Mode of action of a xylanase and its significance for the structural investigation of the branched l-arabino-d-glucurono-d-xylan from redwood (Sequoia sempervirens)

The substitution pattern of the water-soluble l-arabino-(4-O-methyl-d-glucurono)-d-xylan from redwood (Sequoia sempervirens) has been studied by enzymic degradation. Exhaustive hydrolysis by an endo-xylanase (EC 3.2.1.8) from a Basidiomycete Sporotrichum dimorphosporum left a residue accounting for 20% of the original d-xylan. In the dialyzable material, oligosaccharides having arabinose or 4-O-methylglucuronic acid residues attached to the non-reducing d-xylosyl end-group of xylobiose or xylotriose, respectively, were the smallest branched oligomers released. Action of the xylanase appears to involve a region of the polysaccharide backbone having three xylosyl residues. A mode of action is proposed that requires unsubstituted hydroxyl groups at C-2, C-3, and C-2′ of a xylobiosyl residue. The binding site seems to correspond to a shallow cavity. The composition and structure of the final residue of attack shows that the enzyme has no action when the xylosyl residues branched through O-2 are separated by only one, unsubstituted xylose residue. This pattern of action, the nature of the dialyzable products, and the production of a final residue in which the substituents are accumulated, suggest that the arabinosyl and glucosyl-uronic groups are irregularly distributed on the main chain of the xylan from redwood and that in some regions they are in close vicinity when not actually on adjacent xylosyl residues.