Cyanobacteria (blue-green algae) are photoautotrophic prokaryotes which include a large variety of species with varied morphological, physiological, and biochemical properties. It is well known that some kinds of cyanobacteria produce large amounts of exopolysaccharides. Although the quantity and biochemical properties are diverse depending on the species, these polysaccharides are thought to play important roles in desiccation tolerance, UV tolerance, nutrient uptake, and interactions with other organisms in the establishment of symbiosis. Nostoc commune, a nitrogen-fixing filamentous cyanobacterium, also produces copious amounts of polysaccharide around the cells and has remarkable desiccation tolerance. Despite conspicuous features of polysaccharide of Nostoc commune, little is known about its chemical structure due largely to its structural complexity. In general, polysaccharides of cyanobacteria are more complex both in their sugar composition and in their linkage types than those of other bacteria or macroalgae, and at this time only a few structures of cyanobacterial polysaccharides have been proposed. In this study, we investigated the chemical structure of water-soluble polysaccharide from desiccation-tolerant cyanobacterium Nostoc commune. The water-soluble polysaccharide (glycan) was extracted and purified from field-grown Nostoc commune. Sugar compositional analysis revealed that this polysaccharide contained D-glucose, D-galactose, D-xylose, and D-glucuronic acid in the molecular ratio of 8: 4: 4: 1 together with unknown uronic acids and trace amount of ribose. One of the unknown uronic acid was identified as 2,3-O-(1-carboxyethylidene)-D-glucuronic acid by NMR analysis of the methanolysate of the polysaccharide. Partial hydrolysis of the polysaccharide by acid (0.2 N TFA, 90℃, 5h) or by crude enzyme which was obtained from culture brose of Paenibacillus sp. afforded 5 oligosaccharide fragments. After pyridylamination, these oligosaccharides were isolated by reverse phase column chromatography, and structures were elucidated by 2D NMR. By comparison with the structures of these oligosaccharides, we concluded that the polysaccharide had a 5-membered repeating unit viz.→4)-β-D-Glcp-(1→4)-β-D-Xylp-(1→4)-[β-D-GlcAp-(1→6)]-β-D-Glcp-(1→4)-β-D-Galp-(1→, and residues in the repeating unit were partially substituted by 3-O-(1-carboxyethyl)-D-glucuronic acid or 2,3-O-(1-carboxyethylidene)-D-glucuronic acid in the GlcA moiety and by 2-O-Me-Man in the branched Glc moiety as shown in Figure 5.
