Glycosaminoglycan Structure

GAG are long, unbranched, heteropolysaccharide chains composed of a repeating disaccharide unit [acidic sugar–amino sugar]n (Fig. 1). [Note: A single exception is keratan sulfate, which contains galactose rather than an acidic sugar.] The amino sugar is either D-glucosamine or D-galactosamine, in which the amino group is usually acetylated, thus eliminating its positive charge. The amino sugar may also be sulfated on carbon 4 or 6 or on a nonacetylated nitrogen. The acidic sugar is either D-glucuronic acid or its C-5 epimer Liduronic acid (Fig. 2). These uronic sugars contain carboxyl groups that are negatively charged at physiologic pH and, together with the sulfate groups (−SO₄ ²⁻), give GAG their strongly negative nature.

Figure 1: Repeating disaccharide unit of glycosaminoglycans.

Figure 2: Some monosaccharide units found in glycosaminoglycans.