Hydrolytic activity in baker's yeast limits the yield of asymmetric 3-oxo ester reduction

Abstract

Microbial reductions of ketones hold great potential for the production of enantiopure alcohols, as long as highly selective redox enzymes are not interfered with by competing activities. During reduction of ethyl 3-oxobutanoate by baker's yeast (Saccharomyces cerevisiae) to ethyl (S)-3-hydroxybutanoate, a high enantiomeric excess (> 99%) can be obtained. However, reported yields do not exceed 50-70%. In this article, three main causes are shown to be responsible for these low to moderate yields. These are evaporation of the substrate and product esters, absorption or adsorption of the two esters by the yeast cells and hydrolysis of the two esters by yeast enzymes. The hydrolysis products are further metabolized by the yeast. By reducing the evaporation and absorption losses, the reduction yield can easily be improved to about 85%. Improvement of the efficiency of the reduction and hence the reduction/hydrolysis ratio should lead to a further increase in yield.