ASYMMETRIC REDUCTION OF ALIPHATIC AND CYCLIC-KETONES WITH SECONDARY ALCOHOL-DEHYDROGENASE FROM THERMOANAEROBACTER-ETHANOLICUS - EFFECTS OF SUBSTRATE STRUCTURE AND TEMPERATURE

The reduction of aliphatic ketones catalyzed by a secondary alcohol de hydrogenase (SADH) from Thermoanaerobacter ethanolicus affords (S)-alc ohols in high enantiomeric purities, when the chain has six or more ca rbons. With 2-pentanone, the reduction gives (S)-2-pentanol, and (R)-s tereoselectivity is observed in the case of 2-butanone at 37 degrees C . The rate of the reduction of aliphatic methyl ketones decreases abou t three-fold for each additional methylene increment. A temperature de pendent reversal of enantiospeficity is observed in the oxidation of e nantiomers of 2-butanol. A linear dependence of - RT1nE with temperatu re is observed for 2-butanol, 2-pentanol, and 2-hexanol. The cofactor analogues, thionicotinamide adenine dinucleotide phosphate (SNADP) and acetylpyridine adenine dinucleotide phosphate (APADP) gave higher ena ntioselectivity in the reduction of 2-butanone to (R)-2-butanol. For t he reduction of cyclic ketones, SADH is enantiospecific for (S) isomer s of cyclic alkyl ketones, and the transfer of hydrogen is stereoselec tive for the Re face to give (1S) cyclic alcohols. The facial stereosp ecificity of SADH for hydride transfer to NADP was determined by NMR, and it was found to be re-specific ('A face'). In order to explain the stereoselectivity of SADH catalyzed reductions, a model is proposed t hat emphasizes the importance of the stability of substrate conformati on and the steric interaction between substrate, enzyme and coenzyme.