Structural Polysaccharides

A major function of polysaccharides in organisms is providing structural integrity. This is quite important in plants, because they do not use fibrous proteins, such as keratin (see here) or collagen, for this purpose.

Cellulose is a glucan (contains only glucose) and is the major polysaccharide in woody and fibrous plants. It is the most abundant single polymer in the biosphere. Made up of units of D-glucose linked in the 1-> 4 configuration, cellulose forms a planar structure with individual parallel chains held together by hydrogen bonds (Figure 9.21). Most animals cannot digest the 1->4 linkages in cellulose. Among the animals, only ruminants (cows, horses, etc.) contain a symbiotic bacterium with an enzyme, cellulase, that can break cellulose down to glucose. Fungi too contain cellulases. Cellulose is also used as a structural component of some animal cells, such as the marine invertebrates called tunicates.

Other glucans with structural roles include some in fungi, which have glucoses joined by 1->3 or 1->6 bonds.

Other plant polysaccharides include the xylans and the glucomannans (see here). The xylans are polymers of D-xylopyranose, often with substituent groups attached. The glucomannans, on the other hand, are heteropolymers of glucopyranose and mannopyranose joined by 1->4 linkages with 1->6 branches to other substituents. The glucomannans and xylans are often grouped together and called hemicellulose.

Chitin is a homopolymer of N-acetyl-D-glucosamine, with units joined by 1-> 4 bonds. Chitin is found in organisms as diverse as algae, fungi, insects, arthropods, mollusks, and insects.

Glycosaminoglycans 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 (See Figure 9.23). A major function of glycosaminoglycans is formation of a matrix to hold together the protein components of skin and connective tissue in animals. An example is the proteoglycan complex (protein-carbohydrate complex) in cartilage (Figure 9.24b). This structure binds collagen, apparently via electrostatic interactions between the sulfate and/or carboxylate groups of the proteoglycan complex and the basic side chains in collagen.

See also: Saccharides, Oligosaccharides, Polysaccharides, Monosaccharide Nomenclature