Figure 2.

Biosynthetic pathway of DS side chain of PGs by diverse glycosyltransferases, epimerases, and sulfotransferases. Following the biosynthesis of core proteins (green), the common GAG-protein linker region, GlcUAβ1-3Galβ1-3Galβ1-4Xylβ1-, is constructed by XylT, GalT-I, GalT-II, and GlcAT-I on the specific Ser residue(s) of core proteins. After the formation of the linker region, chondroitin synthases build up the chondroitin backbone. Subsequently, the epimerization of GlcUA moieties as well as sulfation of mainly GalNAc and a small proportion of IdoUA residues are catalyzed by DSE as well as D4ST and UST, respectively. Each enzyme and its coding gene are indicated by blue and red characters, respectively.