Evaluating the Pathway for Co-fermentation of Glucose and Xylose for Enhanced Bioethanol Production Using Flux Balance Analysis

Two novel thermotolerant yeasts, Kluyveromyces marxianus NIRE-K1.1 and Kluyveromyces marxianus NIRE-K3.1, evolutionarily adapted for fermentation of glucose and xylose were analyzed for their metabolic fluxes with an objective of maximum ethanol production. Metabolic fluxes were analyzed for these thermotolerant yeasts by co-fermenting glucose/xylose mixture in two different ratios (1:1 and 4:1). Flux balance analysis revealed the active role of pentose phosphate pathway for effective xylose utilization in both yeasts. A comparison between co-fermentation of glucose/xylose mixtures in the ratio of 1:1 and 4:1 (g/L) reveals that the flux from glucose-6-phosphate to ribulose-5-phosphate was approximately 2.56-fold and 3.75-fold higher in 1:1 mixture in K. marxianus NIRE-K1.1 and K. marxianus NIRE-K3.1, respectively. Overall, flux towards pyruvate (for ethanol production) was found to be higher in both glucose/xylose mixtures 1:1 (1.87%) and 4:1 (0.89%) for K. marxianus NIRE-K3.1 than K. marxianus NIRE-K1.1. Tricarboxylic Acid (TCA) cycle was also found to be incomplete for both the isolates which signify that most of the available substrates were utilized for ethanol production rather than biomass formation. Moreover, it was also observed that in both, the ethanol yields were found to be higher in case of K. marxianus NIRE-K3.1 than K. marxianus NIRE-K1.1, however, xylose uptake rates were higher in the later as compared to the former. Thus, this study concludes with the capable potential of both the yeasts for the production of bioethanol from glucose/xylose mixtures with higher yield and is highly correlated to the relative concentration of both xylose and glucose in a mixture.