glycosaminoglycans

(= formerly mucopolysaccharides)

Polysaccharide side-chains of proteoglycans made up of repeating disaccharide units (more than 100) of amino sugars, at least one of which has a negatively charged side-group (carboxylate or sulphate). Commonest are hyaluronate (D-glucuronic acid- N-acetyl-D-glucosamine: MW up to 10 million), chondroitin sulphate (D-glucuronic acid- N-acetyl-D-galactosamine-4 or -6-sulphate), dermatan sulphate (D-glucuronic acid- or L-iduronic acid- N-acetyl-D-galactosamine), keratan sulphate (D-galactose- N-acetyl-D-glucosamine-sulphate), and heparan sulphate (D-glucuronic acid- or L-iduronic acid- N-acetyl-D-glucosamine). Glycosaminoglycan side chains (with the exception of hyaluronate) are covalently attached to a core protein at about every 12 amino acid residues to produce a proteoglycan; these proteoglycans are then non-covalently attached by link proteins to hyaluronate, forming an enormous hydrated space-filling polymer found in extracellular matrix. The extent of sulphation is variable and the structure allows tremendous diversity.