9-20: Bioconversion of D-xylose to D-xylonate with Saccharomyces cerevisiae

Increasing concern about climate change and fluctuation in fossil fuel prices has increased interest in development of new biomass based products. Production of organic acids using yeast is a promising approach to generate building-block chemicals from renewable carbon sources, such as lignocellulosic hydrolysates. Hydrolysed plant biomass typically contains a substantial fraction of D-xylose, which could be converted to ethanol, but might preferably be converted to other products, including D-xylonic acid. D-Xylonic acid can be used as a substitute for D-gluconic acid, e.g. to improve dispersal of concrete, as a polyamide modifier or as a precursor for 1,2,4-butanetriol.

We have demonstrated that D-xylonate can be efficiently produced from D-xylose with Saccharomyces cerevisiae. 17±2 g D-xylonate l^-1 at 0.23 g l^-1 h^-1 was produced from 23 g D-xylose l^-1 (with glucose and ethanol as co-substrates) when expressing an NAD⁺-dependent D-xylose dehydrogenase, XylB, from Caulobacter crescentus. D-Xylonate titre and production rate were improved and xylitol production reduced, compared to strains expressing genes encoding Trichoderma reesei or pig liver NADP⁺-dependent D-xylose dehydrogenases. However, the production led to an intracellular accumulation of D-xylonate (up to 70 mg g^-1) and xylitol (up to 18 mg g^-1) and to a decreased viability of the D-xylonate producing cells. To reduce xylitol production, xylB was also expressed in a strain from which the major aldose reductase, encoded by GRE3, had been deleted. An industrial S. cerevisiae strain expressing XylB produced 43 g D-xylonate l^-1 from 49 g D-xylose l^-1, with an initial production rate of 0.44 g l^-1 h^-1.