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WIREs Energy Environ.
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Hydrolysis and fermentation for cellulosic ethanol production

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Second‐generation bioethanol produced from various lignocellulosic materials, such as wood, agricultural, or forest residues, has the potential to be a valuable substitute for, or a complement to, gasoline. At least three major factors—rapidly increasing atmospheric CO2 levels, dwindling fossil fuel reserves, and their rising costs—suggest that we now need to accelerate research plans to make greater use of plant‐based biomass for energy production and as a chemical feedstock as part of a sustainable energy economy. Optimizing the production of bioethanol to be competitive with petrochemical fuels is the main challenge for the underlying process development. The exhaustive research on enzyme technology during the latest years, resulting in significant advances in the field, show the importance of the enzymatic hydrolysis for a profitable ethanol production process. On the other hand, the persisting challenges in biomass pretreatment, which are the initial steps in most process designs, show the remarkable recalcitrance of the lignocellulosic materials to biological degradation. The recent scientific trends show toward an integrated overall bioconversion process in which fermentation technology and genetic engineering of ethanologenic microorganisms aim not only at maximizing yields and productivities but also at widening the range of fermentation products and applications. This article is categorized under: Bioenergy > Science and Materials
Cellulose structure is formed by β‐(1,4)‐linked d‐glucose units, where adjacent d‐glucoses are flipped making cellobiose the fundamental repeating unit. The inter‐ and intramolecular hydrogen bonds (shown as dots) and van der Waals interactions form recalcitrance microfiber structures.
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Microbial conversion of glucose to ethanol under anaerobic conditions. The enzymes catalysing the main biochemical steps are indicated. Most microorganisms catabolize glucose through the glycolytic pathway (EMP). Although there are many aerobic bacterial species that use the ED pathway, Zymomonas is the only known microbial genus that uses this pathway under anaerobic conditions. LFP, pyruvate formate lyase; LDH, lactate dehydrogenase; PEP: phosphoenolpyruvate; PPP, pentose phosphate pathway; DHAP, dihydroxyacetone phosphate; KDPG, 2‐keto‐3‐deoxy‐6‐phosphogluconate.
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An overview of cellulose hydrolysis by the synergistic action of cellulolytic enzymes; the β‐1,4‐endoglucanases (EG5) catalyze the hydrolysis of the main chain of cellulose located in the amorphous region, resulting in nonoxidized chain ends, whereas polysaccharide monooxygenases of GH family 61 (CEL61) catalyze oxidatively possibly the crystalline region, resulting in oxidized chain ends. Cellobiohydrolases hydrolyze cellulose chain ends from the reducing (CBH7) or nonreducing (CBH6) end in a processive manner to produce cellobiose or oxidized cellobiose, depending on the preceded family of enzymes that made the nick on cellulose surface. The processive action of cellobiohydrolases generates a majority of cellobiose that could be further hydrolyzed to d‐glucose by β‐d‐glucosidases (BGL3).
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