FIG. 1.

Redox cofactors in the metabolic pathway for d-xylose fermentation. d-Xylose is converted to d-xylulose through an NADPH-utilizing reductase and NAD⁺-utilizing dehydrogenase. d-Xylulose is then phosphorylated to the pentose phosphate intermediate d-xylulose 5-phosphate (X5P). The products of the pentose phosphate pathway are fructose-6-phosphate (F6P) and GAP. GAP is reduced through an NADP-GAPDH, encoded by GDP1, or by the endogenous NAD-GAPDH, depending on cofactor availability. In the following reactions involving an NADH requiring alcohol dehydrogenase, equimolar amounts of CO₂ and ethanol are derived. A competing pathway for NADP⁺ is the oxidative part of the pentose phosphate pathway. Glucose-6-phosphate (G6P) is derived from fructose-6-phosphate and can enter the oxidative part of the pentose phosphate pathway through G6PDH, which is encoded by the ZWF1 gene. G6P is oxidized, thereby generating NADPH and CO₂. The deletion of ZWF1 prevents this reaction.