Does the Cellulose-Binding Module Move on the Cellulose Surface?

Exoglucanases are key enzymes required for the efficient hydrolysis of crystalline cellulose. It has been proposed that exoglucanases hydrolyze cellulose chains in a processive manner to produce primarily cellobiose. Usually, two functional modules are involved in the processive mechanism: a catalytic module and a carbohydrate-binding module (CBM). In this report, single molecule tracking techniques were used to analyze the molecular motion of CBMs labeled with quantum dots (QDs) and bound to cellulose crystals. By tracking the single QD, we observed that the family 2 CBM from Acidothermus cellulolyticus (Ac CBM2) exhibited linear motion along the long axis of the cellulose fiber. This apparent movement was observed consistently when different concentrations (25 μM to 25 nM) of Ac CBM2 were used. Although the mechanism of Ac CBM2 motion remains unknown, single-molecule spectroscopy has been demonstrated to be a promising tool for acquiring new fundamental understanding of cellulase action.