Fungal Polysaccharides: Structure, Classification, Extraction, and Food Applications

The basic constituent units of fungal polysaccharides are sugars or uronic acids, including xylose, fucose, rhamnose, mannose, galactose, glucose, arabinose, and glucuronic acid. The primary structure of fungal polysaccharides encompasses the composition of sugar residues, their arrangement sequence, the linkage types between adjacent sugar residues, as well as the presence or absence of branches, branch positions, and branch chain lengths. The primary backbone structure of fungal polysaccharides is typically a homopolysaccharide, meaning it is composed of a single type of monosaccharide unit.

Currently, two main types have been identified:

1. Glucans: These are primarily linked by β-1,3-glycosidic bonds in the main chain, with a small number of β-1,4-glycosidic bonds or other glycosidic bonds. Side chains are usually oligomers linked via β-1,6-glycosidic bonds or β-1,3-glycosidic bonds. Examples include lentinan (from shiitake mushrooms) and schizophyllan (from split gill mushrooms).

2. Mannans: Their main chains are primarily linked by α-glycosidic bonds. Examples include polysaccharides from Cordyceps sinensis (Cordyceps) and Tremella fuciformis (silver ear mushroom).

The higher-order structure, also known as the spatial conformation, is the structure through which polysaccharides exert their physiological or pharmacological effects. This includes secondary, tertiary, and quaternary structures.

• Secondary Structure: The spatial conformation formed by the polysaccharide backbone chain through hydrogen bonding, creating various aggregates.

• Tertiary Structure: An ordered spatial conformation formed by non-covalent interactions between hydroxyl, amino, carboxyl, and other groups on the monosaccharide residues of the polysaccharide chain.

• Quaternary Structure: Aggregates formed by the non-covalent association of multiple polysaccharide chains.

Based on the composition of monosaccharides in the main chain, fungal polysaccharides are mainly classified into two categories: heteropolysaccharides and homopolysaccharides. This classification not only reflects the structural characteristics but is also closely related to their functional properties.

Heteropolysaccharides

Heteropolysaccharides are complex polysaccharides composed of two or more different types of monosaccharide units. Common constituent monosaccharides include glucose, mannose, galactose, xylose, and fructose. Their structural diversity endows them with a wide range of functional properties.

• Chitosan: A typical fungal heteropolysaccharide derived from the deacetylation of chitin. It is mainly found in the exoskeletons of crustaceans and the cell walls of certain fungi. Its structure features N-acetylglucosamine and glucosamine units, providing excellent biocompatibility, biodegradability, and antibacterial activity. These properties make it valuable in food preservation (e.g., antibacterial packaging films), medicine (e.g., wound dressings, drug delivery systems), and tissue engineering.

• Pullulan: A linear heteropolysaccharide secreted by the fungus Aureobasidium pullulans. Its molecular structure is characterized by maltotriose repeating units (composed of α-1,4-glycosidic bonds) linked by α-1,6-glycosidic bonds. This unique structure imparts exceptional film-forming properties and plasticity, making it an ideal material for edible coatings, encapsulation of active ingredients, drug delivery, and tissue repair.

• β-Glucans: A class of branched polysaccharides formed by β-1,3 and β-1,6 glycosidic linkages. In the food sector, they serve as high-quality dietary fiber and possess immunomodulatory functions, allowing their use as functional food additives and nutritional supplements. Their specific molecular structure enables interaction with receptors on immune cell surfaces, thereby exerting immune-enhancing effects.

Homopolysaccharides

Compared to heteropolysaccharides, homopolysaccharides are composed of a single type of monosaccharide, resulting in relatively simpler structures but unique functions.