D-LACTATE DEHYDROGENASE IS A MEMBER OF THE D-ISOMER-SPECIFIC 2-HYDROXYACID DEHYDROGENASE FAMILY - CLONING, SEQUENCING, AND EXPRESSION IN ESCHERICHIA-COLI OF THE D-LACTATE DEHYDROGENASE GENE OF LACTOBACILLUS-PLANTARUM

The gene encoding D-lactate dehydrogenase (D-lactate: NAD+ oxidoreductase, EC 1.1.1.28) of Lactobacillus plantarum has been sequenced, and expressed in Escherichia coli cells with an inducible expression plasmid, in which the 5'-noncoding region of the gene was replaced with the tac promoter. Comparison of the sequence Of D-lactate dehydrogenase with L-lactate dehydrogenases, including the L. plantarum L-lactate dehydrogenase, showed no significant homology. In contrast, the D-lactate dehydrogenase is homologous to E. coli D-3-phosphoglycerate dehydrogenase and Lactobacillus casei D-2-hydroxyisocaproate dehydrogenase. This indicates that D-lactate dehydrogenase is a member of a new family of 2-hydroxyacid dehydrogenases recently proposed, being distinct from L-lactate dehydrogenase and L-malate dehydrogenase, and strongly suggests that the new family consists Of D-iSomer-stereospecific enzymes. In the reductive reaction, the enzyme showed a broad substrate specificity, although pyruvate was the most favorable of all 2-ketocarboxylic acids tested. In particular, hydroxypyruvate is effectively reduced by the enzyme, the reaction rate, and K(m) value being comparable to those in the case of pyruvate, indicating that the enzyme has not only D-lactate dehydrogenase activity but also D-glycerate dehydrogenase activity. The conserved residues in this family appear to be the residues involved in the substrate binding and the catalytic reaction, and thus to be targets for site-directed mutagenesis.