Figure 5. Biosynthetic pathway of HS and relevant to the hexasaccharide model compounds.

HS biosynthesis is initiated from the polysaccharide containing -GlcA-GlcNAc- repeating units, followed by the modifications of N-deacetylase/N-sulphotransferase, C₅-epimerase, 2-O-sulphotransferase, 6-O-sulphotransferase and 3-O-sulphotransferase. The 2-O-sulphotransferase modification can be incomplete, resulting the polysaccharide product consists of IdoA2S-containing domain and IdoA-containing domain. Hexasaccharide 2 and 6 represent these two domain structures. Hexasaccharide 3 and 7 represent the domain structures after 6-O-sulphotransferase modification, and hexasaccharide 4 and 8 represent the structures after 3-O-sulphotransferase modification. The wavy lines represent the linker between domains in HS polysaccharide with unknown structures.