Article

Simultaneous co-fermentation of mixed sugars: A promising strategy for producing cellulosic ethanol

Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL 61821, USA.
Trends in Biotechnology (Impact Factor: 10.04). 02/2012; 30(5):274-82. DOI: 10.1016/j.tibtech.2012.01.005
Source: PubMed

ABSTRACT The lack of microbial strains capable of fermenting all sugars prevalent in plant cell wall hydrolyzates to ethanol is a major challenge. Although naturally existing or engineered microorganisms can ferment mixed sugars (glucose, xylose and galactose) in these hydrolyzates sequentially, the preferential utilization of glucose to non-glucose sugars often results in lower overall yield and productivity of ethanol. Therefore, numerous metabolic engineering approaches have been attempted to construct optimal microorganisms capable of co-fermenting mixed sugars simultaneously. Here, we present recent findings and breakthroughs in engineering yeast for improved ethanol production from mixed sugars. In particular, this review discusses new sugar transporters, various strategies for simultaneous co-fermentation of mixed sugars, and potential applications of co-fermentation for producing fuels and chemicals.

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Available from: Soo Rin Kim, Feb 05, 2015
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    • "Certain fermentative microorganisms are known to utilize sugars sequentially with a preference of glucose over xylose (Jojima et al., 2010; Kim et al., 2012). In order to investigate if the xylAB strain also has such preference, photomixotrophic cultures with a starting cell density of 0.6 OD 730 were tested with 10 mM glucose or xylose, or an equal molar mixture totaling 20 mM. "
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    ABSTRACT: Hydrolysis of plant biomass generates a mixture of simple sugars that is particularly rich in glucose and xylose. Fermentation of the released sugars emits CO2 as byproduct due to metabolic inefficiencies. Therefore, the ability of a microbe to simultaneously convert biomass sugars and photosynthetically fix CO2 into target products is very desirable. In this work, the cyanobacterium, Synechocystis 6803, was engineered to grow on xylose in addition to glucose. Both the xylA (xylose isomerase) and xylB (xylulokinase) genes from E. coli were required to confer xylose utilization, but a xylose-specific transporter was not required. Introduction of xylAB into an ethylene-producing strain increased the rate of ethylene production in the presence of xylose. Additionally, introduction of xylAB into a glycogen-synthesis mutant enhanced production of keto acids. Isotopic tracer studies found that nearly half of the carbon in the excreted keto acids was derived from the engineered xylose metabolism, while the remainder was derived from CO2 fixation. Copyright © 2015. Published by Elsevier Inc.
    Metabolic Engineering 06/2015; DOI:10.1016/j.ymben.2015.06.002 · 8.26 Impact Factor
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    • "Recently, cellodextrin transporter and an intracellular b-glucosidase (BGL) were introduced in host microorganism to accumulate little amount of glucose in the medium and to enhance the overall consumption of cellodextrin and non-glucose sugars (Kim et al., 2012). In this regard, R. kratochvilovae HIMPA1 species is a novel oleaginous yeast that is capable of simultaneous sugar utilization. "
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    ABSTRACT: Novel strategy for enhancing TAG accumulation by simultaneous utilization of fermentable and non-fermentable carbon sources as substrate for cultivation of oleaginous yeast Rhodosporidium kratochvilovae HIMPA1 were undertaken in this investigation. The yeast strain showed direct correlation between the size of lipid bodies, visualized by BODIPY stain (493-515nm) and TAG accumulation when examined on individual fermenting and non-fermenting carbon sources and their mixtures. Maximum TAG accumulation (μm) in glucose (2.38±0.52), fructose (4.03±0.38), sucrose (4.24±0.45), glycerol (4.35±0.54), xylulose (3.94±0.12), and arabinose (2.98±0.43) were observed. Synergistic effect of the above carbon sources (fermentable and non-fermentable) in equimolar concentration revealed maximum lipid droplet size of 5.35±0.76μm and cell size of 6.89±0.97μm. Total lipid content observed in mixed carbon sources was 9.26g/l compared to glucose (6.2g/l). FAME profile revealed enhanced longer chain (C14:0-C24:0) fatty acids in mix carbon sources. Copyright © 2015 Elsevier Ltd. All rights reserved.
    Bioresource Technology 02/2015; 188. DOI:10.1016/j.biortech.2015.02.062 · 5.04 Impact Factor
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    • "Two main strategies have been commonly applied to solve this problem: the cloning of a xylose reductase and a xylitol dehydrogenase which are linked to the same coenzyme or the cloning of a xylose isomerase which converts xylose directly into its isomer xylulose. Nevertheless, yeasts engineered through that strategy to ferment xylose still do it slowly and accumulate xylitol (Kim et al. 2012). Thus, additional genetic modifications have been carried out as an attempt to increase the specific consumption of xylose as well as the rate and yield of production of ethanol: i) overexpression of the enzymes necessary for the conversion of xylulose into glycolysis intermediates; ii) deletion of the endogenous aldose reductase which converts xylose into xylitol; iii) overexpression of heterologous xylose transporters (Cai et al. 2012). "
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    ABSTRACT: We have recently demonstrated that heterologous expression of a bacterial xylose isomerase gene (xylA) of Burkholderia cenocepacia enabled a laboratorial Saccharomyces cerevisiae strain to ferment xylose anaerobically, without xylitol accumulation. However, the recombinant yeast fermented xylose slowly. In this study, an evolutionary engineering strategy was applied to improve xylose fermentation by the xylA-expressing yeast strain, which involved sequential batch cultivation on xylose. The resulting yeast strain co-fermented glucose and xylose rapidly and almost simultaneously, exhibiting improved ethanol production and productivity. It was also observed that when cells were grown in a medium containing higher glucose concentrations before being transferred to fermentation medium, higher rates of xylose consumption and ethanol production were obtained, demonstrating that xylose utilization was not regulated by catabolic repression. Results obtained by qPCR demonstrate that the efficiency in xylose fermentation showed by the evolved strain is associated, to the increase in the expression of genes HXT2 and TAL1, which code for a low-affinity hexose transporter and transaldolase, respectively. The ethanol productivity obtained after the introduction of only one genetic modification and the submission to a one-stage process of evolutionary engineering was equivalent to those of strains submitted to extensive metabolic and evolutionary engineering, providing solid basis for future applications of this strategy in industrial strains.
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