Processive Degradation of Crystalline Cellulose by a Multimodular Endoglucanase via a Wirewalking Mode

Processive hydrolysis of crystalline cellulose by cellulases is a critical step for lignocellulose deconstruction. The classic Trichoderma reesei exoglucanase TrCel7A, which has a closed active-site tunnel, starts each processive run by threading the tunnel with a cellulose chain. Loop regions are necessary for tunnel conformation, resulting in weak thermostability of fungal exoglucanases. However, endoglucanase CcCel9A, from the thermophilic bacterium Clostridium cellulosi, comprises a glycoside hydrolase (GH) family 9 module with an open cleft and five carbohydrate-binding modules (CBMs) and hydrolyzes crystalline cellulose processively. How CcCel9A and other similar GH9 enzymes bind to the smooth surface of crystalline cellulose to achieve processivity is still unknown. Our results demonstrate that the C-terminal CBM3b and three CBMX2s enhance productive adsorption to cellulose, while the CBM3c adjacent to the GH9 is tightly bound to 11 glucosyl units, thereby extending the catalytic cleft to 17 subsites, which facilitates decrystallization by forming a supramodular binding surface. In the open cleft, the strong interaction forces between substrate-binding subsites and glucosyl rings enable cleavage of the hydrogen bonds and extraction of a single cellulose chain. In addition, subsite -4 is capable of drawing the chain to its favored location. Cellotetraose is released from the open cleft as the initial product to achieve high processivity, which is further hydrolyzed to cellotriose, cellobiose and glucose by the catalytic cleft of the endoglucanase. On this basis, we propose a wirewalking mode for processive degradation of crystalline cellulose by an endoglucanase, which provides insights for rational design of industrial cellulases.