Substrate specificity of endoglucanases: what determines xyloglucanase activity?
Endoglucanases from Trichoderma viride differ in their activity and mode of action towards xyloglucans. In order to explain the basis for their different behavior, the number of substrate-binding sites of three endoglucanases (endoI, endoIV, and endoV) were determined using bond cleavage frequencies of both normal and reduced cellodextrins and Ko/K(m). EndoIV differed from other endoglucanases described so far, in having at least nine putative binding sites. The specificities of the three endoglucanases towards various xyloglucans derived from apple fruit and potato were determined. Also, the release of oligosaccharides from these substrates in time was monitored. It was concluded that the endoglucanases prefer to bind unbranched glucosyl residues. Because most xyloglucans are composed of XXXG-type of building units, distant subsites are needed to bind xyloglucan. Having at least nine substrate-binding sites, endoIV seems to be well equipped to degrade xyloglucans which was confirmed by its high xyloglucanase activity.