Limited proteolysis of the cellobiohydrolase I from Trichoderma reesei

Limited proteolysis of the cellobiohydrolase I (CBH I, 65 kDa) from Trichoderma reesei by papain yields a core protein (56 kDa) which is fully active against small, soluble substrates such as the chromophoric glycosides derived from the cellodextrins and lactose. Activity against an insoluble substrate, such as Avicel, is however completely lost and concomitantly decreased adsorption onto this microcrystalline cellulose is observed. The peptide (10 kDa), initially split off during proteolysis, is identified as the heavily glycosylated carboxy‐terminal of the native CBH I. Depending on the experimental conditions the core protein is further nicked in between disulfide bonds, but its properties and stability do not appreciably differ from those of intact CBH I. These results lead to the proposal of a bifunctional organisation of the CBH I: one domain, corresponding to the carboxyterminal, acts as a binding site for insoluble cellulose and the other, localised in the core protein, contains the active (hydrolytic) site.