CRYOCRYSTALLOGRAPHY OF 3-ISOPROPYLMALATE DEHYDROGENASE FROM THERMUS-THERMOPHILUS AND ITS CHIMERIC ENZYME

The crystal structures of thermostable enzyme, 3-isopropylmalate dehyd rogenase of Thermus thermophilus (10T) and a chimeric enzyme between T . thermophilus and Bacillus subtilus with one point mutation (cS82R), were determined at both 100 and 150 K. At the cryogenic condition, the volume of the unit cell decreased by 5% as a result of a contraction in the solvent region. Although the overall structures of both enzymes at low temperature were the same as that of 10T at room temperature, interactions between two domains and between two subunits in a functio nal dimer of cS82R were significantly altered. The decrease in the ave rage temperature factor of 10T at low temperature and no significant d ecrease for cS82R suggested that the structure of the engineered enzym e (cS82R) may have many conformational substates even at low temperatu re, while the native enzyme (10T) at low temperature has a more defini te conformation than that at room temperature. The location of water m olecules around the enzyme molecule and the calculation of the radii o f gyration suggested that cS82R had a weaker. hydration than 10T.