Cellulosome-Enhanced Conversion of Biomass: On the Road to Bioethanol

Abstract

The cellulosome was first isolated on the basis of the cellulose-binding function of the anaerobic thermophilic bacterium Clostridium thermocellum. Cellulose-binding domains promote hydrolysis of different sites on crystalline cellulose. The cellulose utilization systems in cellulosome-producing bacteria include over 100 different genes that must be orchestrated and timely expressed. The concept of directly converting biomass to ethanol by a mixed clostridial fermentation was fashionable some 30 years ago when it was found that the product pattern of C. thermocellum in favor of ethanol could become almost quantitative in stable coculture with another ethanol-producing anaerobe. The action of the exocellular protuberance-bound cellulosome may serve to delay or limit diffusional loss of the hydrolyzed sugar to the environment and/or competing bacteria. Consolidated bioprocessing (CBP) was recently extended for direct production of ethanol in yeast by cloning an endoglucanase and a s-glucosidase in Saccharomyces cerevisiae. At present, C. thermocellum, as a very potent cellulolytic, anaerobic thermophile, still seems to be the microorganism of choice for future bioethanol production from biomass. Theoretically, C. thermocellum can be engineered metabolically to produce better yields of ethanol or other products. Eventually, yeast cell surfaces may be modified to contain designer cellulosomes for direct ethanol conversion. The combination of CBP with the designer cellulosome concept may ultimately provide optimized degradation of specific cellulosic feedstocks for bioethanol production.