= Proteoglycan
A <proteoglycan> consists of a core <protein> bearing one or more covalently attached <glycosaminoglycan> chains. These chains are <polysaccharides> containing <amino sugars>. Thus the molecule combines a sequence-defined <polypeptide> with long carbohydrate components, whose number and composition can vary.<Proteoglycan in the IUPAC Gold Book>[P04899, protein bearing glycosaminoglycans; G02659, carbohydrate-chain definition]{ref}
An illuminating example comes from <cartilage>, where individual <proteoglycans> can associate with <hyaluronic acid> into larger aggregates. A <proteoglycan> molecule and an aggregate of many such molecules are different organizational levels: a change in aggregate size need not mean that every core <protein> has changed length.
In 1974, researchers digested bovine cartilage aggregates with <enzymes> and separated the resulting components by <chromatography>. They isolated a core-protein-derived fraction and a smaller linking-protein fraction that both retained binding to <hyaluronic acid>. Disassembly and selective recovery of binding components helped distinguish the carbohydrate-rich molecule from the interactions holding a larger aggregate together.<Aggregation of Cartilage Proteoglycans: III. Characteristics of the Proteins Isolated from Trypsin Digests of Aggregates by D. Heinegård and others>[Journal of Biological Chemistry 249, pp. 4250–4256; abstract, digestion, separation and retained hyaluronic-acid binding]{ref}
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