 Glycosaminoglycan

A glycosaminoglycan is a long, unbranched polysaccharide built from repeating two-sugar units containing an amino sugar derivative. The partner is commonly a uronic acid; keratan sulfate instead contains galactose. Many glycosaminoglycans carry sulfate groups and occur as carbohydrate chains of proteoglycans. A proteoglycan is the larger protein–carbohydrate assembly, while a glycosaminoglycan is the sugar chain.^{[ref] [ref]}

Hyaluronic acid is a useful comparison: it alternates glucuronic acid with N-acetylglucosamine, is nonsulfated, and is not covalently attached to a protein core. Thus neither “sulfated” nor “covalently protein-bound” defines every member of the family. At physiological pH its carboxylate groups give the chain negative charge, helping explain its interaction with water and surrounding ions. ^[ref]

The distinction matters in experiments on the endothelial glycocalyx. Inhibition or removal of a particular sugar-chain component can change the environment through which blood cells move without removing the entire endothelial cell membrane. Heparinase treatment in microvessels altered capillary hematocrit, supporting a role for the surface carbohydrate layer in blood-cell distribution; it does not establish that every glycosaminoglycan produces the same effect.^[ref]