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Cellulose degradation by Clostridium thermocellum

The thermophilic, anaerobic bacterium Clostridium thermocellum produces a cellulase system having a very high specific activity against crystalline cellulose. Thus, it provides a good model system in order to understand the essential features required for the degradation of natural cellulosic substrates. The work improves the feasibility of enzymic conversion of cellulosic materials into sugars by gaining a better understanding of the factors that effect the efficiency of conversion by the enzymes responsible. The released sugars can be used as a carbon source for fermentations generating chemicals, pharmaceuticals and fuels, as well as in modification of animal feed. Such plant cell wall degrading enzymes also find applications in fibre processing, vegetable oil extraction, flavour release, textiles, detergent and chemicals.

Three endoglucanases and one xylanase have been characterized in detail, including the determination of their 3-dimensional structure. The results can now be utilized to model other enzymes belonging to the same families, as determined by primary sequence analysis. The basic building principle of the cellulosome has been elucidated. Catalytic subunits of the complex contain a conserved, non catalytic region acting as a docking domain. This region, which has been termed dockerin domain, mediates attachment of the catalytic components to a large glycoprotein termed CipA, which acts as a scaffolding component and as a cellulose binding factor. The sequence of CipA contains a cellulose-binding domain and 9 highly conserved, reiterated segments. These segments correspond to a series of receptors, termed cohesin domains. By combining two types of cohesin domains, it should be possible to engineer new scaffolding proteins on which polypeptides can be targeted to bind at specific sites depending on their dockerin domain.

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Institut Pasteur
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