The Crystal Structures of Mutated 3-Isopropylmalate Dehydrogenase from Thermus thermophilus HB8 and Their Relationship to the Thermostability of the Enzyme

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The structures of two mutant forms (G240A and L246E/V249M) of 3-isopropylmalate dehydrogenase from Thermus thermophilus HB8 were studied by X-ray crystallography. In the case of G240A, the replacement of glycine by alanine at residue 240 was expected to decrease the thermostability as a result of abnormal contacts between the methyl group of alanine and the peptide chain. However, the normal van der Waals' contacts were achieved owing to a shift in a bundle of β-strands that yielded a vacant space for the alanine residue. The extended hydrogen bonds within the β-sheet are the major reason for the decreased thermostability of G240A. The mutations in L246E/V249M are located in an α-helix region which is involved in subunit-subunit contact via hydrophobic interaction. Loosening of the subunit-subunit contact owing to ionic repulsion was the major cause of the lower heat stability of L246E/V249M.