pH, giving a pK of 5.7. The pH-independent value of V/K(aspartate)E(t) is 3 M(-1) s(-1), 12500-fold lower than that obtained with L-malate. The dissociation constant for aspartate as a competitive inhibitor of malate is 60 mM at neutral pH, allowing an estimate of about 0.18 s(-1) for V/E(t) with L-aspartate compared to a value of 39 s(-1) obtained with L-malate. The deuterium isotope effect on V/K(aspartate) is pH independent over the range 5.1-6.9 with an average value of 3.3. Data suggest that the monoanion of L-aspartate binds to enzyme and that the same general base, general acid mechanism that is responsible for the oxidative decarboxylation of malate to pyruvate applies to the oxidative decarboxylation of aspartate to iminopyruvate. In addition, the oxidation step appears to be largely rate determining with aspartate as the substrate.