Cellulose Binding in the Processive Cellulase Cel7A
Cellobiohydrolases effectively degrade cellulose and are of biotechnological interest because they can convert lignocellulosic biomass to fermentable sugars. Using a fluorescence-based method for real-time measurements of complexation and decomplexation, the authors followed the processive cellulase Cel7A and its insoluble substrate, cellulose. Wild type Cel7A and several variants in which one or two of four highly conserved Trp residues in the binding tunnel had been replaced with Ala, were investigated.
The results of this analysis suggested a transition state for complexation and decomplexation in which the reducing end of the ligand is close to the tunnel entrance, whereas the rest of the binding tunnel is empty. This structure defines the highest free-energy barrier of the overall catalytic cycle and hence governs the turnover rate of this industrially important enzyme.