17446: Clostridium thermocellum cellulases – what are we missing?

The efficient hydrolysis of biomass to 5 carbon and 6 carbon sugars is limited by the lack of affordable, high specific activity enzymes.  Clostridium thermocellum (Cth) utilizes a high molecular weight complex of enzymes called the cellulosome to degrade biomass; the enzyme complex has been reported to be 50-fold more active on a weight basis than the corresponding Trichoderma reesei enzymes.   The Cth genome has been sequenced and is predicted to have genes for over 60 potential biomass-degrading enzymes associated with the cellulosome, and another 18 enzymes that are noncellulosomal. We have cloned, expressed, and characterized 14 Cth cellulases and evaluated their ability alone and together to degrade amorphous cellulose, crystalline celluloses and biomass.   The ability of the enzymes to rapidly degrade amorphous cellulose and minimal activity on crystalline cellulose and biomass suggests that cellulosomes may not be active directly on crystalline cellulose.  Rather, cellulosomes may act cooperatively to first decrystallize individual cellulose chains, followed by rapid hydrolysis of the amorphous chain.