3)D-Gal((3 1—> 4)3,6-anhydro-L-Gal2S(a 1 repeats

FIGURE 7-23 The structure of agarose. The repeating unit consists of d-galactose (^1 n4)-linked to 3,6-anhydro-l-galactose (in which an ether ring connects C-3 and C-6). These units are joined by (a1n3) glycosidic links to form a polymer 600 to 700 residues long. A small fraction of the 3,6-anhydrogalactose residues have a sulfate ester at C-2 (as shown here).

tures in turn associate with each other to form a gel— a three-dimensional matrix that traps large amounts of water. Agarose gels are used as inert supports for the electrophoretic separation of nucleic acids, an essential part of the DNA sequencing process (p. 8-24). Agar is also used to form a surface for the growth of bacterial colonies. Another commercial use of agar is for the capsules in which some vitamins and drugs are packaged; the dried agar material dissolves readily in the stomach and is metabolically inert.

Glycosaminoglycans Are Heteropolysaccharides of the Extracellular Matrix

The extracellular space in the tissues of multicellular animals is filled with a gel-like material, the extracellular matrix, also called ground substance, which holds the cells together and provides a porous pathway for the diffusion of nutrients and oxygen to individual cells. The extracellular matrix is composed of an interlocking meshwork of heteropolysaccharides and fibrous proteins such as collagen, elastin, fibronectin, and laminin. These heteropolysaccharides, the glycosaminoglycans, are a family of linear polymers composed of repeating disac-charide units (Fig. 7-24). One of the two monosaccharides is always either ^-acetylglucosamine or ^-acetyl-galactosamine; the other is in most cases a uronic acid, usually d-glucuronic or l-iduronic acid. In some gly-cosaminoglycans, one or more of the hydroxyls of the amino sugar are esterified with sulfate. The combination of sulfate groups and the carboxylate groups of the uronic acid residues gives glycosaminoglycans a very high density of negative charge. To minimize the repulsive forces among neighboring charged groups, these molecules assume an extended conformation in solution. The specific patterns of sulfated and nonsulfated sugar residues in gly-cosaminoglycans provide for specific recognition by a

Glycosaminoglycan Repeating disaccharide

Number of disaccharides per chain






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