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[Display omitted] • A glycoprotein (SGP1) was isolated from Sipunculus nudus for the first time. • SGP1 is a protein-dominate glycoprotein with O-glycosidic bonds containing 92.8% protein. • SGP1 was a predominantly β-sheet glycoprotein. • SGP1 exhibited strong immune-enhancing activity by promoting proliferation, phagocytosis and the secretions of NO, IL-1β, TNF-α and IL-6 on RAW264.7 cells. • SGP1 exerts immune-enhancing activity via NF-κB signaling pathway. Sipunculus nudus , an edible marine invertebrate, has long been used as traditional Chinese medicine in folk remedies. In order to assess the immunoregulatory activity of glycoproteins in Sipunculus nudus and conduct a structure–activity relationship, a glycoprotein (SGP1) with molecular mass of 9.26 kDa was purified from Sipunculus nudus , and its chemical structure as well as immune-enhancing activity was investigated in this study. Structure analysis revealed that SGP1, a protein-dominate glycoprotein with O-glycosidic bonds, contained 92.8 % protein and 3.1 % saccharide. GC–MS result indicated that the saccharide moieties of SGP1 basically consisted of lyxose (Lyx), xylose (Xyl) as well as glucose (Glu) at a molar proportion of 0.87:4.16:1.36. The fourier transform infrared specoscopy (FT-IR) result proved that SGP1 have a typical characteristic of glycoprotein. Besides, circular dichroism (CD) result showed that SGP1 contained 4.1 % α-helix, 42.5 % β-sheet, 21.4 % β-turn, and 32.0 % random coil, indicating it's mainly a β-sheet glycoprotein. The amino acid sequence of SGP1 shared a similarity to the Myohemerythrin (sp|Q5K473|HEMTM) with protein sequence coverage of 28.3 %. Moreover, the activity evaluation results showed that SGP1 exhibited significant immune-enhancing activity to the RAW 264.7 macrophages by promoting macrophages proliferation, enhancing phagocytic capacity, and simultaneously stimulating the secretions of nitric oxide (NO), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) via NF-κB pathways. In this study, SGP1 as a novel glycoprotein had an obvious immune-enhancing activity to macrophages, and thus could be applied in the functional foods as a potential immunopotentiator for the hypoimmune population. [ABSTRACT FROM AUTHOR]