Xylose utilization in Saccharomyces cerevisiae during conversion of hydrothermally pretreated lignocellulosic biomass to ethanol.

With growing interest in alternative fuels to minimize carbon and particle emissions, research continues on the production of lignocellulosic ethanol and on the development of suitable yeast strains. However, great diversities and continued technical advances in pretreatment methods for lignocellulosic biomass complicate the evaluation of developed yeast strains, and strain development often lags industrial applicability. In this review, recent studies demonstrating developed yeast strains with lignocellulosic biomass hydrolysates are compared. For the pretreatment methods, we highlight hydrothermal pretreatments (dilute acid treatment and autohydrolysis), which are the most commonly used and effective methods for lignocellulosic biomass pretreatment. Rather than pretreatment conditions, the type of biomass most strongly influences the composition of the hydrolysates. Metabolic engineering strategies for yeast strain development, the choice of xylose-metabolic pathway, adaptive evolution, and strain background are highlighted as important factors affecting ethanol yield and productivity from lignocellulosic biomass hydrolysates. A comparison of the parameters from recent studies demonstrating lignocellulosic ethanol production provides useful information for future strain development.

Comparative global metabolite profiling of xylose-fermenting Saccharomyces cerevisiae SR8 and Scheffersomyces stipitis.

Bioconversion of lignocellulosic biomass into ethanol requires efficient xylose fermentation. Previously, we developed an engineered Saccharomyces cerevisiae strain, named SR8, through rational and inverse metabolic engineering strategies, thereby improving its xylose fermentation and ethanol production. However, its fermentation characteristics have not yet been fully evaluated. In this study, we investigated the xylose fermentation and metabolic profiles for ethanol production in the SR8 strain compared with native Scheffersomyces stipitis. The SR8 strain showed a higher maximum ethanol titer and xylose consumption rate when cultured with a high concentration of xylose, mixed sugars, and under anaerobic conditions than Sch. stipitis. However, its ethanol productivity was less on 40 g/L xylose as the sole carbon source, mainly due to the formation of xylitol and glycerol. Global metabolite profiling indicated different intracellular production rates of xylulose and glycerol-3-phosphate in the two strains. In addition, compared with Sch. stipitis, SR8 had increased abundances of metabolites from sugar metabolism and decreased abundances of metabolites from energy metabolism and free fatty acids. These results provide insights into how to control and balance redox cofactors for the production of fuels and chemicals from xylose by the engineered S. cerevisiae.