Mode of Action of Exo- and Endo-Type Cellulases from Irpex lacteus in the Hydrolysis of Cellulose with Different Crystallinities.

The mode of action of two highly purified cellulases of exo- and endo-types from Irpex lacteus was investigated by using pure cellulosic substrates with different crystallinities derived from cellulose I. Exo-type cellulase saccharified all celluloses more effectively than endo-type enzyme, and the saccharification activities of both cellulases similarly increased with decreasing crystallinity of cellulose. The ^-DP-towering activity of exo-type cellulase remained similar for celluloses with higher crystallinity, while this cellulase showed a degradation mode resembling that of the endo-type enzyme for the substrates with lower crystallinity. Compared with exo-type cellulase, endo-type cellulase remarkably decreased the ^-DP of cellulose with higher crystallinity, while this activity was abated for cellulose with lower crystallinity. Thus, the effects of both cellulases became similar in the degradation of amorphous substrates such as H₃PO₄-treated cellulose. Endo-type cellulase produced several kinds of cellooligosaccharide from all kinds of cellulose used, while the product of the exo-type enzyme was only cellobiose from crystalline cellulose such as cotton and cotton linter even after a 12-h incubation period. The results indicate that each cellulase shows a typical mode of action (exo or endo) for crystalline cellulose, but that their characteristic modes of attack may change with decreasing crystallinity of cellulose.