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 dehydrogenase (SADH) from Thermoanaerobacter ethanolicus affords (S)-alcohols in high enantiomeric purities, when the chain has six or more carbons. With 2-pentanone, the reduction gives (S)-2-pentanol, and (R)-stereoselectivity is observed in the case of 2-butanone at 37 degrees C. The rate of the reduction of aliphatic methyl ketones decreases about three-fold for each additional methylene increment. A temperature dependent reversal of enantiospeficity is observed in the oxidation of enantiomers of 2-butanol. A linear dependence of - RT1nE with temperature 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 enantioselectivity in the reduction of 2-butanone to (R)-2-butanol. For the reduction of cyclic ketones, SADH is enantiospecific for (S) isomers of cyclic alkyl ketones, and the transfer of hydrogen is stereoselective for the Re face to give (1S) cyclic alcohols. The facial stereospecificity 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 that emphasizes the importance of the stability of substrate conformation and the steric interaction between substrate, enzyme and coenzyme.