Promotion Effect of Xylose Co-substrate on Stability of Catalytic System for Asymmetric Redox of (R,S)-1- Phenyl-1,2-ethanediol to Its (S)-Enantiomer by Candida parapsilosis

The asymmetric redox of (R,S)-1-phenyl-1,2-ethanediol to its optically active (S)-enantiomer catalyzed by Candida parapsilosis in an aqueous phase was investigated. A new efficient and economical cofactor regeneration system was established using D-xylose as the co-substrate by analyzing the reaction mechanism and the metabolic pathway of sugars in yeast. The results showed that the xylose co-substrate was advantageous for the stability of the catalytic system in a repetitive batch mode. In the presence of xylose (8 g/L), the optical purity and the yield of the (S)-enantiomer were increased by 14% and 10%, respectively. Furthermore, whole cells could be reused 3–4 times and the enantiomeric excess of the (S)-enantiomer remained at a high level of 98%. The effects of xylose on the accumulation of 2-hydroxyacetophenone in repetitive batch experiments and the asymmetric reaction catalyzed by carbonyl reductases from recombinant Escherichia coli were investigated. It was proposed that xylose improved the stability of the catalytic system by strengthening the (S)-carbonyl reductase system, in which NADPH that is necessary for the asymmetric reaction could be regenerated via the pentose phosphate pathway of xylose metabolism.