Different protein families adapt to higher temperatures by different sets of structural devices. Regarding the structural parameters, the only generally observed rule is an increase in the number of ion pairs with increasing growth temperature. Other parameters show just a trend, whereas the number of hydrogen bonds and the polarity of buried surfaces exhibit no clear-cut tendency to change with growth temperature. Proteins from extreme thermophiles are stabilized in different ways to moderately thermophilic ones. The preferences of these two groups are different with regards to the number of ion pairs, the number of cavities, the polarity of exposed surface and the secondary structural composition.

ABSTRACT3-Isopropylmalate dehydrogenase (IPMDH, E.C. 1.1.1.85) from the thermophilic bacterium Thermus thermophilus HB8 is homologous to IPMDH from the mesophilic Escherichia coli, but has an approximately 17°C higher melting temperature. Its temperature optimum is 22-25°C higher than that of the E. coli enzyme; however, it is hardly active at room temperature. The increased conformational rigidity required to stabilize the thermophilic enzyme against heat denaturation might explain its different temperature-activity profile. Hydrogen͞deuterium exchange studies were performed on this thermophilic-mesophilic enzyme pair to compare their conformational f lexibilities. It was found that Th. thermophilus IPMDH is significantly more rigid at room temperature than E. coli IPMDH, whereas the enzymes have nearly identical f lexibilities under their respective optimal working conditions, suggesting that evolutionary adaptation tends to maintain a ''corresponding state'' regarding conformational f lexibility. These observations confirm that conformational f luctuations necessary for catalytic function are restricted at room temperature in the thermophilic enzyme, suggesting a close relationship between conformational f lexibility and enzyme function.