title: Reduced oxidative pentose phosphate pathway flux in recombinant xylose-utilizing Saccharomyces cerevisiae strains improves the ethanol yield from xylose.
abstract: In recombinant, xylose-fermenting Saccharomyces cerevisiae, about 30% of the consumed xylose is converted to xylitol. Xylitol production results from a cofactor imbalance, since xylose reductase uses both NADPH and NADH, while xylitol dehydrogenase uses only NAD(+). In this study we increased the ethanol yield and decreased the xylitol yield by lowering the flux through the NADPH-producing pentose phosphate pathway. The pentose phosphate pathway was blocked either by disruption of the GND1 gene, one of the isogenes of 6-phosphogluconate dehydrogenase, or by disruption of the ZWF1 gene, which encodes glucose 6-phosphate dehydrogenase. Decreasing the phosphoglucose isomerase activity by 90% also lowered the pentose phosphate pathway flux. These modifications all resulted in lower xylitol yield and higher ethanol yield than in the control strains. TMB3255, carrying a disruption of ZWF1, gave the highest ethanol yield (0.41 g g(-1)) and the lowest xylitol yield (0.05 g g(-1)) reported for a xylose-fermenting recombinant S. cerevisiae strain, but also an 84% lower xylose consumption rate. The low xylose fermentation rate is probably due to limited NADPH-mediated xylose reduction. Metabolic flux modeling of TMB3255 confirmed that the NADPH-producing pentose phosphate pathway was blocked and that xylose reduction was mediated only by NADH, leading to a lower rate of xylose consumption. These results indicate that xylitol production is strongly connected to the flux through the oxidative part of the pentose phosphate pathway.
authors.affiliation: Department of Applied Microbiology, Lund University, 221 00 Lund, Sweden.
authors.forename: Marie
authors.initials: M
authors.lastname: Jeppsson
authors.forename: Björn
authors.initials: B
authors.lastname: Johansson
authors.forename: Bärbel
authors.initials: B
authors.lastname: Hahn-Hägerdal
authors.forename: Marie F
authors.initials: MF
authors.lastname: Gorwa-Grauslund
mesh_terms: D000431:Ethanol / Q000378:metabolism*; D005285:Fermentation; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D005818:Genetic Engineering / Q000379:methods; D010084:Oxidation-Reduction; D010427:Pentose Phosphate Pathway / Q000235:genetics / Q000502:physiology*; D011995:Recombination, Genetic; D012441:Saccharomyces cerevisiae / Q000201:enzymology* / Q000235:genetics*; D014994:Xylose / Q000235:genetics / Q000378:metabolism*
chemical_list: D005656:Fungal Proteins; D000431:Ethanol; D014994:Xylose