Xylitol does not inhibit xylose fermentation by engineered Saccharomyces cerevisiae expressing xylA as severely as it inhibits xylose isomerase reaction in vitro

Efficient fermentation of xylose, which is abundant in hydrolysates of lignocellulosic biomass, is essential for producing cellulosic biofuels economically. While heterologous expression of xylose isomerase in Saccharomyces cerevisiae has been proposed as a strategy to engineer this yeast for xylose fermentation, only a few xylose isomerase genes from fungi and bacteria have been functionally expressed in S. cerevisiae. We cloned two bacterial xylose isomerase genes from anaerobic bacteria (Bacteroides stercoris HJ-15 and Bifidobacterium longum MG1) and introduced them into S. cerevisiae. While the transformant with xylA from B. longum could not assimilate xylose, the transformant with xylA from B. stercoris was able to grow on xylose. This result suggests that the xylose isomerase (BsXI) from B. stercoris is functionally expressed in S. cerevisiae. The engineered S. cerevisiae strain with BsXI consumed xylose and produced ethanol with a good yield (0.31 g/g) under anaerobic conditions. Interestingly, significant amounts of xylitol (0.23 g xylitol/g xylose) were still accumulated during xylose fermentation even though the introduced BsXI might not cause redox imbalance. We investigated the potential inhibitory effects of the accumulated xylitol on xylose fermentation. Although xylitol inhibited in vitro BsXI activity significantly (K (I) = 5.1 +/- 1.15 mM), only small decreases (less than 10%) in xylose consumption and ethanol production rates were observed when xylitol was added into the fermentation medium. These results suggest that xylitol accumulation does not inhibit xylose fermentation by engineered S. cerevisiae expressing xylA as severely as it inhibits the xylose isomerase reaction in vitro.