Repeated-batch fermentations of xylose and glucose-xylose mixtures using a respiration-deficient Saccharomyces cerevisiae engineered for xylose metabolism

Xylose-fermenting Saccharomyces strains are needed for commercialization of ethanol production from lignocellulosic biomass. Engineered Saccharomyces cerevisiae strains expressing XYL1, XYL2 and XYL3 from Pichia stipitis, however, utilize xylose in an oxidative manner, which results in significantly lower ethanol yields from xylose as compared to glucose. As such, we hypothesized that reconfiguration of xylose metabolism from oxidative into fermentative manner might lead to efficient ethanol production from xylose. To this end, we generated a respiration-deficient (RD) mutant in order to enforce engineered S. cerevisiae to utilize xylose only through fermentative metabolic routes. Three different repeated-batch fermentations were performed to characterize characteristics of the respiration-deficient mutant. When fermenting glucose as a sole carbon source, the RD mutant exhibited near theoretical ethanol yields (0.46gg^-1) during repeated-batch fermentations by recycling the cells. As the repeated-batch fermentation progressed, the volumetric ethanol productivity increased (from 7.5 to 8.3gL^-1h^-1) because of the increased biomass from previous cultures. On the contrary, the mutant showed decreasing volumetric ethanol productivities during the repeated-batch fermentations using xylose as sole carbon source (from 0.4 to 0.3gL^-1h^-1). The mutant did not grow on xylose and lost fermenting ability gradually, indicating that the RD mutant cannot maintain a good fermenting ability on xylose as a sole carbon source. However, the RD mutant was capable of fermenting a mixture of glucose and xylose with stable yields (0.35gg^-1) and productivities (0.52gL^-1h^-1) during the repeated-batch fermentation. In addition, ethanol yields from xylose during the mixed sugar fermentation (0.30gg^-1) were higher than ethanol yields from xylose as a sole carbon source (0.21gg^-1). These results suggest that a strategy for increasing ethanol yield through respiration-deficiency can be applied for the fermentation of lignocellulosic hydrolyzates containing glucose and xylose.