11-06: Studies of the impact of cellulose microfibrillar properties on productive engagement by cellulases

In the bioconversion of lignocellulosic biomass to fuels and chemicals, the crux lies in the challenges in dissociation of microfibrils and depolymerization of cellulose. The reaction mechanisms of cellulose hydrolysis by cellulases are yet unsolved, leaving the industry to rely on costly trial-and-error scale-up strategies. The elusiveness of the reaction mechanisms is due to complexities in the physical properties of cellulose and the complexities in the surface interactions between cellulase and cellulose. Cellobiohydrolases, the workhorse in synergistic mixtures of cellulases, are informative probes in the study of cellulase-cellulose interactions, such as to elucidate substrate properties that impact hydrolysis rates. Recent evidences suggest that the majority of the population of cellobiohydrolases bound to cellulose surfaces is inactive (non-productively bound) after a brief initial transient phase. Combining biochemical analyses with force microscopy to track changes in cellulose fibrils with hydrolysis rates, we have observed that bacterial cellulose fibrils undergo large changes in the supramolecular structure due to Trichoderma reesei Cel7A (TrCel7A) cellobiohydrolase activity.  Larger assemblies of hydrogen-bond associated elementary fibrils are fibrillated to increase cellulose surface area, however, a concurrent decline in cellulase binding and hydrolysis rates suggest that reactive sites (reducing-ends) for TrCel7A did not increase. Here, we present results from our work seeking to understand physical and surface properties of cellulose microfibrils that impact productive binding of cellulases.