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      Development of microorganisms for cellulose-biofuel consolidated bioprocessings: metabolic engineers’ tricks

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          Abstract

          Cellulose waste biomass is the most abundant and attractive substrate for “biorefinery strategies” that are aimed to produce high-value products (e.g. solvents, fuels, building blocks) by economically and environmentally sustainable fermentation processes. However, cellulose is highly recalcitrant to biodegradation and its conversion by biotechnological strategies currently requires economically inefficient multistep industrial processes. The need for dedicated cellulase production continues to be a major constraint to cost-effective processing of cellulosic biomass.

          Research efforts have been aimed at developing recombinant microorganisms with suitable characteristics for single step biomass fermentation (consolidated bioprocessing, CBP). Two paradigms have been applied for such, so far unsuccessful, attempts: a) “native cellulolytic strategies”, aimed at conferring high-value product properties to natural cellulolytic microorganisms; b) “recombinant cellulolytic strategies”, aimed to confer cellulolytic ability to microorganisms exhibiting high product yields and titers.

          By starting from the description of natural enzyme systems for plant biomass degradation and natural metabolic pathways for some of the most valuable product (i.e. butanol, ethanol, and hydrogen) biosynthesis, this review describes state-of-the-art bottlenecks and solutions for the development of recombinant microbial strains for cellulosic biofuel CBP by metabolic engineering. Complexed cellulases (i.e. cellulosomes) benefit from stronger proximity effects and show enhanced synergy on insoluble substrates (i.e. crystalline cellulose) with respect to free enzymes. For this reason, special attention was held on strategies involving cellulosome/designer cellulosome-bearing recombinant microorganisms.

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          Most cited references75

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          Non-fermentative pathways for synthesis of branched-chain higher alcohols as biofuels.

          Global energy and environmental problems have stimulated increased efforts towards synthesizing biofuels from renewable resources. Compared to the traditional biofuel, ethanol, higher alcohols offer advantages as gasoline substitutes because of their higher energy density and lower hygroscopicity. In addition, branched-chain alcohols have higher octane numbers compared with their straight-chain counterparts. However, these alcohols cannot be synthesized economically using native organisms. Here we present a metabolic engineering approach using Escherichia coli to produce higher alcohols including isobutanol, 1-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol and 2-phenylethanol from glucose, a renewable carbon source. This strategy uses the host's highly active amino acid biosynthetic pathway and diverts its 2-keto acid intermediates for alcohol synthesis. In particular, we have achieved high-yield, high-specificity production of isobutanol from glucose. The strategy enables the exploration of biofuels beyond those naturally accumulated to high quantities in microbial fermentation.
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            Consolidated bioprocessing of cellulosic biomass: an update.

            Biologically mediated processes seem promising for energy conversion, in particular for the conversion of lignocellulosic biomass into fuels. Although processes featuring a step dedicated to the production of cellulase enzymes have been the focus of most research efforts to date, consolidated bioprocessing (CBP)--featuring cellulase production, cellulose hydrolysis and fermentation in one step--is an alternative approach with outstanding potential. Progress in developing CBP-enabling microorganisms is being made through two strategies: engineering naturally occurring cellulolytic microorganisms to improve product-related properties, such as yield and titer, and engineering non-cellulolytic organisms that exhibit high product yields and titers to express a heterologous cellulase system enabling cellulose utilization. Recent studies of the fundamental principles of microbial cellulose utilization support the feasibility of CBP.
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              How biotech can transform biofuels.

              For cellulosic ethanol to become a reality, biotechnological solutions should focus on optimizing the conversion of biomass to sugars.
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                Author and article information

                Journal
                Comput Struct Biotechnol J
                Comput Struct Biotechnol J
                CSBJ
                Computational and Structural Biotechnology Journal
                Research Network of Computational and Structural Biotechnology (RNCSB) Organization
                2001-0370
                08 November 2012
                2012
                : 3
                : e201210007
                Affiliations
                [a ]Department of Life Sciences and Systems Biology, University of Torino, Via Accademia Albertina 13, 10123 Torino, Italy
                Author notes
                Citation
                Mazzoli R (2012) Development of microorganisms for cellulose-biofuel consolidated bioprocessings: metabolic engineers’ tricks. Computational and Structural Biotechnology Journal. 3 (4): e201210007. doi: http://dx.doi.org/10.5936/csbj.201210007
                [* ] Corresponding author. E-mail address: roberto.mazzoli@ 123456unito.it (Roberto Mazzoli)
                Article
                CSBJ-3-e201210007
                10.5936/csbj.201210007
                3962139
                737c537f-aaf7-45b0-a83c-71c29eee01b1
                © Mazzoli.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly cited.

                History
                : 01 September 2012
                : 22 October 2012
                : 24 October 2012
                Categories
                Mini Review

                metabolic engineering,butanol,ethanol,hydrogen,cellulosome,cellulase

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