Dynamic visualization of an enzyme hydrolyzing cellulose

Nature has evolved a class of enzymes called cellulases that can digest cellulose in biomass releasing energy rich sugars for biofuel production. Understanding the mechanistic details of cellulase–cellulose interactions is crucial for developing efficient systems for converting biomass to useful energy. A significant challenge has been the inability to obtain molecular-level structural information of cellulases while the enzymes are digesting cellulose.  In this work, we studied Trichoderma reesei Cel7A, a processive exocellulase enzyme, free in solution and bound to cellulose. We exploited a unique property of neutrons to obtain the first experimental structural information of a cellulase when bound to cellulose. Using small-angle neutron scattering (SANS) and contrast variation, it was possible to obtain scattering contrast between cellulose and Cel7A by substituting hydrogen atoms of cellulose by deuterium. The results provide insights into the pH dependent structural properties of cellulases free in solution and when bound to a cellulose substrate. Combined SANS and MD simulations of Cel7A in solution reveals a conformational selection mechanism that primes the enzyme for binding to cellulose. In the case of the bound enzyme, the SANS data is consistent with populations of extended and compressed Cel7A structures, however a higher proportion of compact conformations are present compared to the free enzyme. Our data provide the first experimental insights into the mode-of-action of cellulases on cellulose surfaces. The approach described is broadly applicable to studies of other cellulytic enzymes and could lead to new approaches to improve their function impacting the profitability of biomass-based fuels.