Comparing the xylose reductase/xylitol dehydrogenase and xylose isomerase pathways in arabinose and xylose fermenting Saccharomyces cerevisiae strains

Background: Ethanolic fermentation of lignocellulosic biomass is a sustainable option for the production
of bioethanol. This process would greatly benefit from recombinant Saccharomyces cerevisiae strains also
able to ferment, besides the hexose sugar fraction, the pentose sugars, arabinose and xylose. Different
pathways can be introduced in S. cerevisiae to provide arabinose and xylose utilisation. In this study, the
bacterial arabinose isomerase pathway was combined with two different xylose utilisation pathways: the
xylose reductase/xylitol dehydrogenase and xylose isomerase pathways, respectively, in genetically
identical strains. The strains were compared with respect to aerobic growth in arabinose and xylose batch
culture and in anaerobic batch fermentation of a mixture of glucose, arabinose and xylose.
Results: The specific aerobic arabinose growth rate was identical, 0.03 h-1, for the xylose reductase/xylitol
dehydrogenase and xylose isomerase strain. The xylose reductase/xylitol dehydrogenase strain displayed
higher aerobic growth rate on xylose, 0.14 h-1, and higher specific xylose consumption rate in anaerobic
batch fermentation, 0.09 g (g cells)-1 h-1 than the xylose isomerase strain, which only reached 0.03 h-1 and
0.02 g (g cells)-1h-1, respectively. Whereas the xylose reductase/xylitol dehydrogenase strain produced
higher ethanol yield on total sugars, 0.23 g g-1 compared with 0.18 g g-1 for the xylose isomerase strain, the
xylose isomerase strain achieved higher ethanol yield on consumed sugars, 0.41 g g-1 compared with 0.32
g g-1 for the xylose reductase/xylitol dehydrogenase strain. Anaerobic fermentation of a mixture of glucose,
arabinose and xylose resulted in higher final ethanol concentration, 14.7 g l-1 for the xylose reductase/
xylitol dehydrogenase strain compared with 11.8 g l-1 for the xylose isomerase strain, and in higher specific
ethanol productivity, 0.024 g (g cells)-1 h-1 compared with 0.01 g (g cells)-1 h-1 for the xylose reductase/
xylitol dehydrogenase strain and the xylose isomerase strain, respectively.
Conclusion: The combination of the xylose reductase/xylitol dehydrogenase pathway and the bacterial
arabinose isomerase pathway resulted in both higher pentose sugar uptake and higher overall ethanol
production than the combination of the xylose isomerase pathway and the bacterial arabinose isomerase
pathway. Moreover, the flux through the bacterial arabinose pathway did not increase when combined
with the xylose isomerase pathway. This suggests that the low activity of the bacterial arabinose pathway
cannot be ascribed to arabitol formation via the xylose reductase enzyme.