Xyloglucan oligosaccharides with at least two α‐d‐xylose residues act as acceptor substrates for xyloglucan endotransglycosylase and promote the depolymerisation of xyloglucan

A xyloglucan‐derived pentasaccharide. Xyl₂‐Glc₃, was shown by viscometry to promote the depolymerisation of xyloglucan by enzyme extracts from bean (Phaseolus vulgaris L. cv. Canadian Wonder) leaves and pea (Pisum sativum L. cv. Alaska) stems. Xyl₂‐Glc₃ was also shown by a radiochemical assay to act as an acceptor substrate for xyloglucan endotransglycosylase activity (XET: EC 2.4.1.—) present in the same extracts. In both these assays, a heptasaccharide (Xyl₃‐Glc₄) was more effective than Xyl₂‐Glc₃ whereas two isomeric tetrasaccharides (Xyl₁‐Glc₃) were essentially ineffective. The agreement in the structural requirements of the two assays suggests that they share a common basis; we therefore propose that the oligosaccharide‐sensitive enzyme that depolymerises xyloglucan is XET rather than cellulase (EC 3.2.1.4). In the viscometric assay, the penta‐ and heptasaccharides would, according to our interpretation, compete with high molecular weight xyloglucan molecules as acceptor substrates for XET, leading to a decrease in the weight‐average molecular weight of the xyloglucan and, therefore, to a decrease in viscosity. Our results indicate that oligosaccharides have to possess two α‐d‐xylose residues in order to act as acceptor substrates for XET. The non‐reducing end of a high‐molecular weight xyloglucan can also act as an acceptor substrate. Therefore, it is likely that exo‐hydrolysis by α‐d‐xylosidase would destroy the ability of a polysaccharide to act as an acceptor, even though α‐d‐xylosidase may remove only a single xylose residue from each polysaccharide molecule.