ENZYME ENGINEERING ASPECTS OF BIOCATALYSIS - COFACTOR REGENERATION ASEXAMPLE

Reaction engineering is an important tool in the case of cofactor depe nding enzyme-catalyzed reactions. It allows the establishment of condi tions resulting in lower product specific cofactor costs as compared w ith product-specific enzyme costs. This is shown for the stereospecifi c reduction of carbonyl compounds yielding chiral amino acids and alco hols. In continuous processes, cofactor costs can be reduced if the co factor can be retained with in the bioreactor or recycled into it afte r separation of the product. In case of readily water-soluble substrat es it is even possible to recycle the cofactor during a single pass th rough a continuously operated reactor more than 4000 times because nor mally very low cofactor concentrations are sufficient to saturate the enzymes involved. L-tert-Leucine has been produced by reductive aminat ion with a spacetime yield of up to 366 g L(-1) d(-1) in a single cont inuously operated enzyme membrane reactor and a two-stage cascade. Tot al turnover number of the cofactor NAD(+) increased to 4230. (S)-1-Phe nyl-2-propanol was obtained by reduction of the corresponding ketone i n an membrane reactor with integrated extraction of the product. A new alcohol dehydrogenase from Rhodococcus erythropolis was used. A space -time yield of 63 g L(-1) d(-1) and a total turnover number of 1350 ha ve been reached. L-Leucine has been produced using polymer-enlarged NA DH. The total turnover number was 80,000 at a space-time yield of 214 g L(-1) d(-1). (C) 1996 John Wiley & Sons, Inc.