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The Binding Process and Free Energy Characteristics of Cellulose Chain into the Catalytic Domain of Cellobiohydrolase TrCel7A.

Experiments observed that catalytic domain (CD) of Trichoderma reesei Cel7A (TrCel7A) hydrolyzes crystalline cellulose in a processive manner, but the underling binding mechanism is still unknown. Here, through replica exchange molecular dynamics simulations, we find that the loading and sucking-in process of cellulose chain into CD is entropy-driven and enthalpy unfavorable, which firmly relates to the de-solvation of the binding channel of CD. During the loading process, hydrophobic interactions play a dominant role because several aromatic residues have been identified to guide the cellulose chain processing. At the active site, a transition from enthalpy- to entropy-driven is detected for the driving force. Such finding reveals the indispensability of catalytic reaction of the glycosidic bond to provide the energy to drive the movements of cellulose chain. Our study reveals the interaction pictures between cellulose chain and TrCel7A at atomic level, which helps to better understand the catalytic mechanism of TrCel7A.