Visualization of the enzymatic degradation of crystalline cellulose at a molecular level

Abstract

Efficient degradation of cellulose enables us to produce fuels and chemicals from plant resources. However, biochemical conversion of cellulose by cellulase is quite slow, and the reaction becomes a bottleneck of the process. We carried out the real-time visualization of individual cellulase molecules using an advanced version of high-speed atomic force microscopy (HS-AFM), which has sub-second time resolution and nanometer space resolution. Trichoderma reesei cellobiohydrolase I (TrCel7A) molecules were observed to slide unidirectionally along the crystalline cellulose surface, but the movement of individual molecules was halted by other molecules, leading to traffic jams of enzyme molecules. This would decrease the apparent activity of the enzyme, as the hydrolytic action is coupled with molecular movement. Changing the crystalline polymorphic form of the cellulose increased the number of available lanes on the crystalline surface and consequently the number of moving cellulase molecules. Treatment of this crystalline cellulose with another type of Trichoderma cellobiohydrolase, TrCel6A, led to further increase in the number of entrance and exit points on the substrate surface, resulting in a significant increase in the proportion of mobile TrCel7A molecules on the surface.