Microbial cellulose utilization: fundamentals and biotechnology.
01 Sep 2002-Microbiology and Molecular Biology Reviews (American Society for Microbiology)-Vol. 66, Iss: 3, pp 506-577
TL;DR: A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.
Abstract: Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.
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TL;DR: Hydrogen Production by Water−Gas Shift Reaction 4056 4.1.
Abstract: 1.0. Introduction 4044 2.0. Biomass Chemistry and Growth Rates 4047 2.1. Lignocellulose and Starch-Based Plants 4047 2.2. Triglyceride-Producing Plants 4049 2.3. Algae 4050 2.4. Terpenes and Rubber-Producing Plants 4052 3.0. Biomass Gasification 4052 3.1. Gasification Chemistry 4052 3.2. Gasification Reactors 4054 3.3. Supercritical Gasification 4054 3.4. Solar Gasification 4055 3.5. Gas Conditioning 4055 4.0. Syn-Gas Utilization 4056 4.1. Hydrogen Production by Water−Gas Shift Reaction 4056
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TL;DR: Here, the natural resistance of plant cell walls to microbial and enzymatic deconstruction is considered, collectively known as “biomass recalcitrance,” which is largely responsible for the high cost of lignocellulose conversion.
Abstract: Lignocellulosic biomass has long been recognized as a potential sustainable source of mixed sugars for fermentation to biofuels and other biomaterials. Several technologies have been developed during the past 80 years that allow this conversion process to occur, and the clear objective now is to make this process cost-competitive in today's markets. Here, we consider the natural resistance of plant cell walls to microbial and enzymatic deconstruction, collectively known as "biomass recalcitrance." It is this property of plants that is largely responsible for the high cost of lignocellulose conversion. To achieve sustainable energy production, it will be necessary to overcome the chemical and structural properties that have evolved in biomass to prevent its disassembly.
4,035 citations
TL;DR: It is suggested that it is timely to revisit and reinvigorate functional modeling of cellulose hydrolysis and that this would be highly beneficial if not necessary in order to bring to bear the large volume of information available on cellulase components on the primary applications that motivate interest in the subject.
Abstract: Information pertaining to enzymatic hydrolysis of cellulose by noncomplexed cellulase enzyme systems is reviewed with a particular emphasis on development of aggregated understanding incorporating substrate features in addition to concentration and multiple cellulase components. Topics considered include properties of cellulose, adsorption, cellulose hydrolysis, and quantitative models. A classification scheme is proposed for quantitative models for enzymatic hydrolysis of cellulose based on the number of solubilizing activities and substrate state variables included. We suggest that it is timely to revisit and reinvigorate functional modeling of cellulose hydrolysis, and that this would be highly beneficial if not necessary in order to bring to bear the large volume of information available on cellulase components on the primary applications that motivate interest in the subject.
1,852 citations
Cites background or methods from "Microbial cellulose utilization: fu..."
...Considering both the uncertainty of methodologies for measuring CrI as well as conflicting results on the change of CrI during hydrolysis, it is difficult to conclude at this time that CrI is a key determinant of the rate of enzymatic hydrolysis (Lynd et al., 2002; Mansfield et al., 1999)....
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...Cellulose hydrolysis rates mediated by fungal cellulases are typically 3–30 times faster for amorphous cellulose as compared to high crystalline cellulose (Lynd et al., 2002; Table III)....
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...…‘‘hydrothermal’’ process, ‘‘organosolv’’ processes involving organic acid solvents in an aqueous phase, ammonia fiber explosion (AFEX), strong alkali process (Lynd et al., 2002), as well as mechanical treatments such as hammer and ball milling (Millett et al., 1976; Sun and Cheng, 2002)....
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..., 1998); 6) cellulose-enzyme-microbe (CEM) synergism (Lynd et al., 2002); and 7) a proximity synergism due to formation of cellulase complexes (Fierobe et al....
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...(Chang et al., 1981; Klein and Snodgrass, 1993; Lynd et al., 2002; Mansfield et al., 1999)....
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TL;DR: The different technologies for producing fuel ethanol from sucrose-containing feedstocks (mainly sugar cane, starchy materials and lignocellulosic biomass) are described along with the major research trends for improving them.
1,792 citations
Cites background from "Microbial cellulose utilization: fu..."
...Endoglucanases especially act on amorphous cellulose, whereas cellobiohydrolases can act on crystalline cellulose as well (Lynd et al., 2002)....
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...Lynd et al. (2002) report low ethanol concentrations (in the order of 25 g/L) for T. thermo- saccharolyticum cultivated in xylose-based media during batch and continuous cultures....
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...By continuous ion-exchange, over 97% acid recovery is possible (Hamelinck et al., 2005)....
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...These methods are also chosen by Hamelinck et al. (2005) as the more perspective in short-, mid- and long-term evaluations....
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...…ethylene glycol, triethylene glycol) or their mixture with 1% of H2SO4 or HCl; 185–198 C, 30– 60 min, pH = 2.0–3.4 Solvent recovery required Poplar wood Lynd et al. (2002); Pan et al. (2005); Rezzoug and Capart (1996); Sun and Cheng (2002) Almost total hydrolysis of hemicellulose, high yield of…...
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TL;DR: In this paper, the state of the art of hydrolysis-fermentation technologies to produce ethanol from lignocellulosic biomass, as well as developing technologies, are evaluated.
Abstract: The state of the art of hydrolysis-fermentation technologies to produce ethanol from lignocellulosic biomass, as well as developing technologies, is evaluated. Promising conversion concepts for the short-, middle- and long-term are defined. Their technical performance was analysed, and results were used for economic evaluations. The current available technology, which is based on dilute acid hydrolysis, has about 35% efficiency (HHV) from biomass to ethanol. The overall efficiency, with electricity co-produced from the not fermentable lignin, is about 60%. Improvements in pre-treatment and advances in biotechnology, especially through process combinations can bring the ethanol efficiency to 48% and the overall process efficiency to 68%. We estimate current investment costs at 2.1 k€/kW HHV (at 400 MW HHV input, i.e. a nominal 2000 tonne dry/day input). A future technology in a 5 times larger plant (2 GW HHV ) could have investments of 900 k€/kW HHV . A combined effect of higher hydrolysis-fermentation efficiency, lower specific capital investments, increase of scale and cheaper biomass feedstock costs (from 3 to 2 €/GJ HHV ), could bring the ethanol production costs from 22 €/GJ HHV in the next 5 years, to 13 €/GJ over the 10–15 year time scale, and down to 8.7 €/GJ in 20 or more years.
1,683 citations
References
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TL;DR: With the steady increase in sequence and structural data, it is suggested that the enzyme classification system should perhaps be revised.
Abstract: The amino acid sequences of 301 glycosyl hydrolases and related enzymes have been compared. A total of 291 sequences corresponding to 39 EC entries could be classified into 35 families. Only ten sequences (less than 5% of the sample) could not be assigned to any family. With the sequences available for this analysis, 18 families were found to be monospecific (containing only one EC number) and 17 were found to be polyspecific (containing at least two EC numbers). Implications on the folding characteristics and mechanism of action of these enzymes and on the evolution of carbohydrate metabolism are discussed. With the steady increase in sequence and structural data, it is suggested that the enzyme classification system should perhaps be revised.
3,338 citations
3,200 citations
TL;DR: On the basis of a comparison of 482 sequences corresponding to 52 EC entries, 45 families, out of which 22 are polyspecific, can now be defined and has been implemented in the SWISS-PROT protein sequence data bank.
Abstract: 301 glycosyl hydrolases and related enzymes corresponding to 39 EC entries of the I.U.B. classification system have been classified into 35 families on the basis of amino-acid-sequence similarities [Henrissat (1991) Biochem. J. 280, 309-316]. Approximately half of the families were found to be monospecific (containing only one EC number), whereas the other half were found to be polyspecific (containing at least two EC numbers). A > 60% increase in sequence data for glycosyl hydrolases (181 additional enzymes or enzyme domains sequences have since become available) allowed us to update the classification not only by the addition of more members to already identified families, but also by the finding of ten new families. On the basis of a comparison of 482 sequences corresponding to 52 EC entries, 45 families, out of which 22 are polyspecific, can now be defined. This classification has been implemented in the SWISS-PROT protein sequence data bank.
2,046 citations