Enantioselectivities of enzymes involved in the reduction of methylketones by Bakers' yeast

The enantioselectivities of Bakers' yeast alcohol dehydrogenase (YADH) and (S)-diacetylreductase [(S)-DR] in the course of the reduction of C₆-C₁₁-methylketones have been investigated by means of capillary gas chromatographic determination of the optical purities of the formed secondary alcohols. YADH catalyses the reduction of methylketones in vitro enantioselectively to the optically pure (S)-alkan-2-ols; no influence of substrate concentration, pH value, and temperature was observed. On the other hand, (S)-DR leads to chain-length-dependent enantiomeric ratios of secondary alcohols ranging between 99.0% and 92.2% e.e.(S). Intact yeast cells catalyse a partial racemization of optically pure (S)- and (R)-octan-2-ol. These results imply that the moderate optical purities of alkan-2-ols [80%-91.4% e.e.(S)] obtained by the reduction of methylketones employing intact yeast cells are not due to a partial enantioselectivity of YADH. However, the stereochemical outcome of in vivo reductions cannot be exclusively explained by the additional action of (S)-DR. Other oxidoreductases (and/or racemases) must participate in this biotransformation.