Cellulose hydrolysis by the cellulases from Trichoderma reesei: Adsorptions of two cellobiohydrolases, two endocellulases and their core proteins on filter paper and their relation to hydrolysis

Abstract

Separate binding of several purified cellulolytic components of Trichoderma reesei on to filter paper was studied and concomitant hydrolysis rates evaluated. Enhancement of mass transfer from the bulk liquid to the solid substrate by agitation has two different effects on adsorption depending on the type of enzyme: (i) the fraction of cellobiohydrolase II (CBH II) and endoglucanase III (EG III) bound at equilibrium is increased, whereas (ii) the rate but not the extent of cellobiohydrolase I (CBH I) and endoglucanase I (EG I) adsorption is affected. The adsorption of CBH I core, a component lacking the cellulose-binding domain (CBD), is, however, not significantly influenced by mass transfer. The CBH I interdomain peptide (present in CBH I core b) does not participate in adsorption but enhances stability. The adsorption of CBH I core proteins is a fully reversible process whereas that of the intact CBH I is not. Thus, the interaction of the CBD with filter paper apparently accounts for the mass-transfer-limited binding rate and also for the irreversible adsorption of intact CBH I. Adsorption isotherms at 50°C indicate very similar relative association constants for the intact cellulases (0.24-0.30 l/g of cellulose), but drastically reduced values for CBH I core proteins (0.03 l/g of cellulose). The specific activities of adsorbed CBH I and of its core proteins are identical and a linear relationship between adsorption and rates of hydrolysis is found only for these enzymes. Thus, non-productive binding on to cellulose seems evident in the case of CBH II and EG III but not CBH I.