Malate dehydrogenase from the thermophilic green bacterium Chloroflexus aurantiacus: purification, molecular weight, amino acid composition, and partial amino acid sequence.

Malate dehydrogenase (MDH; EC 1.1.1.37) from the thermophilic green nonsulfur bacterium Chloroflexus aurantiacus was purified by a two-step procedure involving affinity chromatography and gel filtration. The enzyme consists of identical subunits which had molecular weights of approximately 35,000. In its active form at 55 degrees C, it formed tetramers. At lower temperatures, inactive dimers and trimers existed. Antibodies against the purified enzyme were produced, and immunotitration and enzyme-linked immunosorbent assays showed that there was an immunochemical homology between the MDH from C. aurantiacus and MDHs from several other bacteria. The amino acid composition of C. aurantiacus MDH was similar to those of other MDHs. The N-terminal amino acid sequence was enriched with hydrophobic amino acids, which showed a high degree of functional similarity to amino acids at the N-terminal ends of both Escherichia coli and Thermus flavus MDHs. The activity of the native enzyme was inhibited by high concentrations of substrate and had temperature and pH optima consistent with the optimal growth conditions for the organism.