How to Turn Yeast Cells into a Sustainable and Switchable Biocatalyst? On-Demand Catalysis of Ketone Bioreduction or Acyloin Condensation

Abstract

In this study, yeast strains were screened and immobilized in a form preserving the multifaceted biocatalytic activity. Immobilization of the silica-supported whole cells of various yeasts, such as Lodderomyces elongisporus, Pichia carsonii, Candida norvegica, and Debaryomyces fabryi, by sol-gel entrapment resulted in easy-to-handle biocatalysts that mediated efficiently different types of synthetic reactions. In the present study, the enantiotope selective reduction of prochiral ketones 1a-d and the acyloin condensation of benzaldehyde 3 were studied, representing two remarkably diverse types of biotransformation. The yeast cell biocatalysts - in the presence of fresh or recovered NADH cofactor - could be applied for continuous-flow bioreduction of ketones 1a-d with moderate to good yields (20 to 92%) and excellent enantiomeric purity (>99%). Immobilized L. elongisporus and P. carsonii cells could also mediate acyloin condensation of benzaldehyde 3 in batch as well as in continuous-flow mode. The switchable biocatalytic activity of the immobilized yeast cells was demonstrated by consecutive biotransformations under continuous-flow conditions involving reduction of phenylacetone 1a to (S)-phenylpropane-2-ol [(S)-2a] first, followed by conversion of benzaldehyde 3 to (R)-1-hydroxy-1-phenylpropan-2-one [(R)-4] and reduction of 1a to (S)-2a again by using the same packed-bed bioreactor.