Stereochemical control in microbial reduction .29. Mechanism of stereochemical control with an additive in the diastereoselective reduction by Geotrichum candidum

The reduction of ethyl 2-methyl-3-oxobutanoate with Geotrichum candidum yields approximately equal amounts of the corresponding syn-(2R,3S)- and anti-(2S,3S)-3-hydroxy esters. When the microbe is incubated with methyl vinyl ketone (MVK) or chloroacetone before it is subjected to the reduction, the corresponding anti-(2S,3S)-hydroxy ester is obtained with exclusive enantio- as well as diastereoselectivity. In order to obtain more insight into the mechanism of this stereochemical control in microbial reduction, beta-keto ester reductases were isolated and purified from the cells of G. candidum. Three dominantly competing enzymes were purified. One of them affords the anti-(2S,3S)-hydroxy ester selectively, whereas the others give the syn-(2R,3S)-hydroxy ester selectively. The rates of enzymatic reductions were measured in the presence and absence of an additive. Chloroacetone and MVK are competitive and suicide substrates, respectively, for the syn-enzymes, whereas they hardly affect the activity of the anti-enzyme. In the presence of an additive, the reduction of ethyl 2-methyl-3-oxobutanoate with G. candidum is catalzsed by the anti-enzyme only.