ABSTRACT
Malate dehydrogenase (MDH) (EC 1.1.1.37) catalyzes the conversion of oxaloacetate and malate. This reaction is important in cellular metabolism, and it is coupled with easily detectable cofactor oxidation/reduction. It is a rather ubiquitous enzyme, for which several isoforms have been identified, differing in their subcellular localization and their specificity for the cofactor NAD or NADP. The nucleotide binding characteristics can be altered by a single amino acid change. Multiple amino acid sequence alignments of MDH show that there is a low degree of primary structural similarity, apart from several positions crucial for catalysis, cofactor binding and the subunit interface. Despite the low amino acids sequence identity their 3-dimensional structures are very similar. MDH is a group of multimeric enzymes consisting of identical subunits usually organized as either dimer or tetramers with subunit molecular weights of 30-35 kDa. MDH has been isolated from different sources including archaea, eubacteria, fungi, plant and mammals.
Subject(s)
Malate Dehydrogenase/chemistry , Malate Dehydrogenase/metabolism , Animals , Archaea/enzymology , Bacteria/enzymology , Fungi/enzymology , Isoenzymes/chemistry , Isoenzymes/metabolism , Macromolecular Substances , Mammals , NAD/metabolism , NADP/metabolism , Plants/enzymology , Protein ConformationABSTRACT
The malate dehydrogenase (MDH) from Streptomyces aureofaciens was purified to homogeneity and its physical and biochemical properties were studied. Its amino-terminal sequence perfectly matched the amino-terminal sequence of the MDH from Streptomyces atratus whose biochemical characteristics have never been determined. The molecular mass of the native enzyme, estimated by size-exclusion chromatography, was 70 kDa. The protein was a homodimer, with a 38-kDa subunit molecular mass. It showed a strong specificity for NADH and was much more efficient for the reduction of oxaloacetate than for the oxidation of malate, with a pH optimum of 8. Unlike MDHs from other sources, it was not inhibited by excess oxaloacetate. This first complete functional characterization of an MDH from Streptomyces shows that the enzyme is very similar in many respects to other bacterial MDHs with the notable exception of a lack of inhibition by excess substrate.
