Linear phycoxyloglucan with oligo-xylose domains in the green seaweed Ulva: a unique hemicellulose

The chlorophycean seaweed genus Ulva (sea lettuce and relatives) possesses cellulose and cell-wall-matrix polysaccharides operationally classified as 'pectin-like' (hot-water-extractable anionic ulvans and glucuronans) and hemicelluloses. Given that Ulva is only distantly related to streptophytes (land-plants plus charophytic algae), their wall polysaccharides are likely to diverge significantly. We therefore explored the little-known structure and properties of Ulva hemicellulose.

Hemicellulose b was alkali-extracted from hot-oxalate-pretreated Ulva linza alcohol-insoluble residue, then digested with endo-β-xylanase, Driselase or endo-cellulase, yielding several oligosaccharides. These were separated by size on gel-permeation and preparative paper chromatography. Oligosaccharides were dissected by acid hydrolysis, glycosidase digestion and alkaline peeling; products were analysed by thin-layer chromatography. Purified oligosaccharides were also characterized by nuclear magnetic resonance (NMR) spectroscopy.

Thirteen di- to pentasaccharides were purified. On acid hydrolysis, they yielded only xylose (Xyl) and/or glucose (Glc). Each was digestible by either β-xylosidase or β-glucosidase, identifying the non-reducing terminus as β-Xyl or β-Glc, respectively. The rate of alkaline peeling, which attacks progressively from the reducing terminus, indicated (1→4) linkages; in each case, the smallest peeling product (i.e. originally the non-reducing terminus) was either Xyl or Glc, concurring with the glycosidase digestion result. Structures were deduced [β-(1→4)-linked Xyl-Xyl, Xyl-Xyl-Xyl, Xyl-Glc, Xyl-Xyl-Glc, Xyl-Xyl-Xyl-Glc, Glc-Glc, Glc-Xyl-Xyl, Glc-Glc-Xyl-Xyl and Xyl-Glc-Glc-Xyl-Xyl] and validated by NMR spectroscopy. Unexpectedly, NMR showed one oligosaccharide to be Glc-3-Glc-4-Glc, with the (1→3)-bond probably arising artefactually by trans-β-glucanase activity of the cellulase. Ulva hemicellulose hydrogen-bonded to cellulose more rapidly than any tested land-plant polysaccharide.

The major hemicellulose of Ulva is a β-(1→4)-linked linear polysaccharide of Xyl and Glc, which we term phycoxyloglucan. The backbone has runs of up to three contiguous β-Xyl and three contiguous β-Glc residues, thus differing fundamentally from land-plant xyloglucan. Its structure confers high affinity for cellulose, suggesting potential biotechnological applications for this seaweed and contributing to the mechanical properties of the cell wall.