Intact and partially acid hydrolyzed cellulose from Acetobacter xylinum were used as model substrates for cellulose hydrolysis by 1,4-b-d-glucan-cellobiohydrolase I (CBH I) and 1,4-b-d-endoglucanase I (EG I) from Trichoderma reesei. A high synergy between CBH I and EG I in simultaneous action was observed with intact bacterial cellulose (BC), but this synergistic effect was rapidly reduced by acid pretreatment of the cellulose. Moreover, a distinct synergistic effect was observed upon sequential endo±exo action on BC, but not on bacterial microcrystalline cellulose (BMCC). A mechanism for endo±exo synergism on crystalline cellulose is proposed where the simultaneous action of the enzymes counteract the decrease of activity caused by undesirable changes in the cellulose surface microstructure.
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-β-D-glucan-cellobiohydrolase I (CBH I) and 1,4-β-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.
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