A NEW MODEL FOR ENZYMATIC-HYDROLYSIS OF CELLULOSE BASED ON THE 2-DOMAIN STRUCTURE OF CELLOBIOHYDROLASE-I

We have studied the kinetics of adsorption to microcrystalline cellulose (Avicel) and its hydrolysis by intact and isolated domains of Trichoderma reesei cellobiohydrolase I (CBH I). The catalytic domain (the core) shows lower adsorption and activity than the complete enzyme, the effect being more pronounced the lower the enzyme concentration and the higher the extent of degradation. Apparent dissociation constants were derived from adsorption isotherms through non-linear regression analysis using a two-site model. Total binding capacities were estimated to be 1.1, 0.17 and 6.0-mu-mol protein/g Avicel (69, 9.3 and 53 mg/g) for intact enzyme, core and binding domain, respectively. The structural characteristics of cellulose that influence enzyme adsorption are discussed and a model is developed for the action of two-domain cellulases. This model may be applicable to other enzymes such as cutinases and amylases, which also have separate catalytic and binding domains and act on insoluble polymeric substrates.