We cloned and sequenced the leuB gene encoding 3-isopropylmalate dehydrogenase from Escherichia coli K-12 (JM103). Errors (33 residues) were found and corrected in the sequence previously reported for the leuB gene of Thermus thermophilus. The three-dimensional structure of the thermophile enzyme and the amino acid sequence comparison suggested that a part of the high stability of the T. thermophilus enzyme is conferred by increased hydrophobic interaction at the subunit-subunit interface. Two residues at the interface of the T. thermophilus enzyme, Leu246 and Val249, are substituted with less hydrophobic residues, Glu and Met, respectively, in the E. coli enzyme, whereas other residues in this region are highly conserved. The mutated T. thermophilus enzyme [L246E, V249M]IPMDH had reduced stability to heat. Two residues of the E. coli dehydrogenase, Glu256 and Met259, were replaced with the corresponding residues from the thermophile sequence. The resulted mutant enzyme was more resistant to heat than the wild-type enzyme.
- [L246E, V249M]IPMDH
mutated Thermus thermophilus enzyme which Leu246 and Val249 were substituted by Glu and Met, respectively
- [E256L, M259V]IPMDH
mutated Escherichia coli enzyme which Glu256 and Met259 were substituted by Leu and Val, Respectively
3-Isopropylmalate dehydrogenase (EC 188.8.131.52)