Figure 1.

Repeating disaccharide units found in different types of glycosaminoglycans. The disaccharide units consist of a hexuronic acid such as gluconorate or iduronate, and a hexosamine such as N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc) (Garret and Grisham 1999; Ancsin 2003). The symbol “@” indicates that the carboxyl group on the 5-carbon can exist in multiple epimerization states. The symbol “*” indicates that the substituent can be either a hydrogen (—H) or a sulfate group (—SO₃⁻). Negatively charged groups are shown in red (—COO⁻) and brown (—SO₃⁻). After synthesis, GAGs are extensively processed in the Golgi by enzymes that catalyze their sulfonation and epimerization. Hyaluronate is the only GAG that is not sulfated, and the only GAG that does not occur in proteoglycans. Heparin and heparan sulfate are shown together. Heparan sulfate is less extensively modified than heparin, has fewer sulfate groups, is less negatively charged, and is more difficult to obtain than heparin. Heparin occurs mainly in the cytoplasmic granules of mast cells, while heparan sulfate is found on cell surfaces and in the extracellular matrix.