Plant cell walls are composed of macromolecular carbohydrate polymers (cellulose, hemicelluloses and pectin) together with the aromatic polymer lignin, where cellulose is the most abundant biopolymer on Earth. Its structure has come under renewed interest in the context of bio-fuel production from enzymatic hydrolysis of lingo-cellulosic biomass. The major problem in this endeavor is the recalcitrant nature of crystalline cellulose.
Clostridium thermocellum has garnered extensive research interest, due to its cellulolytic and ethanologenic abilities. This bacterium is capable of directly converting cellulosic substrates into ethanol. The major extracellular component of the C. thermocellum cellulolytic system is the cellulosome. Cellulosomes are multi-enzyme complexes that liberate massive amounts of cellobiose from cellulose. The cellulase system of this bacterium significantly differs from fungal cellulases, since the entire complement of cellulosomal enzymes is targeted directly en bloc to the crystalline cellulose surface, thereby enhancing the synergistic action of the different enzymes in the complex.
One of the major steps in biological conversion of lignocellulose after pre-treatment is enzymatic cellulose hydrolysis, in which the cellulases hydrolyze cellulose to fermentable sugars. However, during this process the cellulosome of C. thermocellum generates inhibitors.
Until now, reducing sugars — notably cellobiose — were reported to be the inhibitory agents. Here we provide evidence that reactive intermediates consisting of oligo-saccharides or their derivatives (and not cellobiose), are among the major inhibitors of cellulose hydrolysis. These observed inhibitors are currently being studied and characterized further. The study was performed using micro crystalline cellulose as a substrate in the presence of the isolated cellulosome with and without selected beta-glucosidases. The different beta-glucosidases were applied to overcome the inhibition in cellulose hydrolysis. This study will have an impact on the formation of fermentable sugars from bio-mass and in turn on bio-ethanol yields.
