Substrate determinants of the course of tartrate dehydrogenase-catalyzed reactions

The substrate specificity of tartrate dehydrogenase has been probed using a series of alternative substrates to identify the molecular interactions which determine whether a particular substrate undergoes enzyme-catalyzed decarboxylation or not. A series of 3-substituted malate analogs, in which F, Cl, Br, I, SH, or NH2 substituents were placed at the 3R- or 3S-position, was prepared, and the product resulting from the action of tartrate dehydrogenase on each compound was identified. All of the halomalates and both diastereomers of aminomalate underwent oxidative decarboxylation; both diastereomers of 3-thiomalate underwent net nonoxidative decarboxylation. The results were interpreted in terms of a model in which decarboxylation is conformationally controlled. The data are not consistent with a model which suggests that substrates assume the conformation that is necessary to avoid steric crowding between the enzyme and the substituent at the 3-position of the substrate. These data are consistent with a model in which the course of the reaction with (+)-tartrate and meso-tartrate is dictated by the coordination of the substrate hydroxyls to the active site Mn2+. However, the observed reactivities of the 3-methyltartrate diastereomers are not consistent with this model, either: (2R,3R)-3-methyltartrate undergoes oxidative decarboxylation, and (2R,3S)-3-methyltartrate undergoes simple oxidation. These results suggest that for these compounds the conformation is dictated by the positioning of the hydrophobic substituent in a specific binding pocket.(ABSTRACT TRUNCATED AT 250 WORDS)