Abstract: In this study, the structural characteristics, antioxidant, and hypoglycemic activities of glycoprotein from Nostoc sphaeroides Küting (GNSK) were evaluated. GNSK was extracted by ammonium sulfate precipitation and purified through hydroxyapatite and Sephadex G-100 column chromatography, yielding GNSK1. The structural and compositional differences between GNSK and GNSK1 were analyzed using chemical composition analysis, high-performance gel permeation chromatography (HPGPC), β-elimination reaction, ultraviolet (UV), Fourier transform infrared (FTIR), and circular dichroism (CD) spectroscopy. Their antioxidant activities were evaluated using the ferric reducing antioxidant power (FRAP), pyrogallol auto-oxidation, and 1,10-phenanthroline methods, and their hypoglycemic activities were assessed via α-amylase and α-glucosidase inhibition assays. The results indicated that both GNSK and GNSK1 were chromophore-containing glycoproteins dominated by carbohydrate moieties, with total carbohydrate contents 2.10- and 2.27-fold higher than those of protein, respectively. Their weight-average molecular masses were 67.843 and 21.098 kDa, respectively. Both glycoproteins featured O-type glycopeptide linkages and β-pyranose sugar chains. The secondary structure of GNSK was mainly α-helix, whereas that of GNSK1 was dominated by random coils. Both samples contained the same monosaccharides, but their molar ratios of monosaccharides differed. The molar ratio of mannose: glucuronic acid: glucose: galactose: arabinose in GNSK was 1:1.46:2.68:0.97:0.96, compared with 1:1.36:1.21:0.98:0.87 in GNSK1. Furthermore, 17 amino acids were identified in GNSK, whereas methionine was not detected in GNSK1. The proportions of hydrophobic amino acids in GNSK and GNSK1 were 21.75% and 32.59%, respectively. Both of them effectively scavenged superoxide anion and hydroxyl radicals, and inhibited α-amylase and α-glucosidase activities, with the purified glycoprotein fraction exhibiting enhanced antioxidant and hypoglycemic capacities. The antioxidant and hypoglycemic activities of GNSK may be attributed to a combination of factors including glucuronic acid, molecular mass, hydrophobic amino acids, and random coil structures. This study provides a theoretical basis for further development and utilization of N. sphaeroides Küting resources.

Key words: Nostoc sphaeroides Küting; glycoprotein; structure characteristics; antioxidant activity; hypoglycemic effect