Biotechnology and Bioprocess Engineering 2019; 24(6): 924-933  
Evaluating the Pathway for Co-fermentation of Glucose and Xylose for Enhanced Bioethanol Production Using Flux Balance Analysis
Richa Arora1,2, Shuvashish Behera1, Nilesh Kumar Sharma1, and Sachin Kumar1,*
1Biochemical Conversion Division, Sardar Swaran Singh National Institute of Bio-Energy, Kapurthala 144601, India
2Department of Microbiology, School of Bioengineering and Biosciences, Lovely Professional University, Phagwara 144411, India
Correspondence to: Sachin Kumar*
Biochemical Conversion Division, Sardar Swaran Singh National Institute of Bio-Energy, Kapurthala 144601, India
Tel: +91-1822-507415; Fax: +91-1822-255545
E-mail: sachin.biotech@gmail.com; sachin.kumar20@gov.in
Received: January 23, 2019; Revised: August 15, 2019; Accepted: August 16, 2019; Published online: December 31, 2019.
© The Korean Society for Biotechnology and Bioengineering. All rights reserved.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
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.
Keywords: bioethanol, xylose, thermotolerant yeast, Kluyveromyces marxianus, flux balance analysis, incomplete TCA cycle


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