Proteoglycans: their structure, interactions and molecular organization in cartilage.

Abstract

The term ‘proteoglycan’ was introduced in 1967 (see Balazs. 1970) to describe the family of molecules in which glycosaminoglycan chains are linked to protein. However. their rate of diffusion into textbooks of biochemistry has been rather slow, and I will therefore begin this presentation with a general introduction, which I hope will set the basis for their structural identity and indicate how their properties may be of importance in a broad biological context, before describing in more detail studies with proteoglycans from cartilage. Glycosaminoglycans are characteristic components of the extracellular space of vertebrate tissues. They are long unbranched polysaccharide chains which contain many acidic, carboxylate and/or sulphate groups. They do not normally occur as free chains in uiuo, but as proteoglycans, in which many like chains are linked by the reducing end of the terminal sugar residue to a protein. A whole technology was developed for the isolation, identification and quantification of glycosaminoglycan chains before their attachment to proteins was clearly established (Roden et al., 1972). Proteoglycan research thus began as a specialized area of carbohydrate chemistry. It is now apparent that in understanding the biological role of these macromolecules, the structure and interactions of the proteoglycans cannot be assessed by the analysis of the glycosaminoglycan chains alone. However, there are many difficulties in isolating and purifying intact proteoglycans, and it is only in recent years that a beginning has been made in exploring the full diversity of proteoglycan structure. There are seven types of glycosaminoglycan. on the basis of different repeating disaccharide units, which occur in mammalian tissues (Table I ) (see Muir & Hardingham, 1975). With the exception of hyaluronate, they have all been reported to occur linked to protein in different types of proteoglycan. The linkage to protein is via a neutral trisaccharide (Gal-Gal-Xyl) at the reducing end of the glycosaminoglycan chain, and xylose forms an 0-glycosidic bond with a serine residue in the protein. Keratan sulphate contains no neutral trisaccharide, but is linked via an 0-glycosidic bond between N-acetylgalactosamine to The Sixteenth Colworth Medal Lecture