Trichoderma reesei has no true exo-cellulase: all intact and truncated cellulases produce new reducing end groups on...

Adsorption to and formation of insoluble reducing end groups on cellulose was studied for intact enzymes and catalytic domains, 'cores', of the four major cellulases from Trichoderma reesei, CBH I, CBH II, EG I and EG III. Individual enzymes were incubated with NaBH4-reduced, phosphoric acid swollen Avicel (regenerated cellulose) or with filter paper. Adsorption onto regenerated cellulose was rapid (equilibration reached within 2 min), but was slow onto filter paper (not completed after 24 h). On both substrates, less was bound of the core domains than of the intact enzymes. After reaching a maximum in adsorption, all the core domains except CBH I core were released again. In general, the desorption of the core enzymes was much faster than the rate of substrate conversion. All enzymes produced new reducing end groups on both substrates, and thus none of them is a true exo-cellulase. However, both the rate of formation and the amount was considerably higher for the EG enzymes than for the CBH's, which may justify the classification of cellulases into two groups, although the difference is quantitative rather than qualitative. EG III was the most endo-active of the enzymes, and CBH I the least.