A 3D-structural model of unsulfated chondroitin from high-field NMR: 4-sulfation has little effect on backbone conformation

The glycosaminoglycan chondroitin sulfate is essential in human health and disease but exactly how sulfation dictates its 3D-strucutre at the atomic level is unclear. To address this, we have purified homogenous oligosaccharides of unsulfated chondroitin (with and without ¹⁵N-enrichment) and analysed them by high-field NMR to make a comparison published chondroitin sulfate and hyaluronan 3D-structures. The result is the first full assignment of the tetrasaccharide and an experimental 3D-model of the hexasaccharide (PDB code 2KQO). In common with hyaluronan, we confirm that the amide proton is not involved in strong, persistent inter-residue hydrogen bonds. However, in contrast to hyaluronan, a hydrogen bond is not inferred between the hexosamine OH-4 and the glucuronic acid O5 atoms across the β(1→3) glycosidic linkage. The unsulfated chondroitin bond geometry differs slightly from hyaluronan by rotation about the β(1→3) ψ dihedral (as previously predicted by simulation), while the β(1→4) linkage is unaffected. Furthermore, comparison shows that this glycosidic linkage geometry is similar in chondroitin-4-sulfate. We therefore hypothesise that both hexosamine OH-4 and OH-6 atoms are solvent exposed in chondroitin, explaining why it is amenable to sulfation and hyaluronan is not, and also that 4-sulfation has little effect on backbone conformation. Our conclusions exemplify the value of the 3D-model presented here and progress our understanding of glycosaminoglycan molecular properties.