ISOTHERMS FOR ADSORPTION OF CELLOBIOHYDROLASE-I AND-II FROM TRICHODERMA-REESEI ON MICROCRYSTALLINE CELLULOSE

Adsorption to microcrystalline cellulose (Avicel) of pure cellobiohydr olase I and II (CBH I and CBH II) from Trichoderma reesei has been stu died. Adsorption isotherms of the enzymes were measured at 4 degrees C using CBH I and CBH II alone and in reconstituted equimolar mixtures. Several models (Langmuir, Freundlich, Temkin, Jovanovic) were tested to describe the experimental adsorption isotherms. The isotherms did n ot follow the basic (one site) Langmuir equation that has often been u sed to describe adsorption isotherms of cellulases; correlation coeffi cients (R-2) were only 0.926 and 0.947, for CBH I and II, respectively . The experimental isotherms were best described by a model of Langmui r type with two adsorption sites and by a combined Langmuir-Freundlich model (analogous to the Hill equation); using these models the correl ation coefficients were in most cases higher than 0.995. Apparent bind ing parameters derived from the two sites Langmuir model indicated str onger binding of CBH II compared to CBH I; the distribution coefficien ts were 20.7 and 3.7 L/g for the two enzymes, respectively. The bindin g capacity, on the other hand, was higher for CBH I, 1.0 mu mol (67 mg ) per gram Avicel, compared to 0.57 mu mol/g (30 mg/g) for CBH II. The isotherms when analyzed with the combined Langmuir-Freundlich model i ndicated presence of unequal binding sites on cellulose and/or negativ e cooperativity in the binding of the enzyme molecules.