The defining characteristic of the glycoproteins known as proteoglycans is the presence of O-linked acidic polysaccharides known as GAGs (glycosaminoglycans). The backbone of these linear polysaccharides is a repeating disaccharide, comprising N-acetyl hexosamine alternating with β-D-glucuronic acid, α-L-iduronic acid, or galactose. For some GAGs, partial deacetylation, epimerization of glucuronic acid, and substitution with N- and O-sulphates result in highly complex, heterogeneous structures. The interactions with proteins through which GAGs exert their biological effects depend on the resulting sequences. Some proteins, for example antithrombin, have highly specific sequence requirements for their GAG ligand [in this case heparin or HS (heparan sulphate)]; others, for example the fibroblast growth factors, are less demanding. GAGs, in particular HS, play a role as co-receptors for some cytokines. In addition, HS is thought to be important for the localization of cytokines, acting both as a tissue store and as a mediator of morphogen gradient formation in development. The structural determinants of GAG–cytokine interactions are therefore clearly important to understanding the biology of development, wound healing and the immune system. No single paradigm has been identified for such interactions, and the search for general principles underlying involvement of GAGs in cytokine function is at an early stage.

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