title: Tartrate dehydrogenase catalyzes the stepwise oxidative decarboxylation of D-malate with both NAD and thio-NAD.
abstract: Tartrate dehydrogenase catalyzes the divalent metal ion- and NAD-dependent oxidative decarboxylation of D-malate to yield CO(2), pyruvate, and NADH. The enzyme also catalyzes the metal ion-dependent oxidation of (+)-tartrate to yield oxaloglycolate and NADH. pH-rate profiles and isotope effects were measured to probe the mechanism of this unique enzyme. Data suggest a general base mechanism with likely general acid catalysis in the oxidative decarboxylation of D-malate. Of interest, the mechanism of oxidative decarboxylation of D-malate is stepwise with NAD(+) or the more oxidizing thio-NAD(+). The mechanism does not become concerted with the latter as observed for the malic enzyme, which catalyzes the oxidative decarboxylation of L-malate [Karsten, W. E., and Cook, P. F. (1994) Biochemistry 33, 2096-2103]. It appears the change in mechanism observed with malic enzyme is specific to its transition state structure and not a generalized trait of metal ion- and NAD(P)-dependent beta-hydroxy acid oxidative decarboxylases. The V/K(malate) pH-rate profile decreases at low and high pH and exhibits pK(a) values of about 6.3 and 8.3, while that for V/K(tartrate) (measured from pH 7.5 to pH 9) exhibits a pK(a) of 8.6 on the basic side. A single pK(a) of 6.3 is observed on the acid side of the V(max) pH profile, but the pK(a) seen on the basic side of the V/K pH profiles is not observed in the V(max) pH profiles. Data suggest the requirement for a general base that accepts a proton from the 2-hydroxyl group of either substrate to facilitate hydride transfer. A second enzymatic group is also required protonated for optimum binding of substrates and may also function as a general acid to donate a proton to the enolpyruvate intermediate to form pyruvate. The (13)C isotope effect, measured on the decarboxylation of D-malate using NAD(+) as the dinucleotide substrate, decreases from a value of 1.0096 +/- 0.0006 with D-malate to 1.00787 +/- 0.00006 with D-malate-2-d, suggesting a stepwise mechanism for the oxidative decarboxylation of D-malate. Using thio-NAD(+) as the dinucleotide substrate the (13)C isotope effects are 1.0034 +/- 0.0007 and 1.0027 +/- 0.0002 with D-malate and D-malate-2-d, respectively.
mesh_terms: D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D002247:Carbon Isotopes / Q000378:metabolism; D002384:Catalysis; D003903:Deuterium / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008293:Malates / Q000378:metabolism*; D009243:NAD / Q000031:analogs & derivatives* / Q000378:metabolism*; D062907:Oxaloacetic Acid / Q000378:metabolism; D016958:Pseudomonas putida / Q000201:enzymology / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D008293:Malates; D009243:NAD; D062907:Oxaloacetic Acid; C003451:thionicotinamide adenine dinucleotide; C030298:malic acid; D003903:Deuterium; D000429:Alcohol Oxidoreductases; C038481:tartrate dehydrogenase