Key Points
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Cellulose, the main structural component of plant cell walls, is the most abundant carbohydrate polymer in nature and is therefore a key component of the carbon cycle. Anaerobic microorganisms have evolved a system to break down plant cell wall materials, including cellulose, that involves the formation of a large extracellular enzyme complex called the cellulosome.
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Although anaerobic fungi as well as anaerobic bacteria are believed to produce cellulosomes, so far full genetic evidence for their presence has only been obtained in anaerobic bacteria. To date, cellulosomes have been identified in 12 different bacterial species, and this list is expected to continue growing.
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Cellulosomes comprise a fibrillar protein known as the scaffolding protein or scaffoldin with cellulosomal enzyme subunits positioned periodically along the fibrils. Typically, the scaffoldin contains enzyme-binding sites known as cohesins, and a cellulose-binding domain (CBD) or carbohydrate-binding module (CBM). The enzyme subunits bind to the cohesins via cohesin-binding sites known as dockerins. The properties of the scaffoldins vary between species. Additionally, cellulosomes can contain either one or several different scaffoldins that can bind different combination of enzymes. This variation and the presence of many cellulosomal enzymes means that any single microorganism can secrete a variety of cellulosomes with many different compositions. The cellulosomal enzymes include cellulases, hemicellulases, pectinase, chitinase and many ancillary enzymes that can degrade plant cell wall materials.
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In biotechnology, there is great interest in exploiting the properties of cellulosomes. 'Mini-cellulosomes' can be created, which, because they contain specific cohesins, will only bind to specific enzymes. These constructs have been used to great effect in cellulosome research in the study of the synergistic effects of cellulosomal enzymes and the cohesin–dockerin interaction, for example, but it is also hoped that mini-cellulosome constructs could be used in the future to develop artificial metabolic pathways that allow the synthesis of any desired product. There is also great interest in the heterologous expression of cellulosome genes, such that non-cellulose degrading organisms can be converted to cellulose degraders.
Abstract
Cellulose, the main structural component of plant cell walls, is the most abundant carbohydrate polymer in nature. Although abundant, it is extremely difficult to degrade, as it is insoluble and is present as hydrogen-bonded crystalline fibres. Anaerobic microorganisms have evolved a system to break down plant cell walls that involves the formation of a large extracellular enzyme complex called the cellulosome, which consists of a scaffolding protein and many bound cellulases. Cellulosomes have many potential biotechnological applications as the conversion of cellulosic biomass into sugars by cellulosomes could result in the production of high-value products such as ethanol or organic acids from inexpensive renewable resources. Rapid advances in cellulosome research are providing basic information for the development of both in vitro and in vivo systems to achieve such goals.
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The research reported from our laboratory was supported in part by a grant from the US Department of Energy.
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Glossary
- CELLULOSE
-
The most abundant plant polysaccharide consisting of (1→4)β-D-glucan chains hydrogen-bonded to one another along their length.
- HEMICELLULOSE
-
Cross-linking glycans that comprise up to about 30% of plant cell walls; the two major hemicelluloses are xyloglucans and glucuronoarabinoxylans.
- CELLULOSOME
-
An extracellular enzyme complex consisting of a scaffoldin and cellulosomal enzymes that are capable of degrading plant cell walls. Cellulosomes are produced by anaerobic microorganisms.
- CELLULASE
-
Glycosyl hydrolases that degrade cellulose.
- SCAFFOLDIN
-
A scaffolding protein found in cellulosomes containing cohesin domains that bind cellulosomal enzymes.
- COHESIN
-
Domains in the scaffoldin to which cellulosomal enzymes are bound. There are at least three types of cohesins, which vary in amino acid sequence.
- DOCKERIN
-
Duplicated sequences present in cellulosomal enzymes that bind to cohesins. There are at least three types of dockerins, which vary in amino acid sequence.
- AVICEL
-
A commercially available microcrystalline cellulose.
- CELLOBIOSE
-
An individual unit of cellulose.
- SIGMA-A
-
Sigma factors are variable protein components of the bacterial RNA polymerase that influence transcription by determining where the polymerase binds to DNA. In Bacillus, σA is a housekeeping sigma factor, σB an alternative sigma factor that responds to stress and σL the Bacillus subtilis homologue of σ54, the major variant sigma factor in E. coli.
- CATABOLITE REPRESSION
-
Transcriptional repression of a prokaryotic operon by the metabolic products of the enzymes that are encoded by the operon.
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Doi, R., Kosugi, A. Cellulosomes: plant-cell-wall-degrading enzyme complexes. Nat Rev Microbiol 2, 541–551 (2004). https://doi.org/10.1038/nrmicro925
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DOI: https://doi.org/10.1038/nrmicro925
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