Substrate Specificities of Exo- and Endo-Type Cellulases in the Hydrolysis of β-(1→3)- and β-(1→4)-Mixed D-Glucans

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Abstract
An exo-type cellulase (Ex-1) was extracted from Irpex lacteus (Polyporus tulipiferae) and purified essentially to homogeneity. This cellulase attacked cellulosic substrates in an exo-wise fashion to produce almost exclusively cellobiose. In contrast, Ex-1 was found to attack β-glucans having β-(1→3)- and β-(1→4)-mixed linkages in a way similar to an endo-type cellulase. The products formed from barley glucan by Ex-1 were 32-O-β-D-cellobiosyl-cellobiose » 32-O-β-D-glucosyl-cellobiose > cellobiose>cellotriose » glucose in the early stage, but no laminaribiose was produced. An endo-type cellulase (En-1) obtained from the same fungus also hydrolyzed β-glucans but in a typical endo-wise fashion and the products from barley glucan were 32-O-β-D-glucosyl-cellobiose»32-O-β-D-cellobiosyl-cellobiose cellobiose > laminaribiose; no glucose or cellotriose was produced. Thus, it seems likely that En-1 can attack any intramolecular linkage of, β-glucan, while Ex-1 requires the presence of at least cellobiosyl residues adjacent to a β-(1→3)-D-linked glucosyl residue. This finding, together with the mode of hydrolysis of cellulosic substrates by Ex-1, suggests that the stereochemical structure of successive β(1→4)-cellobiosyl residues inserted by β-(1→3)-D-glucosidic linkage is permissible in the action of Ex-1, although this enzyme prefers the β-(1→4)-linked cellobiosyl sequence.