Xylose fermentation by Saccharomyces cerevisiae

Abstract

We have performed a comparative study of xylose utilization in Saccharomyces cerevisiae transformants expressing two key enzymes in xylose metabolism, xylose reductase (XR) and xylitol dehydrogenase (XDH), and in a prototypic xylose-utilizing yeast, Pichia stipitis. In the absence of respiration (see text), baker's yeast cells convert half of the xylose to xylitol and ethanol, whereas P. stipitis cells display rather a homofermentative conversion of xylose to ethanol. Xylitol production by baker's yeast is interpreted as a result of the dual cofactor dependence of the XR and the generation of NADPH by the pentose phosphate pathway. Further limitations of xylose utilization in S. cerevisiae cells are very likely caused by an insufficient capacity of the nonoxidative pentose phosphate pathway, as indicated by accumulation of sedoheptulose-7-phosphate and the absence of fructose-l,6-bisphosphate and pyruvate accumulation. By contrast, uptake at high substrate concentrations probably does not limit xylose conversion in S. cerevisiae X Y L 1 / X Y L 2 transformants.

Enhanced biofuel production through coupled acetic acid and xylose consumption by engineered yeast

“…1). However, this pathway introduces a problematic redox cofactor imbalance because XR can use both NADPH and nicotinamide adenine dinucleotide (NADH), whereas XDH uses NAD þ only, leading to high xylitol accumulation and low ethanol yields [17][18][19][20] . Xylitol formation could be reduced by providing sufficient oxygen 21 .…”

Lipid accumulation from glucose and xylose in an engineered, naturally oleaginous strain of Saccharomyces cerevisiae

“…Glyceraldehyde-3-phosphate is readily isomerized to DHAP which can then enter the lipid pathway by conversion to glycerol-3-phosphate. Previous work has shown that S. cerevisiae has insufficient levels of TAL1 to efficiently ferment xylose (Walfridsson et al, 1995;Hasunuma et al, 2011), causing a reduced capacity of flux through the PPP and a build-up of sedoheptulose-7-phosphate (Kotter and Ciriacy, 1993). Reduced oxidative PPP flux improves ethanol yield (Jeppsson et al, 2002) suggesting that having a slower carbon flux through the PPP allows time for the lipid production pathway enzymes to siphon off carbon and shunt it to lipids rather than ethanol.…”

Microbial conversion of sugars from plant biomass to lactic acid or ethanol

“…To meet the challenge of fermenting all the plant biomass sugars to ethanol, a number of engineered bacteria and yeasts have been produced. Pentose-fermenting S. cerevisiae strains have been generated by adding the xylose metabolic pathway from P. stipitis (Kotter and Ciriacy, 1993) and introducing the xylose isomerase gene either from a bacterium (Thermus thermophilus; Walfridsson et al, 1996) or from an anaerobic fungus (Piromyces sp. ; Kuyper et al, 2003;van Maris et al, 2007).…”