Crystal Structure of an Alcohol Dehydrogenase from the Extreme Thermophile Thermoanaerobium brockii.

The crystal structure of a thermophilic alcohol dehydrogenase TBAD has been determined in complex with sec-butanol as substrate to a resolution of 3.0 A. The enzyme is from Thermoanaerobacter brockii, a bacterium isolated from hot springs in Yellowstone National Park. This tetrameric enzyme has a melting temperature of 98 C. The structure consists of two domains, the catalytic domain and the cofactor-binding domain, with the active site located in a deep cleft at the domain interface. Factors important for the thermostability have been deduced by comparison with the crystal structure of a highly homologous mesophilic alcohol dehydrogenase from Clostridium beijerinckii CBAD. The thermophilic enzyme has a more hydrophilic exterior, a more hydrophobic interior. a smaller surface area, more prolines, alanines, and less serines than CBAD. Furthermore, in the thermophilic enzyme the number of all types of intersubunit interactions is increased more salt bridges, hydrogen bonds and hydrophobic interactions. All these effects combined can account for the 32 deg C higher melting temperature of the thermophilic enzyme.