A proteoglycan (also called a mucopolysaccharide is a protein glycosylated? by one or more (up to about 100) glycosaminoglycans. The glycosaminoglycans of the proteoglycans are linear polymers of up to about 200 repeating disaccharide units that consist of a hexosamine (D-glucosamine or D-galactosamine) alternating with a uronic acid (D-glucuronic or L-iduronic) or a neutral sugar (D-galactose). The hexosamines are usually N-acetylated, and in some of the glycosaminoglycans the D- glucosamine is N-sulfated. Varying degrees of sulfation occur in other positions of the hexosamines as well as on the L-iduronic acid. The chain of repeating units is linked to the protein by an oligosaccharide of a structure different from the repeating units.
This linkage region is identical in most of the proteoglycans (chondroitin sulfates, dermatan sulfate, heparin, and heparan sulfate), but is different in keratan sulfate. This last glycosaminoglycan contains repeating units of N-acetyllactosamine that are O-sulfated. Because of their content of uronic acid and/or ester sulfate, the glycosaminoglycans of the proteoglycans are anionic polyelec- trolytes and have been referred to as 'acidic glycosaminoglycans' (equivalent to the older term 'acid mucopolysaccharides'). Proteoglycans may also contain one or more oligosaccharides of structures similar to those found in other glycoproteins.