THE CRYSTAL-STRUCTURES OF MUTATED 3-ISOPROPYLMALATE DEHYDROGENASE FROM THERMUS-THERMOPHILUS HB8 AND THEIR RELATIONSHIP TO THE THERMOSTABILITY OF THE ENZYME

The structures of two mutant forms (G240A and L246E/V249M) of 3-isopro pylmalate dehydrogenase from Thermus thermophilus HB8 were studied by X-ray crystallography. In the case of G240A, the replacement of glycin e by alanine at residue 240 was expected to decrease the thermostabili ty as a result of abnormal contacts between the methyl group of alanin e and the peptide chain, However, the normal van der Waals' contacts w ere achieved owing to a shift in a bundle of beta-strands that yielded a vacant space for the alanine residue, The extended hydrogen bonds w ithin the beta-sheet are the major reason for the decreased thermostab ility of G240A. The mutations in L246E/V249M are located in an alpha-h elix region which is involved in sub unit-subunit contact via hydropho bic interaction, Loosening of the subunit-subunit contact owing to ion ic repulsion was the major cause of the lower heat stability of L246E/ V249M.