Enzymatic transformation and anti-tumor activity of Sargassum horneri fucoidan

•Fucoidan from S. horneri was fractionated via enzyme treatment.•Structures of its high molecular weight products were elucidated.•Fucoidan and its derivatives’ antitumor effects were determined. Structure of the fucoidan from Sargassum horneri and products of its enzymatic transformation with molecular weight over 20 kDa were investigated. Fucoidan was hydrolyzed by recombinant fucoidanase FFA1 and its fraction of higher molecular weight was fractionated using anion-exchange chromatography, resulting in three sulphated polysaccharides of various molecular weight (63−138 kDa). Their structures were analyzed using NMR spectroscopy, showing the fucoidan (ShF) to be a branched polysaccharide with the backbone consisting of the repeating →3-α-l-Fucp(2SO3−)-1→4-α-l-Fucp(2,3SO3−)-1→ fragment and side chains including the α-l-Fucp-1→2-α-l-Fucp-1→ or α-l-Fucp-1→3-α-l-Fucp(4SO3−)-1→ fragments attached to the main chain at C4. The fragment F3 differing by molecular weight and side chain from other fucoidans fragments possessed the most significant anticancer and radiosensitizing activities.