The P. stipitis XR, which converts xylose to xylitol, prefers the cofactor NADPH over NADH by a factor of approximately 100 [23]. In yeast, NADPH is primarily formed in the oxidative PPP converting glucose-6-phosphate to ribulose-5-phosphate. Therefore, genes coding for enzymes in the oxidative PPP were deleted in order to decrease NADPH concentration in the cell and

thus force XR to use NADH instead of NADPH, which was demonstrated by the deletion of ZWF1, coding for glucose-6-phosphate dehydrogenase (G6PDH) [114] (strain TMB3255, Table 2), [115] (strain H2723, Table 1). In­creased ethanol formation at the expense of not only xylitol formation but also the xylose consumption rate was observed [114] (strains TMB3001 and TMB3255, Tables 2 and 3). In a follow-up study, the G6PDH activity was in­stead fine-tuned, which enabled the design of strains with increased ethanol yield and reasonable xylose consumption rate [116] (strains TMB3256 and TMB3037, Table 2, Fig. 4). However, in an industrial context, it is worth notic­ing that the ZWF1 deletion increases the sensitivity toward lignocellulose hydrolysates, possibly due to the limited intracellular NADPH concentration, which is important for inhibitor tolerance [116,117].

4.5.2