[Adsorption of cellulolytic enzymes on cellulose and the kinetics of the adsorbed enzymes. Two modes for interaction of the enzymes with the insoluble substrate].

The affinity of cellulolytic enzymes of various origin for microcrystalline cellulose (MCC) in a column type reactor has been studied. It was shown that there exist two types of the enzymes differing in their ability to bind to cellulose and to degrade MCC. One group of cellulases is characterized by weak affinity for MCC and by a rather low degree of conversion of this substrate into soluble products, but shows a burst of soluble products during the initial period of the hydrolysis. On the contrary, the other group of the enzymes irreversibly binds to cellulose and shows a noticeable ability to solubilize MCC. The results obtained suggest that the weak binding reflects a productive enzyme--cellulose interaction by the enzyme active center, like its interaction with soluble polymeric substrates. The tight binding, on the other hand, is less productive and reflects the interaction of peripheral parts of the enzymes with the cellulose surface. The hydrolytic action of the tightly adsorbed cellulases proceeds on the substrate surface consecutively, without leaving the insoluble substrate between the catalytic acts and the enzymes might steadily penetrate into the cellulose matrix. This in turn might induce mechanical fragmentation of the substrate. The rate of action of adsorbed cellulases can be limited by their diffusion along the surface or into the cellulose matrix. A decrease of the ionic strength weakens the cellulase affinity for cellulose, which in its turn leads to an increase in the initial burst of the soluble products of cellulose hydrolysis and a respective decrease of efficiency of cellulose conversion into glucose. It is concluded that the principal factor which determines the ability of cellulases to degrade crystalline cellulose is the affinity (i. e. degree of adsorption) of the enzymes for the insoluble substrate.