Optimized mixtures of recombinant Humicola insolens cellulases for the biodegradation of crystalline cellulose

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Abstract
The digestion of bacterial cellulose ribbons by ternary mixtures of enzymes consisting of recombinant cellulases (two cellobiohydrolases, Cel6A and Cel7A, and the endoglucanase Cel45A) from Humicola insolens was investigated over a wide range of mixture composition. The extent of digestion was followed by soluble sugar release (saccharification) analysis together with transmission electron microscopy (TEM) observations. It was found that the addition of minute quantities of Cel45A induced a spectacular increase in saccharification of the substrate with either Cel7A or the mixture of Cel6A and Cel7A. Conversely, only a moderate saccharification resulted from the mixing of Cel45A and Cel6A. This difference is believed to originate from (1) the occasional endo character of Cel6A and (2) the competition of Cel6A and Cel45A for the substrate sites that are sensitive to endo activity. Interestingly, the mixture of enzymes giving rise to the highest saccharification rate did not always correspond to mixtures of enzymes generating the highest synergy. TEM images revealed that the bacterial cellulose ribbons became at the same time cut and narrowed down under the action of an optimized mixture of the three enzymes.