Metabolic engineering applications to renewable resource utilization.
TL;DR: The conversion of both cellulose and hemicellulose for the production of fuel ethanol is being studied intensively, with a view to developing a technically and economically viable bioprocess.
About: This article is published in Current Opinion in Biotechnology.The article was published on 2000-04-01. It has received 492 citations till now. The article focuses on the topics: Hemicellulose & Bioconversion.
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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
7,067 citations
TL;DR: The prospects included are fermentation technology converting xylose to ethanol, cellulase enzyme utilized in the hydrolysis of lignocellulosic materials, immobilization of the microorganism in large systems, simultaneous saccharification and fermentation, and sugar conversion into ethanol.
Abstract: In recent years, growing attention has been devoted to the conversion of biomass into fuel ethanol, considered the cleanest liquid fuel alternative to fossil fuels. Significant advances have been made towards the technology of ethanol fermentation. This review provides practical examples and gives a broad overview of the current status of ethanol fermentation including biomass resources, microorganisms, and technology. Also, the promising prospects of ethanol fermentation are especially introduced. The prospects included are fermentation technology converting xylose to ethanol, cellulase enzyme utilized in the hydrolysis of lignocellulosic materials, immobilization of the microorganism in large systems, simultaneous saccharification and fermentation, and sugar conversion into ethanol.
1,610 citations
Cites background from "Metabolic engineering applications ..."
...With the inevitable depletion of the world’s energy supply, there has been an increasing worldwide interest in alternative sources of energy ( Aristidou and Penttila 2000; Jeffries and Jin 2000; John 2004; Kerr 1998; Wheals et al. 1999; Zaldivar et al. 2001)....
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...With the inevitable depletion of the world’s energy supply, there has been an increasing worldwide interest in alternative sources of energy (Aristidou and Penttila 2000; Jeffries and Jin 2000; John 2004; Kerr 1998; Wheals et al. 1999; Zaldivar et al. 2001)....
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TL;DR: There has been much industrial interest in xylan and its hydrolytic enzymatic complex, as a supplement in animal feed, for the manufacture of bread, food and drinks, textiles, bleaching of cellulose pulp, ethanol and xylitol production.
Abstract: Xylan is the principal type of hemicellulose. It is a linear polymer of beta-D-xylopyranosyl units linked by (1-4) glycosidic bonds. In nature, the polysaccharide backbone may be added to 4-O-methyl-alpha-D-glucuronopyranosyl units, acetyl groups, alpha-L-arabinofuranosyl, etc., in variable proportions. An enzymatic complex is responsible for the hydrolysis of xylan, but the main enzymes involved are endo-1,4-beta-xylanase and beta-xylosidase. These enzymes are produced by fungi, bacteria, yeast, marine algae, protozoans, snails, crustaceans, insect, seeds, etc., but the principal commercial source is filamentous fungi. Recently, there has been much industrial interest in xylan and its hydrolytic enzymatic complex, as a supplement in animal feed, for the manufacture of bread, food and drinks, textiles, bleaching of cellulose pulp, ethanol and xylitol production. This review describes some properties of xylan and its metabolism, as well as the biochemical properties of xylanases and their commercial applications.
1,214 citations
Cites background from "Metabolic engineering applications ..."
...In recent years, the biotechnological use of xylans and xylanases has grown remarkably (Bhat 2000; Aristidou and Pentillä 2000; Subramaniyan and Prema 2000, 2002; Beg et al. 2000, 2001; Techapun et al. 2003)....
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TL;DR: The key role that process design plays during the development of cost-effective technologies is recognized through the analysis of major trends in process synthesis, modeling, simulation and optimization related to ethanol production.
1,015 citations
Cites background from "Metabolic engineering applications ..."
...However, the reader can consult the illustrative works of Aristodou and Penttilä (2000), Lynd et al....
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01 Aug 2015
TL;DR: Mechanisms and recent advances in pretreatment, cellulases production and second-generation ethanol production processes are described here.
Abstract: Production of liquid biofuels, such as bioethanol, has been advocated as a sustainable option to tackle the problems associated with rising crude oil prices, global warming and diminishing petroleum reserves. Second-generation bioethanol is produced from lignocellulosic feedstock by its saccharification, followed by microbial fermentation and product recovery. Agricultural residues generated as wastes during or after processing of agricultural crops are one of such renewable and lignocellulose-rich biomass resources available in huge amounts for bioethanol production. These agricultural residues are converted to bioethanol in several steps which are described here. This review enlightens various steps involved in production of the second-generation bioethanol. Mechanisms and recent advances in pretreatment, cellulases production and second-generation ethanol production processes are described here.
813 citations
Cites background from "Metabolic engineering applications ..."
...Aristidou and Penttila (2000) reported that the total cost of cellulosic ethanol will be dropped from more than Table 8 Bioethanol production from major agroresidues Biomass Fermenting microorganism Ethanol yield or titre Wheat straw Pichia stipitis NRRL Y-7124 (strain adapted to acid hydrolysate…...
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References
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TL;DR: The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity and an increasing number of three-dimensional structures are becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes.
Abstract: Cellulolytic microorganisms play an important role in the biosphere by recycling cellulose, the most abundant carbohydrate produced by plants. Cellulose is a simple polymer, but it forms insoluble, crystalline microfibrils, which are highly resistant to enzymatic hydrolysis. All organisms known to degrade cellulose efficiently produce a battery of enzymes with different specificities, which act together in synergism. The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity. In spite of a considerable diversity, sequence comparisons show that the catalytic cores of cellulases belong to a restricted number of families. Within each family, available data suggest that the various enzymes share a common folding pattern, the same catalytic residues, and the same reaction mechanism, i.e. either single substitution with inversion of configuration or double substitution resulting in retention of the β-configuration at the anomeric carbon. An increasing number of three-dimensional structures is becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes. In addition to catalytic domains, many cellulolytic enzymes contain domains not involved in catalysis, but participating in substrate binding, multi-enzyme complex formation, or possibly attachment to the cell surface. Presumably, these domains assist in the degradation of crystalline cellulose by preventing the enzymes from being washed off from the surface of the substrate, by focusing hydrolysis on restricted areas in which the substrate is synergistically destabilized by multiple cutting events, and by facilitating recovery of the soluble degradation products by the cellulolytic organism. In most cellulolytic organisms, cellulase synthesis is repressed in the presence of easily metabolized, soluble carbon sources and induced in the presence of cellulose. Induction of cellulases appears to be effected by soluble products generated from cellulose by cellulolytic enzymes synthesized constitutively at a low level. These products are presumably converted into true inducers by transglycosylation reactions. Several applications of cellulases or hemicellulases are being developed for textile, food, and paper pulp processing. These applications are based on the modification of cellulose and hemicellulose by partial hydrolysis. Total hydrolysis of cellulose into glucose, which could be fermented into ethanol, isopropanol or butanol, is not yet economically feasible. However, the need to reduce emissions of greenhouse gases provides an added incentive for the development of processes generating fuels from cellulose, a major renewable carbon source.
1,327 citations
TL;DR: Ethanol produced from cellulosic biomass is examined as a large-scale transportation fuel and a cost-competitive process appears possible in a decade, with conversion economics the key obstacle to be overcome.
Abstract: Ethanol produced from cellulosic biomass is examined as a large-scale transportation fuel. Desirable features include ethanol's fuel properties as well as benefits with respect to urban air quality, global climate change, balance of trade, and energy security. Energy balance, feedstock supply, and environmental impact considerations are not seen as significant barriers to the widespread use of fuel ethanol derived from cellulosic biomass. Conversion economics is the key obstacle to be overcome. In light of past progress and future prospects for research-driven improvements, a cost-competitive process appears possible in a decade.
896 citations
TL;DR: This strain efficiently fermented both glucose and xylose, which is essential for economical conversion of lignocellulosic biomass to ethanol, and achieved through a combination of the pentose phosphate and Entner-Doudoroff pathways.
Abstract: The ethanol-producing bacterium Zymomonas mobilis was metabolically engineered to broaden its range of fermentable substrates to include the pentose sugar xylose. Two operons encoding xylose assimilation and pentose phosphate pathway enzymes were constructed and transformed into Z. mobilis in order to generate a strain that grew on xylose and efficiently fermented it to ethanol. Thus, anaerobic fermentation of a pentose sugar to ethanol was achieved through a combination of the pentose phosphate and Entner-Doudoroff pathways. Furthermore, this strain efficiently fermented both glucose and xylose, which is essential for economical conversion of lignocellulosic biomass to ethanol.
835 citations
TL;DR: Limitations of xylose utilization in S. cerevisiae cells are very likely caused by an insufficient capacity of the non-oxidative pentose phosphate pathway, as indicated by accumulation of sedoheptulose-7-phosphate and the absence of fructose-1,6-bisph phosphate and pyruvate accumulation.
Abstract: We have performed a comparative study of xylose utilization in Saccharomyces cerevisiae transformants expressing two key enzymes in xylose metabolism, xylose reductase (XR) and xylitol dehydrogenase (XDH), and in a prototypic xylose-utilizing yeast, Pichia stipitis. In the absence of respiration (see text), baker's yeast cells convert half of the xylose to xylitol and ethanol, whereas P. stipilis cells display rather a homofermentative conversion of xylose to ethanol. Xylitol production by baker's yeast is interpreted as a result of the dual cofactor dependence of the XR and the generation of NADPH by the pentose phosphate pathway. Further limitations of xylose utilization in S. cerevisiae cells are very likely caused by an insufficient capacity of the non-oxidative pentose phosphate pathway, as indicated by accumulation of sedoheptulose-7-phosphate and the absence of fructose-1,6-bisphosphate and pyruvate accumulation. By contrast, uptake at high substrate concentrations probably does not limit xylose conversion in S. cerevisiae XYL1/XYL2 transformants.
663 citations
TL;DR: The bioconversion process of lignocellulosics to ethanol could be successfully developed and optimized by aggressively applying the related novel science and technologies to solve the known key problems of conversion process.
646 citations