The effect of fibril length and architecture on the accessibility of reducing ends of cellulose Iα to Trichoderma reesei Cel7A

Abstract

The origin of the recalcitrance of cellulose fibrils to enzymatic hydrolysis is still poorly understood. In this study we examined the role of cellulose fibril lengths and fibril architecture, i.e. the fibrillar structure from lateral association of cellulose microfibrils, on the accessibility to a reducing-end specific cellobiohydrolase, Trichoderma reesei Cel7A (TrCel7A). Cellulose Iα fibrils from Gluconacetobacter xylinus and Cladophora aegagropila showed contrasting digestibility by TrCel7A. Where the bacterial cellulose (BC) fibrils from G. xylinus were rapidly hydrolyzed to near completion by TrCel7A (>99 %) in 120 h, under identical reaction conditions, TrCel7A hydrolysis of the algal cellulose (AC) fibrils from C. aegagropila was slow and limited (~30 %). Mechanically decreasing fibril lengths and increasing average reducing end concentrations by high intensity ultrasonication did not affect the hydrolysis rates of either BC or AC by TrCel7A. Moreover, ultrasonicated AC remained significantly less digestible by TrCel7A than BC despite higher available reducing-end concentrations. In contrast to previous observations of extensive fibrillation of BC by TrCel7A hydrolysis, AC fibrils subjected to hydrolysis by TrCel7A remained associated. The hydrolysis of AC fibrils by TrCel7A roughened the topography of the fibril surfaces in a manner suggesting erosion of microfibrils at the fibril surface. We speculate that the compact cross-sections of the AC microfibrils result in tightly associated fibrils that hinder enzyme access to available reducing ends while the flat, ribbon cross section of the BC microfibrils result in more loosely associated fibrils that more easily dissociate during hydrolysis to improve accessibility and overall digestibility by TrCel7A.