The initial kinetics of hydrolysis by cellobiohydrolases I and II is consistent with a cellulose surface-erosion model.

Introduction of a novel method for the quantification of the cellobiose released made it possible to follow the initial stage of hydrolysis of bacterial microcrystalline cellulose (BMCC) by cellobiohydrolases 1,4-beta-D-glucan-cellobiohydrolase I (CBH I) and 1,4-beta-D-glucan-cellobiohydrolase II (CBH II) from Trichoderma reesei. A drastic retardation of the rate of the hydrolysis was observed already at a very low degree of conversion. Earlier-suggested retardation factors, such as product inhibition by cellobiose or enzyme inactivation, could be discounted as primary causes for the pattern. A model including steric hindrance by non-productive binding and erosion of the cellulose surface during the processive action of exoenzymes was proposed to describe the rate retardation observed. Simultaneous action of CBH I and CBH II on cellulose was not a prerequisite for synergy between them.