An investigation of the substrate specificity of the xyloglucanase Cel74A from Hypocrea jecorina

The substrate specificity of the xyloglucanase Cel74A from Hypocrea jecorina (Trichoderma reesei) was examined using several polysaccharides and oligosaccharides. Our results revealed that xyloglucan chains are hydrolyzed at substituted Glc residues, in contrast to the action of all known xyloglucan endoglucanases (EC 3.2.1.151). The building block of xyloglucan, XXXG (where X is a substituted Glc residue, and G is an unsubstituted Glc residue), was rapidly degraded to XX and XG (k(cat) = 7.2 s(-1) and Km = 120 microM at 37 degrees C and pH 5), which has only been observed before with the oligoxyloglucan-reducing-end-specific cellobiohydrolase from Geotrichum (EC 3.2.1.150). However, the cellobiohydrolase can only release XG from XXXGXXXG, whereas Cel74A hydrolyzed this substrate at both chain ends, resulting in XGXX. Differences in the length of a specific loop at subsite + 2 are discussed as being the basis for the divergent specificity of these xyloglucanases.