Metabolic engineering of Saccharomyces cerevisiae for bioconversion of d-xylose to d-xylonate

An NAD ⁺-dependent d-xylose dehydrogenase, XylB, from Caulobacter crescentus was expressed in Saccharomyces cerevisiae, resulting in production of 17±2g d-xylonate l ^-1 at 0.23gl ^-1h ^-1 from 23g d-xylose l ^-1 (with glucose and ethanol as co-substrates). d-Xylonate titre and production rate were increased and xylitol production decreased, compared to strains expressing genes encoding T. reesei or pig liver NADP ⁺-dependent d-xylose dehydrogenases. d-Xylonate accumulated intracellularly to ~70mgg ^-1; xylitol to ~18mgg ^-1. The aldose reductase encoding gene GRE3 was deleted to reduce xylitol production. Cells expressing d-xylonolactone lactonase xylC from C. crescentus with xylB initially produced more extracellular d-xylonate than cells lacking xylC at both pH 5.5 and pH 3, and sustained higher production at pH 3. Cell vitality and viability decreased during d-xylonate production at pH 3.0. An industrial S. cerevisiae strain expressing xylB efficiently produced 43g d-xylonate l ^-1 from 49g d-xylose l ^-1.