Systematic strain construction and process development: Xylitol production by Saccharomyces cerevisiae expressing Candida tenuis xylose reductase in wild-type or mutant form

•Saccharomyces cerevisiae biocatalyst expressing Candida tenuis xylose reductase.•Effect of enzyme expression on xylitol yield and productivity analysed.•Effect of coenzyme specificity of xylose reductase on xylitol yield analysed.•Fed-batch process for xylitol production using glucose as co-substrate.

A novel Saccharomyces cerevisiae whole-cell biocatalyst for xylitol production based on Candida tenuis xylose reductase (CtXR) is presented. Six recombinant strains expressing wild-type CtXR or an NADH-specific mutant were constructed and evaluated regarding effects of expression mode, promoter strength, biocatalyst concentration and medium composition. Intracellular XR activities ranged from 0.09 U mgProt−1 to 1.05 U mgProt−1 but did not correlate with the strains’ xylitol productivities, indicating that other factors limited xylose conversion in the high-activity strains. The CtXR mutant decreased the biocatalyst’s performance, suggesting use of the NADPH-preferring wild-type enzyme when (semi-)aerobic conditions are applied. In a bioreactor process, the best-performing strain converted 40 g L−1 xylose with an initial productivity of 1.16 g L−1 h−1 and a xylitol yield of 100%. The obtained results underline the potential of CtXR wild-type for xylose reduction and point out parameters to improve “green” xylitol production.