Revised structure model of norovirus-binding fucoidan from Undaria pinnatifida: oligofucose chains branch off from a β6-galactane.

Fucoidans are discussed as antiviral agents, and fucoidan from Undaria pinnatifida (UpF), in particular has gained interest as potential food additive in antinoroviral strategies. As the competitive blocking activity of antinoroviral agents increases with the valency of terminal nonreducing fucose on the competitor, an effective processing of fucoidans to inhibitory oligosaccharides will depend on basic structural features of the polysaccharide. We demonstrate increased antiviral binding activity of processed low-mass UpF generated by hydrothermal degradation contrasting with decreased efficacy of low-mass fucoidan from Fucus vesiculosus. As this finding is in conflict with current structural models of UpF, we undertook a re-investigation of the glycan backbone in UpF. Applying solvolytical desulfation combined with enzymatic cleavage of low-mass fucoidan by endo-β6-galactanase and terminal labeling of oligosaccharides by deutero-reduction and bis-5-phenyl-3-methyl-1-pyrazolone (PMP) substitution, evidence from mass spectrometry and methylation linkage analysis of the oligosaccharides indicates that fucoses and galactoses in the glycan backbone are organized in homomeric blocks, where oligo-fucoses branch off from a galactane-type core: Fuc(1-3Fuc)n1-3[Gal(1-6Gal)n1-6]Gal(1-6Gal)n.