Bioconversion of d-xylose to d-xylonate with Kluyveromyces lactis

d-Xylonate was produced from d-xylose using Kluyveromyces lactis strains which expressed the gene for NADP+-dependent d-xylose dehydrogenase from Trichoderma reesei (xyd1). Up to 19±2 g d-xylonate l−1 was produced when K. lactis expressing xyd1 was grown on 10.5 g d-galactose l−1 and 40 g d-xylose l−1. Intracellular accumulation of d-xylonate (up to ∼70 mg [g biomass]−1) was observed. se was metabolised to d-xylonate, xylitol and biomass. Oxygen could be reduced to 6 mmol O2 l−1 h−1 without loss in titre or production rate, but metabolism of d-xylose and xylitol were more efficient when 12 mmol O2 l−1 h−1 were provided. se uptake was not affected by deletion of either the d-xylose reductase (XYL1) or a putative xylitol dehydrogenase encoding gene (XYL2) in xyd1 expressing strains. K. lactis xyd1ΔXYL1 did not produce extracellular xylitol and produced more d-xylonate than the xyd1 strain containing the endogenous XYL1. K. lactis xyd1ΔXYL2 produced high concentrations of xylitol and significantly less d-xylonate than the xyd1 strain with the endogenous XYL2.