Polysaccharides

Polysaccharides are polymers of monosaccharide units. The monomeric units of a polysaccharide are usually all the same (called homopolysaccharides), though there are exceptions (called heteropolysaccharides). In some cases, the monomeric units are modified monosaccharides. Polysaccharides differ in the composition of the monomeric unit, the linkages between them, and the ways in which branches from the chains occur. Common polymers, their monomeric units, and linkages/branches are shown below:

Polysaccharide Name | Monomeric Unit | Linkages
Glycogen | D-Glucose | 1->6 branches
Cellulose | D-Glucose | 1->4
Chitin | N-Acetyl-D-glucosamine | 1->4
Amylopectin | D-Glucose | 1->6 branches
Amylose | D-Glucose | 1->4

Linkages between the individual units require special enzymes to break them down. For example, the 1-> 4 linkages between glucose units in glycogen, amylose, and amylopectin, are readily broken down by all animals, but only ruminant animals (cows, horses, etc.) contain symbiotic bacteria with an enzyme (cellulase) that can break down the 1-> 4 linkages between individual glucose units in cellulose. As a result, the huge amount of cellulose in the biosphere is unavailable as an energy source to most animals.

The secondary structure of the polysaccharides range from the helical structure of amylose (Figure 9.20) to the planar structure of cellulose (Figure 9.21). Branching affects the secondary structure of a polysaccharide, as shown for amylopectin in Figure 9.19.

Polysaccharides are used to some extent for energy storage in almost all higher organisms. Animals use glycogen. Plants use starch, which is composed of amylose and amylopectin. In both plants and animals, the polysaccharides used for energy storage are readily broken down into monomeric units that can be rapily metabolized to produce ATP. In addition to polysaccharides used for energy storage, plants use different polysaccharides, such as cellulose, for structural purposes in their cell walls. The exoskeleton of many arthropods and mollusks is composed of chitin, a polysaccharide of N-acetyl-D-glucosamine.

Polysaccharides containing a single sugar, such as glucose, are referred to as glucans. Others, which contain only mannose, are called mannans. Still others, containing only xylose, are called xylans.

Another group of polysaccharides of importance is the glycosaminoglycans. These are heteropolysaccharides containing either N-acetylgalactosamine or N-acetylglucosamine as one of their monomeric units. Examples include chondroitin sulfates and keratan sulfates of connective tissue, dermatan sulfates of skin, and hyaluronic acid. All of these are acidic, through the presence of either sulfate or carboxylate groups. Examples are shown in Figure 9.23.

Hyaluronic Acid also acts in the body as a viscosity-increasing agent or lubricating agent in the vitreous humor of the eye and synovial fluid of joints.

Heparin is yet another highly sulfated glycosaminoglycan. Part of the repeating unit of its complex chain is shown here. Heparin is used medicinally to inhibit clotting in blood vessels.

Bacteria contain polysaccharides in their cell walls in the form of peptidoglycans. In the case of Gram-positive bacteria, the long polysaccharide chains are strictly alternating copolymers of N-acetylglucosamine (NAG) and N-acetylmuramic Acid (NAM). A tetrapeptide is attached to the lactic acid moiety of the NAM.

See also: Saccharides, Structural Polysaccharides, Oligosaccharides, Monosaccharide Nomenclature, Biosynthesis of Polysaccharides (from Chapter 16), Biosynthesis of Other Polysaccharides (from Chapter 16), Biosynthesis of Amino Sugars (from Chapter 16), Biosynthesis of Glycoconjugates (from Chapter 16)