Simultaneous consumption of pentose and hexose sugars: an optimal microbial phenotype for efficient fermentation of lignocellulosic biomass
Jae-Han Kim
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David E. Block
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David A. Mills
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D. E. Block Department of Chemical Engineering and Materials Science, University of California
, One Shields Avenue,
Davis, CA 95616, USA
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) 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
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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)