Simultaneous consumption of pentose and hexose sugars: an optimal microbial phenotype for efficient fermentation of lignocellulosic biomass

Applied Microbiology and Biotechnology, Nov 2010

Lignocellulosic biomass is an attractive carbon source for bio-based fuel and chemical production; however, its compositional heterogeneity hinders its commercial use. Since most microbes possess carbon catabolite repression (CCR), mixed sugars derived from the lignocellulose are consumed sequentially, reducing the efficacy of the overall process. To overcome this barrier, microbes that exhibit the simultaneous consumption of mixed sugars have been isolated and/or developed and evaluated for the lignocellulosic biomass utilization. Specific strains of Escherichia coli, Saccharomyces cerevisiae, and Zymomonas mobilis have been engineered for simultaneous glucose and xylose utilization via mutagenesis or introduction of a xylose metabolic pathway. Other microbes, such as Lactobacillus brevis, Lactobacillus buchneri, and Candida shehatae possess a relaxed CCR mechanism, showing simultaneous consumption of glucose and xylose. By exploiting CCR-negative phenotypes, various integrated processes have been developed that incorporate both enzyme hydrolysis of lignocellulosic material and mixed sugar fermentation, thereby enabling greater productivity and fermentation efficacy.

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Simultaneous consumption of pentose and hexose sugars: an optimal microbial phenotype for efficient fermentation of lignocellulosic biomass

Jae-Han Kim 0 David E. Block 0 David A. Mills 0 0 D. E. Block Department of Chemical Engineering and Materials Science, University of California , One Shields Avenue, Davis, CA 95616, USA 1 ) Robert Mondavi Institute for Wine and Food Science, Department of Viticulture and Enology, University of California , One Shields Avenue, Davis, CA 95616, USA Lignocellulosic biomass is an attractive carbon source for bio-based fuel and chemical production; however, its compositional heterogeneity hinders its commercial use. Since most microbes possess carbon catabolite repression (CCR), mixed sugars derived from the lignocellulose are consumed sequentially, reducing the efficacy of the overall process. To overcome this barrier, microbes that exhibit the simultaneous consumption of mixed sugars have been isolated and/or developed and evaluated for the lignocellulosic biomass utilization. Specific strains of Escherichia coli, Saccharomyces cerevisiae, and Zymomonas mobilis have been engineered for simultaneous glucose and xylose utilization via mutagenesis or introduction of a xylose metabolic pathway. Other microbes, such as Lactobacillus brevis, Lactobacillus buchneri, and Candida shehatae possess a relaxed CCR mechanism, showing simultaneous consumption of glucose and xylose. By exploiting CCR-negative phenotypes, various integrated processes have been developed that incorporate both enzyme hydrolysis of lignocellulosic material and mixed sugar fermentation, thereby enabling greater productivity and fermentation efficacy. Keyword Lactobacillus brevis . Carbon catabolite repression . Simultaneous carbohydrate utilization . Lignocellulosic biomass . SSMSF - Use of lignocellulosic biomass is currently under intensive study as an alternative growth substrate for bio-based chemical and energy production. Despite the advantages in sustainability and availability, commercial use of lignocellulose is still problematic. Due to the complexity of lignocellulosic materials, hydrolysis of hemicellulose and cellulose into five- and six-carbon sugars has to be carried out prior to, or concurrently with, the fermentation. During the fermentation of sugars released by hydrolysis, microorganisms tend to selectively utilize a preferred sugar, usually glucose. This preferential consumption of sugar, termed carbon catabolite repression (CCR), makes it challenging to design and efficiently control the fermentation processes using lignocellulosic biomass as a feedstock. The development of microorganisms to ferment sugars released from lignocellulosic biomass, either through selection of new strains or by genetic engineering of traditional strains, has generally focused on generating maximum product yield from total available sugar. However, the pattern of substrate utilization is also important in overall process design as simultaneous use of all sugars is likely to lead to a shorter and more productive process. In this review, we present several microbial strains that can utilize mixed sugars simultaneously and discuss the advantages in lignocellulosic biomass utilization in terms of fermentation process design. Fermentation of mixed sugars derived from lignocellulosic biomass Lignocellulose is one of the structural materials of the plant cell wall that contains the heterogeneous complex of cellulose, hemicellulose, and lignin (Buchanan et al. 2000). Cellulose is a -1,4-linked homopolymer of glucose, whereas hemicellulose is a heteropolymer of hexoses (glucose) and pentoses (mainly xylose and arabinose). Lignin, another component of lignocellulose, is a heterogeneous polymer of phenylpropanoid units that gives the tensile strength of plant material (Boerjan et al. 2003). To use a lignocellulosic biomass as a substrate for a biobased chemical production, hydrolysis of cellulose and hemicellulose has to be achieved prior to the fermentation of the resulting mixed sugars. Hemicellulose can be chemically degraded at high pressure and high temperature by chemicals such as ammonia (Ammonia fiber explosion: AFEX) or at ambient temperature by dilute sulfuric acid (<1.2%) (Dale and Moreira 1982; Holtzapple et al. 1992; Ropars et al. 1992; Schell et al. 1991). However, cellulose fiber is hydrolyzed to glucose enzymatically by the commercially available cellulases and cellobiases. Lignocellulosic biomass is comprised of up to 45% hemicellulose. Therefore, in order to achieve maximum product yield and productivity, a complete utilization of mixed sugars derived from hemicellulose is essential (Saha 2003). While many fermentation microbes are able to utilize pentose sugars, catabolism of these pentose sugars is typically suppressed by glucose derived from cellulose (Stulke and Hillen 1999). This selective and sequential utilization of mixed sugars by most microbes makes the fermentation process complex and often reduces the yields and productivity on the target biomass (Bothast et al. 1999). When sugars are consumed sequentially during the fedb (...truncated)


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Jae-Han Kim, David E. Block, David A. Mills. Simultaneous consumption of pentose and hexose sugars: an optimal microbial phenotype for efficient fermentation of lignocellulosic biomass, Applied Microbiology and Biotechnology, 2010, pp. 1077-1085, Volume 88, Issue 5, DOI: 10.1007/s00253-010-2839-1