Stereospecific reductions of ketones and oxo-acid esters using continuously growing cultures of Thermoanaerobium brockii

Thermoanaerobium brockii grown in continuous culture at 72°C reduced 3-ketoacid esters and 2-formyl-acid esters stereospecifically, in some cases yielding the opposite enantiomer compared with other microbial systems. The ethyl esters of 3-oxo-butanoic, 3-oxo-pentanoic and 3-oxo-hexanoic acid were reduced to give the resulting 3-S-(+)-hydroxy-acid esters. The 2-formyl-substituted ethyl esters of propanoic, butanoic, 3-methyl-butanoic, and 2-phenyl-acetic acid were reduced to the corresponding non-racemic optically active 3-hydroxy esters. Although growth was significantly inhibited and the cells were even lysed, 4-C1-3-oxo-butanoic acid methyl ester, and 2-formyl-propanoic acid ethyl ester were also reduced by T. brockii with good optical yields. The reaction, rates of reductions by growing cells were generally higher than so far reported for other microbial or enzymatic systems: up to 5 g g −1 h- −1, although they depended on the nature of the carbonyl compounds added. Short-term tolerance of the strain towards carbonyl compounds (and products) was higher than long-term tolerance as shown by different responses of T. brockii to pulses and to continuous addition of educt. The technique allowed the reduction of water-immiscible reactants, too. A simple kinetic model was used to quantify the reductions.