Galactose branching modulates the action of cellulase on seed storage xyloglucans

Xyloglucan is a cell wall polysaccharide that has a main β-d-(1→4)-glucan backbone branched at regular intervals with α(1→6) linked d-xylopyranosyl or β-d-galactopyranosyl(1→2)d-xylopyranosyl residues. These residues compromise almost all the structure of xyloglucan when it occurs as a storage polymer in seeds. This polymer is susceptible to the action of endo-β(1,4)glucanase (cellulase) at the non-branched glucosyl residues and this enzyme has been widely used in the study of the structure of xyloglucan. Storage xyloglucans from Copaifera langsdorffii and Hymenaea courbaril were hydrolysed with cellulase and the equilibrium reached after 24 h was analysed by gel filtration and high performance anion exchange chromatography. The equilibrium contained limit digest oligosaccharides (LDOs) and dimers and trimers of these LDOs that, despite their susceptibility to the enzyme, were not hydrolysed. Our data suggest that the side-chains of the polysaccharides can modulate the recognition of the fragments of xyloglucan by the enzyme: (1) the presence of a β-galactosidase in the system avoided the accumulation of these dimmers and trimmers, (2) less-branched LDOs are attacked preferentially by the enzyme, (3) polymers with different fine structures are hydrolysed at a different rates by the same enzyme. Considering that the branching pattern of the polysaccharide seems to have direct influence on the interaction of the enzyme with the substrate, we suggest that the structure of the polysaccharide would code at least part of the information required for its own metabolism.