ACTION OF EXO-TYPE AND ENDO-TYPE CELLULASES FROM IRPEX-LACTEUS ON VALONIA CELLULOSE

Cellulolytic mode of action of the two highly purified exo- and endo-type cellulases from Irpex lacteus on pure Valonia cellulose was investigated. Electron microscopy substantiated that both cellulases are adsorbed preferentially into the internal parts of microfibrils in the network structure of the cellulose at initial stages before enzymatic hydrolysis, and that the adsorption ratio of both cellulases onto the external surfaces of microfibrils increased with incubation time although this tendency was less remarkable with the exo-type cellulase than with the endo-type one. The exo-type cellulase exhibited relatively high activity producing cellobiose throughout 12-h incubation, while the endo-type cellulase produced small amounts of cellooligosaccharides. The degree of polymerization was far more suppressed by the endo-type cellulase than by the exo-type one. Degradation by the cellulases in typical exo- and endo-fashions yielded quite different morphological patterns in the microfibrils. Exo-type cellulase loosened the network structure of microfibrils and made them slightly thinner, while endo-type cellulase caused conspicuous swelling and dissolution of individual microfibrils.