1-09: Effects of expression of NADH-dependent xylose reductase on ethanol fermentation from xylose in Saccharomyces cerevisiae

Xylose is a second most abundant carbohydrate in lignocellulosic biomass. However Saccharomyces cerevisiae, a robust ethanol producer cannot utilize xylose as a carbon source. In this study, two genes coding for NADH-dependent XR mutant (XR^mut) and NAD⁺-dependent XDH derived from Pichia stipitis were introduced into S. cerevisiae D452-2 in order to balance cofactor utilization in the xylose metabolism. Additionally, the gene coding for xylulokinase (XK) from S. cerevisiae was overexpressed by the chromosomal δ-integration to construct the S. cerevisiae D452-2/YIpXR^mut-XDH^wt+pδneoXK (SX3) strain. P. stipitis transaldolase, a key enzyme in the pentose phosphate pathway linking with xylose metabolism, was also overexpressed in the SX3/p423PsTAL1 strain. In the result of microaerobic batch fermentation with YEPD containing 2 % glucose and 5 % xylose, the ethanol productivity of 0.19 g/L-hr and the ethanol yield of 0.25 g/g xylose by the SX3/p423PsTAL1 were obtained which were 3.2 and 1.5 times higher than the corresponding parameters of SX3. To minimize the inhibitory effect of acetate on ethanol fermentation, the ALD6 gene coding for acetaldehyde dehydrogenase was disrupted to construct the SX3/p423PsTAL1+ald6Δ strain. Under the same fermentation conditions, the SX3/p423PsTAL1+ald6Δ strain showed the ethanol productivity of 0.24 g/L-hr and ethanol yield of 0.28 g /g xylose. Finally, in oxygen-limited fed-batch fermentation with YEPD containing 2 % glucose and 9 % xylose, the SX3/p423PsTAL1+ald6Δ strain showed the increased ethanol production performance such as productivity from 0.07 g/L-hr to 0.25 g/L-hr, and yield from 0.24 g/g xylose to 0.39 g/g xylose.