To understand the role of the amino acid residue at position 172 in the conformational stability, four mutant enzymes of Thermus thermophilus 3- isopropylmalate dehydrogenase in which Ala172 was replaced with Asp, Glu, Asn, and Gln were prepared by site-directed mutagenesis. Three mutants were more stable than the wild-type enzyme. No significant change in catalytic properties was found in the mutant enzymes. The molecular modeling studies suggested that the enhanced thermostability of the mutant enzymes resulted from the formation of extra electrostatic interactions and/or improvement of hydrophobic packing of the interior core.
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