STRUCTURAL COMPARISON OF TWO HIGHLY HOMOLOGOUS THERMOPHILIC BACTERIAL ALCOHOL DEHYDROGENASES. Yakov Korkhin, Felix Frolow, Oren Bogin, Moshe Peretz, A. Joseph Kalb(Gilboa) and Yigal Burstein Faculty of Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel.

The NADP-dependent alcohol dehydrogenases from Thermoanaerobium brockii (TBAD) and Clostridium beijerinckii (CBAD) have 75% sequence identity and yet they differ by 26C in their inactivation temperatures (T_1/2^60min.= 93C for TBAD and T_1/2^60mi.= 67C for CBAD). The structures of TBAD and CBAD in the holo-enzyme form have been solved and refined to a resolution of 2.5 and 2.05 respectively. The overall three-dimensional structures are highly homologous with RMSD_C[[alpha]]= 0.6. and RMSD _{all atoms}= 1.0. Structural differences between the two enzymes are discussed in terms of their role in thermostabilization. Based on the 3D-structures, certain point mutations have been engineered and their consequences for enzyme stability have been assessed.