Insight into the Underlying Synergy between Exo–Lytic Cellulases

Abstract Lignocellulosic biomass can be converted into biofuels and biochemicals, through a synergetic degradation by cellulases. The underlying cooperative mechanism is of paramount importance for the development of efficient enzyme cocktails. The synergy between exo–lytic cellulases, however, still remains poorly understood. In the present contribution, we decipher the synergism by investigation of the enzyme–enzyme interactions (EEIs) between two exo–lytic cellulases––Talaromyces emersonii Cel7A (TeCel7A) and Trichoderma reesei Cel6A (TrCel6A), and between TeCel7A and other main components in the cocktails, and propose a novel synergistic way. The enzymes are found to be apt to bind around the eight substrate–enclosing loops (SELs) of TeCel7A, of which the TrCel6A possesses the strongest binding energy with TeCel7A. The combination between TeCel7A and TrCel6A is further investigated experimentally by Microscale Thermophoresis, confirming the existence of their interactions. Due to the EEIs, the flexibility of the SELs, which mediate dissociation of TeCel7A from cellulose, is increased. We further found that the improved flexibility of loop B3 is pivotal to accelerate dethreading process, helping improving enzymatic hydrolytic efficiency. In view of our theoretical and experimental results here and previous experimental phenomena, the carbohydrate binding modules of the exo–lytic enzymes, which enable them to absorb on the same plane of the substrate, are conjectured to enhance the degree of the EEIs. This work brings to light an underlying synergy between exo–lytic cellulases, and is conducive to a systematic understanding of the synergetic actions in cellulase cocktails.