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Glycosaminoglycans (GAGs) are linear polysaccharide chains consisting of repeating disaccharide units and form proteoglycans by covalently attaching to their core proteins. Chondroitin sulfate (CS) is a glycosaminoglycan with the disaccharide unit GalNAc(b1-4)GlcA(b1-3), modified with ester-linked sulfate at certain positions. Dermatan sulfate (DS) is a modified form of CS, in which a portion of D-glucuronate residues is epimerized to L-iduronates. CS and DS are linked to serine residues in core proteins via a linkage tetrasaccharide formed by the transfer of xylose and three more residues. The assembly process of CS is initiated by the transfer of N-acetylgalactosamine to the linkage tetrasaccharide. The polymerization step is catalyzed by bifunctional enzymes (chondroitin synthases) that have both b-3 glucuronosyltransferase and b1-4 N-acetylgalactosaminyltransferase activities. Chondroitin polymerization also requires the action of the chondroitin polymerizing factor. Sulfation of chondroitin in vertebrates is a complex process, with multiple sulfotransferases involved in 4-O sulfation and 6-O sulfation of N-acetylgalactosamine residues. Additional enzymes exist for epimerization of glucuronic acid to iduronic acid in DS, sulfation at the C-2 position of the uronic acids, and other patterns of sulfation found in unusual species of chondroitin.
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