| Proteoglycans are gel-forming components of the ECM. Some proteoglycans are located on the cell surface, where they bind growth factors and other ECM components. They are composed of peptide chains containing covalently bound sugars (see Fig. 25.1). However, the peptide chains of proteoglycans are usually more rigid and extended than the protein portion of the glycoproteins, and the proteoglycans contain much larger amounts of carbohydrate - typically >95% carbohydrate. The sugar chains are linear, unbranched oligosaccharides that are much longer than those of the glycoproteins, and may contain more than 100 sugar residues in a chain. Furthermore, the oligosaccharide chains of proteoglycans have a repeating disaccharide unit, usually composed of an amino sugar and a uronic acid. Proteoglycan oligosaccharide chains are polyanionic because of the many negative charges of the carboxyl groups of the uronic acids, and from sulfate groups attached to some of the hydroxyl or amino groups of the sugars.
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