Stereospecific hydrogenations with immobilized microbial cells or enzymes

Stereospecific hydrogenations according to the general scheme [formula: see text] are of interest from a preparative and mechanistic point of view. Proteus mirabilis is suitable for the hydrogenation of a-keto-acids to R-hydroxy-acids, and a Clostridium strain for the reduction of enoates. Both have been immobilized in formaldehyde crosslinked gelatin and the latter also in polyacrylamide. Immobilized as well as free cells showed usually half lives of 100-200 h. The immobilized cells could be separated from the products and reused. In order to hydrogenate enoates stereospecifically, formate dehydrogenase and enoate reductase have been separately immobilized and coimmobilized on controlled pore glass. The yields for the separately immobilized enzymes were about 30 per cent and 70-80 per cent, respectively. The measured rate of the coupled system with immobilized enzymes was compared with the calculated rate, taking into account effects of pore diffusion for the pyridine nucleotide. Under operational conditions the half-life of the immobilized formate dehydrogenase was 36 h versus 45 h for the free enzyme. The corresponding values for the enoate reductase turned out to be about 17 h versus about 15 h. So far the immobilized as well as the free enzymes seem to be less stable than immobilized or free cells.