Tuesday, May 1, 2012
Napoleon Ballroom C-D, 3rd fl (Sheraton New Orleans)
Saccharomyces cerevisiae has been engineered for the production of ethanol from xylose, a prevalent sugar in cellulosic biomass hydrolyzates. It has been generally known that the heterologous expression of xylose reductase (XYL1) and xylitol dehydrogenase (XYL2), or of xylose isomerase (xylA), either case of which being accompanied by the overexpression of xylulose kinase (XKS1 or XYL3), are the prerequisites for S. cerevisiae to efficiently utilize xylose. In this study, we propose another strategy, of developing efficient xylose-fermenting S. cerevisiae through overexpressing a gene that codes for endogenous aldose reductase (GRE3) instead of XYL1. It was previously hypothesized that as compared to xylose reductase (XYL1) using NADPH and NADH as dual cofactors, NADPH-linked aldose reductase (GRE3) would aggravate cellular redox balance since xylitol dehydrogenase (XYL2) uses NAD+ exclusively. However, we found that the redox imbalance was resolved by a minimal amount of oxygen. Under limited aerobic conditions, the strain overexpressing GRE3 with XYL2 and XYL3 (DGX23) was able to ferment xylose as well as a mixture of glucose and xylose with even higher yields and productivities than an isogenic strain expressing XYL1 with XYL2 and XYL3 (DX123). This is the first report showing efficient xylose fermentation of engineered S. cerevisiae by overexpressing GRE3. Our observation also suggests that the cofactor imbalance between aldose reductase (GRE3) and xylitol dehydrogenase (XYL2) does not have to be a hindering factor in efficient xylose fermentation.