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xylose residue linkage to a specific serine residue in the protein core, with the exception that KS chains are attached to proteins via N- and O-linked glyco-sidic linkages.

Hyaluronan is very large and can have a molecular weight of up to 10 000 kDa. It assumes a randomly kinked, coil structure which occupies a large solution volume and endows solutions with high viscosity. These molecules can self-associate and form networks; in addition, a number of proteoglycans can form supramolecular aggregates with HA. It therefore has an important role in the development and function of many tissues.

The advent of cloning and sequencing techniques has greatly increased the understanding of the protein core of proteoglycans and now a number of the protein cores can be classified into families on the basis of their domain structure. In the case of the small matrix proteoglycans (decorin, biglycan, fibromodulin) the characteristic features of the protein core are the leucine-rich repeats, the presence of which accounts for some of the protein-protein interactions that these small proteoglycans exhibit. The large aggregating proteoglycans, aggrecan (found in cartilage) and versican (found in skin) also share similarities in the protein core, principal of which is the hyaluronate-binding region at the N-terminus. A similar domain is also found in the cell surface proteoglycan CD44.

The highly acidic and hydrophilic GAG chains might be expected to dominate the functional properties of these molecules, and while much of the tissue hydration caused by these molecules is attributable to this part of the molecule, specific interactions are mediated by both the GAG chains and the protein core. These include regulating protease activity, binding growth factors and mediating interactions with other matrix components.

Interactions and hydration of proteoglycans are key determinants of articular cartilage function, where the ability to withstand compressive forces serves to protect the underlying bone. In cartilage, proteoglycans, such as aggrecan, form a supra-molecular aggregate with hyaluronan and this interaction is stabilized by link protein (Figure 2). This helps maintain a high concentration of proteoglycans in cartilage, which is important in determining the hydration of the tissue. Collagen fibers provide the scaffold that resists tensile forces, while the proteoglycan aggregates provide a hydrated, viscous gel that absorbs compressive forces.

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