Revisiting the phenomenon of cellulase action : not all endo-and exo-cellulase interactions are synergistic

Abstract

The conventional endo–exo synergism model has extensively been supported in literature, which is based on the perception that endoglucanases (EGs) expose or create accessible sites on the cellulose chain to facilitate the action of processive cellobiohydrolases (CBHs). However, there is a lack of information on why some bacterial and fungal CBHs and EGs do not exhibit synergism. Therefore, the present study evaluated and compared the synergistic relationships between cellulases from different microbial sources and provided insights into how different GH families govern synergism. The results showed that CmixA2 (a mixture of TlCel7A and CtCel5A) displayed the highest effect with BaCel5A (degree of synergy for reducing sugars and glucose of 1.47 and 1.41, respectively) in a protein mass ratio of 75–25%. No synergism was detected between CmixB1/B2 (as well as CmixC1/C2) and any of the EGs, and the combinations did not improve the overall cellulose hydrolysis. These findings further support the hypothesis that “not all endo-to exo-cellulase interactions are synergistic”, and that the extent of synergism is dependent on the composition of cellulase systems from various sources and their compatibility in the cellulase cocktail. This method of screening for maximal compatibility between exo- and endo-cellulases constitutes a critical step towards the design of improved synergistic cellulose-degrading cocktails for industrial-scale biomass degradation.