Engineering the redox reactions of Saccharomyces cerevisiae for improved pentose fermentation

The two most widespread pentose sugars in our biosphere are D-xylose and L-arabinose. The pentose catabolic pathways are relevant for microorganisms living on decaying plant material and also in biotechnology when cheap raw materials such as plant hydrolysates are fermented to ethanol. Pentose, i.e. D-xylose and L-arabinose fermentation to ethanol with recombinant S. cerevisiae is slow and has a low yield. One reason is that the catabolism of these pentoses through the corresponding fungal pathways creates an imbalance of redox cofactors. The process, although redox neutral, requires NADPH which must be regenerated in a separate process. To facilitate the NADPH regeneration, the recently discovered gene GDP1 coding for a fungal NADP GAPDH was expressed in a S. cerevisiae strain with the D-xylose pathway. Glucose 6-phosphate dehydrogenase is the main path for NADPH regeneration, however it causes futile CO2 production and creates a redox imbalance on the pathway for anaerobic fermentation to ethanol. The deletion of the corresponding gene, zwf1, in combination with overexpression of GDP1 stimulated D-xylose fermentation with respect to rate and yield. The CO2 over ethanol ratio decreased from 2.5 to 1.3 and the ethanol over xylitol ratio increased from 0.9 to 3; i.e. less CO2 and xylitol were produced. Through redox engineering a yeast strain, which was mainly producing xylitol and CO2 from D-xylose, was converted to a strain producing mainly ethanol.