Handbook of Glycosyltransferases and Related Genes pp 1681-1686 | Cite as
Map 4: Biosynthetic Pathways of Proteoglycans
Proteoglycans (PGs) are comprised of one or more glycosaminoglycan (GAG) chains covalently linked to specific residues of a core protein. GAGs are long, unbranched polysaccharides containing a number of repeating disaccharide units composed of N-acetylated or N-sulfated d-hexosamine and either hexuronic acid (d-glucuronic acid or l-iduronic acid) or galactose. The major GAGs are classified by the composition of their disaccharide repeats: heparan sulfate (HS), chondroitin sulfate (CS), dermatan sulfate (DS), and keratan sulfate (KS). CS, DS, and HS are covalently attached to the serine residues of a core protein via a common tetrasaccharide (GlcA-Gal-Gal-Xyl). These GAG chains are then elongated by the stepwise addition of N-acetylhexosamine and glucuronic acid residues by specific glycosyltransferase enzymes. In the case of KS, the GAGs are initiated as N-linked or O-linked oligosaccharides and extended by repeated addition of N-acetylglucosamine and galactose.