Abstract
BackgroundSimultaneous cofermentation of glucose and xylose mixtures would be a cost-effective solution for the conversion of cellulosic biomass to high-value products. However, most yeasts ferment glucose and xylose sequentially due to glucose catabolite repression. A well known thermotolerant yeast, Kluyveromyces marxianus, was selected for this work because it possesses cost-effective advantages over Saccharomyces cerevisiae for biofuel production from cellulosic biomass.ResultsIn the present study, we employed a directed evolutionary approach using 2-deoxyglucose to develop a thermotolerant mutant capable of simultaneous cofermentation of glucose and xylose by alleviating catabolite repression. The selected mutant, K. marxianus SBK1, simultaneously cofermented 40 g/L glucose and 28 g/L xylose to produce 23.82 g/L ethanol at 40 °C. This outcome corresponded to a yield of 0.35 g/g and productivity of 0.33 g/L h, representing an 84% and 129% improvement, respectively, over the parental strain. Interestingly, following mutagenesis the overall transcriptome of the glycolysis pathway was highly downregulated in K. marxianus SBK1, except for glucokinase-1 (GLK1) which was 21-fold upregulated. Amino acid sequence of GLK1 from K. marxianus SBK1 revealed three amino acid mutations which led to more than 22-fold lower enzymatic activity compared to the parental strain.ConclusionsWe herein successfully demonstrated that the cofermentation of a sugar mixture is a promising strategy for the efficient utilization of cellulosic biomass by K. marxianus SBK1. Through introduction of additional biosynthetic pathways, K. marxianus SBK1 could become a chassis-type strain for the production of fuels and chemicals from cellulosic biomass.
Highlights
Simultaneous cofermentation of glucose and xylose mixtures would be a cost-effective solution for the conversion of cellulosic biomass to high-value products
Kahar et al demonstrated that mutant S. cerevisiae (M2) produced 5.5 g/L ethanol from 20 g/L glucose and 16 g/L xylose with a final yield of 0.15 g/g ethanol [12]
A well known thermotolerant yeast, K. marxianus, is a robust strain used for industrial ethanol production [18] and was previously reported to grow and ferment in environments up to 50 °C [19]
Summary
Simultaneous cofermentation of glucose and xylose mixtures would be a cost-effective solution for the conversion of cellulosic biomass to high-value products. A well known thermotolerant yeast, Kluyveromyces marxianus, was selected for this work because it possesses cost-effective advantages over Saccharomyces cerevisiae for biofuel production from cellulosic biomass. Over the past few decades, research has become more focused on alternative fuel resources due to economic and environmental challenges Among these resources, biofuels, including ethanol, are the most well known substitutes for liquid transportation fuels [1, 2]. Simultaneous saccharification and fermentation (SSF), or simultaneous saccharification and cofermentation (SSCF) processes are time- and costeffective strategies often utilized today [15, 16] In these processes, thermotolerant yeasts are necessary, because saccharification, generally considered as the overall ratedetermining step in SSF or SSCF, is performed by cellulase enzymes with high optimal reaction temperatures (45–55 °C) [14, 17]. K. marxianus has various benefits over mesophilic yeasts, such as high growth rates, reduced cooling costs, and a broad spectrum of substrates, including xylose [20, 21]
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