Clostridium thermocellum ATCC 27405
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Photo: Bayer, E. A. and R. L. Lamed
Clostridium thermocellum is an anaerobic, thermophilic, cellulolytic, and ethanogenic bacterium capable of directly converting cellulosic substrate into ethanol. The bacterium can potentially be utilized in a biomass energy process. In this bacterium, degradation of the cellulosic materials is carried out by a large extracellular cellulase system called the cellulosome. The cellulosome is an extremely complicated protein complex consisting of nearly 20 different catalytic subunits ranging in size from about 40 to 180 kDa with a total molecular weight of millions. The key feature of the cellulosome is the nonhydrolytic "scaffoldin" subunit that integrates the various catalytic subunits into the complex through interactions between its repetitive "cohesin" domains and complementary "dockerin" domains on the catalytic subunits. It is now known that many different cellulolytic bacteria and fungi produce extracellular multienzyme complexes similar to the cellulosome. The C. thermocellum cellulosome is the best-characterized cellulase complex and thus serves as a paradigm. Continuing research on the cellulosome and its producer will provide crucial information for better understanding the cellulolytic reaction, a key process in biomass conversion.