(626a) 13C Metabolic Flux Analysis of Xylose Metabolism in Saccharomyces Cerevisiae

Cellulosic biofuel plays an increasingly important role in sustainable energy supply and greenhouse gas emissions reduction. Among all the endeavors for producing cellulosic biofuels, one of the most promising strategies is to engineer yeast strains to utilize xylose for bioethanol production.  To this end, a heterologous xylose pathway identified from fungal species is often introduced into Saccharomyces cerevisiae by functionally expressing xylose reductase (XR), xylitol dehydrogenase (XDH), and xylulose kinase (XKS). Despite extensive in vitro studies on the fungal xylose pathway, the intracellular metabolism rewiring in response to the heterologous xylose pathway remains largely unknown.  In this study, we applied ¹³C metabolic flux analysis to systemically investigate the flux distributions in a series of recombinant S. cerevisiae strains.  It was found that the oxidative pentose phosphate pathway was active and played a dominant role in producing NADPH for the heterologous xylose pathway. The TCA cycle activity was strong and tightly correlated with the requirements of maintenance energy and biomass yield. Compared to Scheffersomyces stipitis, more carbon fluxes were diverted into the oxidative pentose phosphate pathway and the TCA cycle in the recombinant S. cerevisiae strains. Based on in silico simulations, reducing the maintenance energy can be an important strategy to improve ethanol production. Meanwhile, the cofactor-balanced xylose pathway could be more beneficial to the xylose-based ethanol production than its counterparts, by providing a wider range of fermentation conditions for optimal fermentation conditions. As indicated by the in silico simulations, the advantages of using cofactor-balanced pathway are more significant when oxygen uptake level is lower.