Polysaccharide Definition and Functions

A polysaccharide is a type of carbohydrate. It is a polymer made of chains of monosaccharides that are joined by glycosidic linkages. Polysaccharides are also known as glycans. By convention, a polysaccharide consists of more than ten monosaccharide units, while an oligosaccharide consists of three to ten linked monosaccharides.

The general chemical formula for a polysaccharide is C_x(H₂O)_y. Most polysaccharides consist of six-carbon monosaccharides, resulting in a formula of (C₆H₁₀O₅)_n. Polysaccharides may be linear or branched. Linear polysaccharides can form rigid polymers, such as cellulose in trees. Branched forms are often soluble in water, such as gum arabic.

Homopolysaccharide vs. Heteropolysaccharide

Polysaccharides may be classified according to their composition as either homopolysaccharides or heteropolysaccharides.

A homopolysaccharide or homoglycan consists of one sugar or sugar derivative. For example, cellulose, starch, and glycogen are all composed of glucose subunits. Chitin consists of repeating subunits of N-acetyl-D-glucosamine, which is a glucose derivative.

A heteropolysaccharide or heteroglycan contains more than one sugar or sugar derivative. In practice, most heteropolysaccharides consist of two monosaccharides (disaccharides). They are often associated with proteins. A good example of a heteropolysaccharide is hyaluronic acid, which consists of N-acetyl-D-glucosamine linked to glucuronic acid (two different glucose derivatives).

Polysaccharide Structure

Polysaccharides form when monosaccharides or disaccharides link together by glycosidic bonds. The sugars participating in the bonds are called residues. The glycosidic bond is a bridge between the two residues consisting of an oxygen atom between two carbon rings. The glycosidic bond results from a dehydration reaction (also termed a condensation reaction). In the dehydration reaction a hydroxyl group is lost from a carbon of one residue while a hydrogen is lost from a hydroxyl group from another residue. A water molecule (H₂O) is removed and the carbon of the first residue joins to the oxygen from the second residue.

Specifically, the first carbon (carbon-1) of one residue and the fourth carbon (carbon-4) of the other residue are linked by the oxygen, forming the 1,4 glycosidic bond. There are two types of glycosidic bonds, based on the stereochemistry of the carbon atoms. An α(1→4) glycosidic bond forms when the two carbon atoms have the same stereochemistry or the OH on carbon-1 is below the sugar's ring. A β(1→4) linkage forms when the two carbon atoms have different stereochemistry or the OH group is above the plane.

The hydrogen and oxygen atoms from residues form hydrogen bonds with other residues, potentially resulting in extremely strong structures.

Polysaccharide Functions

The three main functions of polysaccharides are providing structural support, storing energy, and sending cellular communication signals. The carbohydrate structure largely determines its function. Linear molecules, like cellulose and chitin, are strong and rigid. Cellulose is the primary support molecule in plants, while fungi and insects rely on chitin. Polysaccharides used for energy storage tend to be branched and folded upon themselves. Because they are rich in hydrogen bonds, they are usually insoluble in water. Examples of storage polysaccharides are starch in plants and glycogen in animals. Polysaccharides used for cellular communication are often covalently bonded to lipids or proteins, forming glycoconjugates. The carbohydrate serves as a tag to help the signal reach the proper target. Categories of glycoconjugates include glycoproteins, peptidoglycans, glycosides, and glycolipids. Plasma proteins, for example, are actually glycoproteins.

Chemical Test

A common chemical test for polysaccharides is the periodic acid-Schiff (PAS) stain. Periodic acid breaks the chemical bond between adjacent carbons not participating in a glycosidic linkage, forming a pair of aldehyde. The Schiff reagent reacts with the aldehydes and yields a magenta purple color. PAS staining is used to identify polysaccharides in tissues and diagnose medical conditions that alter carbohydrates.

Sources

• Campbell, N.A. (1996). Biology (4th ed.). Benjamin Cummings. ISBN 0-8053-1957-3.
• IUPAC (1997). Compendium of Chemical Terminology - The Gold Book (2nd ed.). doi:10.1351/goldbook.P04752
• Matthews, C. E.; Van Holde, K. E.; Ahern, K. G. (1999). Biochemistry (3rd ed.). Benjamin Cummings. ISBN 0-8053-3066-6.
• Varki, A.; Cummings, R.; Esko, J.; Freeze, H.; Stanley, P.; Bertozzi, C.; Hart, G.; Etzler, M. (1999). Essentials of Glycobiology. Cold Spring Har J. Cold Spring Harbor Laboratory Press. ISBN 978-0-87969-560-6.