Primary structure and chain conformation of fucoidan extracted from sea cucumber Holothuria tubulosa

Knowledge of the structure of polysaccharides is essential for understanding and controlling their functional properties. In this study, fucoidan was extracted from Holothuria tubulosa, a species of sea cucumber that has recently become commercially important. The primary structure of the H. tubulosa fucoidan was clarified using a combination of enzymatic degradation, methylation analysis, and NMR, and its chain conformation was further elucidated by utilizing high performance size exclusion chromatography combined with multiple angle laser light scattering and viscometry. The fucoidan was composed of a tetrafucose repeating unit [→ 3-α-l-Fucp2(OSO₃^-)-1 → 3-α-l-Fucp2,4(OSO₃^-)-1 → 3-α-l-Fucp-1 → 3-α-l-Fucp2(OSO₃^-)-1 →]. Its sulfate content was determined to be 31.2 ± 1.6% and the weight-average molecular mass was 1567.6 ± 34.1 kDa. The molecule adapted a random coil conformation in 0.15 M NaCl solution (pH 7.4) at 25 °C, with a root-mean-square radius of 63.9 ± 1.8 nm and a hydrodynamic radius of 44.5 ± 4.5 nm. This is the first report on the chain conformation of sea cucumber fucoidan.