Biomedical Engineering Reference
In-Depth Information
fermentation and improve the productivity of the system, but the impact of lignin
on the fermentation cannot be overcome.
4.1.3 Consolidated Bioprocessing
Cellulases are produced separately and added to hydrolyze the cellulose compo-
nent of pretreated biomass for the SHCF and SSCF processes, which is one of the
major barriers for cost reduction of bioethanol due to the high cost of the enzyme
as well as the high enzyme dosage required by the processes. In nature, many
organisms, particularly microorganisms, can utilize native cellulose as a carbon
source and energy to support their growth and metabolism, through synthesis and
secretion of unique cellulases and subsequent hydrolysis of cellulose by the syn-
ergic functions of these enzymes [
55
]. Such a natural phenomenon has inspired
scientists to develop mimic systems, either an individual microorganism or a
microbial community, to produce ethanol and other chemicals directly from
lignocellulosic biomass, even without pretreatment. All problems found with the
biochemical conversion of lignocellulosic biomass seem to be solvable by this
so-called consolidated bioprocessing (CBP) strategy, which was evolved from the
concept of direct microbial conversion [
56
].
However, no natural microorganism is available for commercial production of
bioethanol with the CBP strategy. Thus, the development of CBP strains is the core of
the CBP process. Currently, both bacterial and yeast species have been explored for
this purpose with the following strategies: (1) engineering a cellulase producer to be
ethanologenic, and (2) engineering an ethanologen to be cellulolytic [
57
]. For the
first strategy, anaerobic cellulolytic bacteria from the genus Clostridium are good
candidates [
58
], and the targets for the metabolic engineering of this species include
increasing ethanol titer by improving ethanol tolerance through rational designs
based on the understanding of the mechanisms underlying its response to ethanol
inhibition and random approaches such as the selection of mutants through an
evolutionary adaptation procedure, and on the other hand improving ethanol yield by
blocking the synthesis of major by-products, as illustrated by the progress with the
thermophilic bacterium Thermoanaerobacterium saccharolyticum [
59
]. As for the
second strategy, the primary concerns are expression and secretion of functional
cellulases in ethanologenic species, particularly S. cerevisiae, which has been
engineered with genes encoding glycoside hydrolases including cellulases and
hemicellulases through cell surface display techniques [
60
,
61
]. Unfortunately,
expression of the cellobiohydrolases (CBH I and CBH II) from Trichoderma reesei is
generally poor, not to mention the challenges of engineering the species with more
other enzymes or pathways required by the efficient production of bioethanol.
Theoretically, the CBP strategy can completely eliminate cellulase production
and integrate all three major steps of the bioconversion into a single cell. However,
there are many unknowns to be elucidated in order to make it significant in
the production of bioethanol and other biofuels. For example, the production of
cellulolytic enzymes, hydrolysis of cellulose and hemicelluloses and fermentation
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