2.1  Source and structure of polysaccharides
The main sources of polysaccharides are endogenous glycosaminoglycans (GAGs), Marine polysaccharides and terrestrial plant polysaccharides, especially polysaccharides from Chinese herbal medicines. GAGs are naturally-derived linear polysaccharides that are expressed in the intracellular compartments, cell surface, and extracellular environments, and they interact with various molecules to regulate many cellular processes associated with health and disease [22]. GAGs are comprised distinct O-linked disaccharide units, which are typically composed of a combination of iduronic acid, glucuronic acid, glucosamine, galactose or galactosamine monosaccharides [23,24]. The widely studied GAGs mainly include chondroitin sulfate (CS), heparan sulfate (HS) and heparin (HP) in animal tissues (Fig. 3 ) [22,25]. GAG chains are in most cases sulfated, except hyaluronan (HA) (Fig. 3), which are biodegradable and non-immunogenic in the body [26,27]. The chemical structures of typical GAGs are shown in Fig. 3.
Fig. 3 The structures of several polysaccharides (GAGs, marine polysaccharides, and terrestrial plant polysaccharides).
Marine organisms are rich sources of polysaccharides. Chitosan is a linear, positive-charged, alkaline polysaccharide repeating by glucosamine and N-acetylglucosamine (Fig. 3) [28,29], derived from the shells of shrimps and crustacean or the cell walls of fungi [30,31]. Marine algae products have been applied in traditional Chinese herbal medicine for a long time [11], and contain a variety of polysaccharides, including carrageenan, fucoidan, and alginate. Carrageenans are sulphated linear polysaccharides composed of repeating disaccharide units with alternating 3-linked β-d-galactopyranose (G-units) and 4-linked α-galactopyranose (D-units) or 3,6-anhydro-α- galactopyranose (AnGal-units) [[32], [33], [34]], which are extracted from certain red algae containing 15–40% ester sulfate with an average molecular weight above 100 kDa [35,36]. The three commercial most important and widely distributed carrageenans are iota (ι-, G4S-DA2S), kappa (κ-, G4S-DA) and lambda (λ-, G2S-D2S, 6S)-carrageenan (Fig. 3) [37]. Fucoidan is a fucose-enriched and sulfated polysaccharide extracted from brown algae [11,38], which is composed of L-fucose, sulfate groups and small proportions of D-xylose, D-mannose, D-galactose, and D-glucuronic acid in different sources of brown algae (Fig. 3) [[38], [39], [40]]. Alginate, an acidic and linear polysaccharide extracted from brown algae, is consisted of alternating β-D-mannuronic acid (M) and α- L-guluronic acid (G) residues [41]. Polyguluronate sulfate (PGS) (Fig. 3) is a low molecular weight sulfated brown algae polysaccharide obtained by chemical sulfation of polyguluronate (PG) with about 1.5 sulfate per sugar residue [42,43].
Astragalus polysaccharide (APS) is the most important bioactive component isolated from a Chinese traditional herbal medicine of Astragalus membranaceus, which is composed of glucose, mannose, d-glucose, and D-galactose (Fig. 3) [[44], [45], [46]]. Radix Isatidis (RI) is also a kind of traditional Chinese herbal medicine with significant antiviral effect, and polysaccharide is its main active component [47,48]. The polysaccharide from RI is mainly composed of mannose, glucose, galactose and arabinose [49]. Mushrooms are used as food for long time in China, and also are drugs in the Orient centuries [50]. Lentinus edodes is one of the most widely edible mushrooms, and is popularly consumed as health foods in Asian countries [50,51]. Among the bioactive components of mushrooms, the Lentinus edodes polysaccharide (lentinan, LNT) is the most extensively investigated with many immune processes, which is generally described as biological response modifiers [52,53]. It consists of a β-(1 → 3)-glucan backbone with β-(1 → 6)-glucosyl side-branching units terminated by mannosyl or galactosyl residues (Fig. 3) [50,51]. Recently, LNT has been widely used as an alternative medicine and dietary supplement in the world [50].