Binding of hemicellulases on isolated polysaccharide substrates

Binding of five fungal xylanases produced by Trichoderma reesei, Aspergillus oryzae, and Aspergillus fumigatus and two mannanases of Bacillus subtilis and T. reesei on insoluble xylan, mannan, and cellulose was studied at different pH values and ionic strengths. For comparison, the binding of cellobiohydrolase I (CBH I) and endoglucanase (EG I) of T. reesei on the same substrates was examined. Ionic interactions appeared to play an important role in the binding of xylan, as most of the enzymes were totally bound on xylan when the pH was below their isoelectric point but remained mainly unbound at pH values above the isoelectric point. The binding on xylan was also clearly sensitive to ionic strength. The xylanases were also incompletely bound on mannan and cellulose, and the two mannanases tested were partially bound on mannan. The two cellulolytic enzymes of T. reesei used in this work are both known to contain a cellulose binding domain and were efficiently bound on cellulose. Interestingly, T. reesei mannanase was also found to bind readily on cellulose. Thus, it most probably contains, instead of a mannan binding domain, a specific cellulose binding domain.