Exploring d-xylose oxidation in Saccharomyces cerevisiae through the Weimberg pathway

Engineering of the yeast Saccharomyces cerevisiae towards efficient d -xylose assimilation has been a major focus over the last decades since d -xylose is the second most abundant sugar in nature, and its conversion into products could significantly improve process economy in biomass-based processes. Up to now, two different metabolic routes have been introduced via genetic engineering, consisting of either the isomerization or the oxido-reduction of d -xylose to d -xylulose that is further connected to the pentose phosphate pathway and glycolysis. In the present study, cytosolic d -xylose oxidation was investigated instead, through the introduction of the Weimberg pathway from Caulobacter crescentus in S. cerevisiae . This pathway consists of five reaction steps that connect d -xylose to the TCA cycle intermediate α-ketoglutarate. The corresponding genes could be expressed in S. cerevisiae , but no growth was observed on d -xylose indicating that not all the enzymes were functionally active. The accumulation of the Weimberg intermediate d -xylonate suggested that the dehydration step(s) might be limiting, blocking further conversion into α-ketoglutarate. Although four alternative dehydratases both of bacterial and archaeon origins were evaluated, d -xylonate accumulation still occurred. A better understanding of the mechanisms associated with the activity of dehydratases, both at a bacterial and yeast level, appears essential to obtain a fully functional Weimberg pathway in S. cerevisiae .