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.
Highlights
In nature, microorganisms have evolved diverse degradative strategies involving an arsenal of enzymatic machinery to assist them in cellulose de-polymerization [1,2]
The results showed that CmixA2 displayed the highest synergistic effect with BaCel5A (DS for reducing sugars: 1.47 and degree of synergy (DS) for glucose: 1.41) in a protein ratio of 75–25%
It was evident that the enzymes are substrate specific, it is apparent that some endo-glucanases may have dual functions due to their processive nature, they display both endo- and exo-glucanase activity
Summary
Microorganisms have evolved diverse degradative strategies involving an arsenal of enzymatic machinery to assist them in cellulose de-polymerization [1,2]. Cellulases belong to a large family of glycosyl hydrolases (GH), which display the ability to cleave β-1,4 glycosidic bonds in cellulose chains via an acid hydrolysis mechanism [1,3]. Based on their amino acid sequence and 3D structural similarities, the catalytic machinery of known cellulases has been classified into different glycoside hydrolase (GH) families; 1, 3, 5, 6, 7, 8, 9, 12, 26, 44, 45, 48, 51 and 74 as documented in the Carbohydrate-Active enZYmes database (CAZy; http://www.cazy.org). Cellulases are traditionally viewed as having either exo- or endo-acting activities, recent evidence supports the idea that many enzymes have evolved activities against a broad range of substrates, due to conformational changes in their active sites [5,6]
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