Figure 4.

The mode of action of the EXT-proteins in heparan sulphate biosynthesis. After a tetrasaccharide linker is synthesised on conserved serine residues of the core protein, EXTL2 and/or EXTL3 initiate the polymerisation of the heparan sulphate chain by the addition of N-acetylglucosamine [60,107]. The EXT1/EXT2 complex subsequently catalyses further elongation of the heparan sulphate chain by adding alternating units of N-acetylglucosamine and glucuronic acid [59-62]. Subsequent deacetylation and sulphation of most N-acetylglucosamines, epimerisation of the glucoronic into iduronic acid and further sulphation result in a large spectrum of structural heterogenic heparan sulphate chains [58,108]. Adapted from Couchman et al [109] and Nybakken et al [110].