Construction of various mutants of xylose metabolizing enzymes for efficient conversion of biomass to ethanol.

We applied protein engineering to construct an efficient biomass-ethanol conversion system using Saccharomyces cerevisiae. Intercellular redox imbalance caused by the different coenzyme specificity of xylose reductase (XR) and xylitol dehydrogenase (XDH) has been thought to be one of the main factors of xylitol excretion. Introduction of NADH-dependant XR generated in this study reduced the xylitol excretion probably because of maintaining the intercellular redox balance. Ethanol fermentation was measured in batch culture under anaerobic conditions. The best strain R276H produced a maximum of 5.94 g/l ethanol with yield of 0.43 g/g from 5 g glucose/l plus 15 g xylose/l.