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.[ref]
Proteoglycan in the IUPAC Gold Book. P04899, protein bearing glycosaminoglycans; G02659, carbohydrate-chain definition
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.[ref]
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
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