The bioconversion of lignocellulosic biomass to fuel ethanol requires both hexose and pentose sugars released to be fully utilized for ethanol production to make this process cost effective.   Baker’s yeast, Saccharomyces cerevisiae, has traditionally been used in industrial ethanol fermentation of sugar and starch-based raw materials. The main challenge for using baker’s yeast in the 2nd generation bioethanol production is its inability to convert xylose, the most abundant pentose sugar in biomass hydrolysate, to ethanol. However, S. cerevisiae is capable of converting xylulose, a ketal isomer of xylose, to ethanol via the non-oxidative pentose phosphate pathway (PPP) followed by glycolysis.

Metabolic engineering approaches have been developed in recent years to introduce key enzymes into S. cerevisiae in order to convert xylose to xylulose. One approach is to introduce xylose isomerase (XI) which isomerizes xylose to xylulose, and also over-express downstream enzymes in the PPP to increase the metabolic fluxes through glycolyis. Even with these efforts, xylose is still not completely consumed. The rate limiting steps, which likely include xylose transport and isomerization, require resolution before efficient conversion of xylose to ethanol will be achieved.