• The initiation of heparan sulfate biosynthesis is catalyzed by an αGlcNAc transferase activity that is termed 'GlcNAcT-I'. Addition of GlcA (HS-GlcAT-II) and GlcNAc (GlcNAcT-II) then ensues to form an HS backbone.
• These activities are mediated by several enzymes of the EXT (EXT-1/2) and EXT-like (EXTL-1/2/3) gene family. EXTL-1, -2 and -3 all have only HexNAc transferase activity, and they were discovered after EXT1/2. EXT1/2 have dual enzyme activity.
• EXTL3 has both GlcNAcT-I activity and GlcNAcT-II activity. Thus, it participate in both chain initiation and extension. EXTL3 knockout is embryonic lethal, it is most likely responsible for adding the first GlcNAc to the tetrasaccharide linker to initiate HS chain extension by EXT1/EXT2. Homozygous mutation of EXTL3 has recently been found to cause severe developmental delay.
• EXTL2 (α1,4-N-Acetylhexosaminyltransferase) catalyzes the transfer of a α(1-4)GlcNAc or alternatively α(1-4)GalNAc residue from UDP-GalNAc/GlcNAc to the core oligosaccharide representing the GAG-protein linker (i.e. GlcNAcT-I activity). EXTL2 is widely distributed in various mammalian tissues and cells as well as in serum. α-GalNAc activity of EXTL2 may suppress HS biosynthesis, as this can no longer be extended by EXT1/EXT2. EXTL2 knockout mice is largely normal but shown various defects when challenged with disease models.
• EXTL1 only has GlcNAcT-II activity.
• EXT1 and EXT2 form a heterodimeric complex of Type II-proteins in cis-Golgi. EXT1-EXT2 interaction and core proteins are critical and required for the heparan chain polymerization reactions.
• EXT1 (Exostosin) and EXT2 both have GlcAT and GlcNAcT activities (i.e. they are bifunctional) and are together responsible for HS chain polymerization. Knocking out either EXT1 or EXT2 is embryonic lethal.
• Even heterozygous mutations in EXT1 or EXT2 led to reduction in HS biosynthesis and can cause a bone disease called Exostosis.