Slowdown of Enzymatic Cellulose Conversion Emerges from Cellulase Mode of Action

The problem of early slowing of the conversion rate in cellulose hydrolysis, and the resulting large amounts of cellulase enzymes required to achieve just useful conversion efficiencies, remains a major obstacle in the development of cellulosic biofuels. While numerous studies have implicated both substrate and enzyme factors, the underlying mechanism of this rate limitation has remained unresolved. Here, using bacterial cellulose as a well-defined model substrate, we demonstrate that the reaction slowdown emerges from the specific mode of substrate degradation imposed by the cellulolytic enzyme system. We introduce nanomechanical mapping by time-lapse in situ atomic force microscopy to characterize at nanometer spatial resolution the change in surface material organization of cellulose due to enzymatic degradation. The layer-by-layer ablation of surface material utilized by fungal cellulases results in the gradual exposure of the nanomechanically stiffer (i.e., more densely organized and hence more resistant) inner core of the cellulose fibrils, which leads to a rapid decline in the conversion rate by these enzymes. Cellulases assembled into stable complexes (the cellulosome) bind almost irreversibly to cellulose. Low dynamics of their adsorption causes stalling of the cellulose degradation as the portion of unproductively bound enzymes increases during the conversion. Together, these findings reveal distinct, system-specific slowdown mechanisms and uncover a functional interplay between substrate nanomechanics and enzyme adsorption dynamics that dictates overall conversion efficiency. By disentangling these coupled rate-limiting factors, this work establishes previously unrecognized molecular design principles for engineering cellulase systems capable of overcoming the intrinsic substrate recalcitrance of crystalline cellulose.