3-isopropylmalate dehydrogenase from chemolithoautotroph Thiobacillus ferrooxidans: DNA sequence, enzyme purification, and characterization

3-Isopropylmalate dehydrogenase encoded by the Thiobacillus ferrooxidans leuB gene was purified to homogeneity from Escherichia coli cells harboring a recombinant plasm id containing the leuB gene. The native enzyme molecule is a dimer of molecular weight 38, 000. The K_m value for 3-isopropylmalate was estimated to be 26 μM and that for NAD⁺ 0.8 mM. The presence of K⁺ or NH₄⁺ is essential for the enzyme reaction. The enzyme is activated about 4-fold by the addition of 1.0 mM Mg²⁺ or Co²⁺. The optimum pH and temperature for the activity are 9.0 and 60*C, respectively. The properties of the enzyme are similar to those of the Salmonella typhimurium and Thermus thermophilus enzymes, except for substrate specificity. T. ferrooxidans 3-isopropylmalate dehydrogenase is able to utilize D- and L-malate as substrates in addition to 3-isopropylmalate. Sequencing of subcloned DNA revealed that the leuB gene consists of a 1, 074 bp open reading frame and encodes 358 amino acid residues corresponding to the subunit (38, 462 Da). The amino acid sequence of 3-isopropylmalate dehydrogenase from T. ferrooxidans and those of some heterotrophic microorganisms have high homology.