Reducing xylitol formation has been a major challenge in xylose fermentation by recombinant S. cerevisiae carrying the P. stipitis xylose pathway enzymes XR and XDH. Xylitol formation has primarily been ascribed to the differ­ence in cofactor requirements of the two enzymes, so that the intracellular concentration of NAD+ controls the amount of xylitol being converted to xy­lulose [21,32,47,74-76]. However, xylitol formation during ethanolic xylose fermentation also depends on the strain background, i. e., the metabolism of the host cell, since for example some strains of P. stipitis do not produce xyl­itol [47,49,50]. Thus, engineering the redox metabolism of the S. cerevisiae host has been given great attention where the aim primarily has been to ma­nipulate the intracellular concentrations and fluxes of cofactors to minimize xylitol formation.

4.5.1