Boost in bioethanol production using recombinant Saccharomyces cerevisiae with mutated strictly NADPH-dependent xylose reductase and NADP(+)-dependent xylitol dehydrogenase.

The xylose-fermenting recombinant Saccharomyces cerevisiae and its improvement have been studied extensively. The redox balance between xylose reductase (XR) and xylitol dehydrogenase (XDH) is thought to be an important factor in effective xylose fermentation. Using protein engineering, we previously successfully reduced xylitol accumulation and improved ethanol production by reversing the dependency of XDH from NAD(+) to NADP(+). We also constructed a set of novel strictly NADPH-dependent XR from Pichia stipitis by site-directed mutagenesis. In the present study, we constructed a set of recombinant S. cerevisiae carrying a novel set of mutated strictly NADPH-dependent XR and NADP(+)-dependent XDH genes with overexpression of endogenous xylulokinase (XK) to study the effects of complete NADPH/NADP(+) recycling on ethanol fermentation and xylitol accumulation. All mutated strains demonstrated reduced xylitol accumulation, ranging 34.4-54.7% compared with the control strain. Moreover, compared with the control strain, the two strains showed 20% and 10% improvement in ethanol production.