Structure and Metabolism of Connective Tissue Proteoglycans

Abstract

With the possible exception of hyaluronic acid, the connective tissue polysaccharides are all synthesized by their parent cells as components of proteoglycans. In these substances, a number of polysaccharide chains are covalently linked to a protein core; e.g., in the proteoglycan of bovine nasal cartilage, which is the prototype of molecules of this kind, close to 100 chondroitin sulfate chains, with a molecular weight of approximately 20,000, and slightly fewer keratan sulfate chains are linked to a core protein (mol. wt. 200,000) which constitutes 7–8% of the entire molecule. In many respects, the proteoglycans are similar to other protein-bound complex carbohydrates, and the conspicuous polysaccharide component per se does not distinguish the proteoglycans from the class of glycoproteins; e.g., there are members of the glycoprotein class, such as the blood group substances, which have a high relative content of carbohydrate consisting of a substantial number of monosaccharide units. Rather, the segregation of the proteoglycans into a separate category is based on a few specific characteristics: (1) each polysaccharide consists of repeating disaccharide units in which a hexosamine, d-glucosamine, or d-galactosamine is always present; (2) all connective tissue polysaccharides except keratan sulfate contain a uronic acid, either d-glucuronic acid or its 5-epimer, l-iduronic acid, or both; (3) ester sulfate groups are present in all members of the group except in hyaluronic acid; in addition, N-sulfate groups are found in heparin and heparan sulfate. Although certain other bipolymers are known to contain ester sulfate, e.g., some epithelial mucins (Horowitz, 1977), these compounds are clearly distinguishable from the connective tissue polysaccharides by the other criteria indicated above. It may also be mentioned that the d-glucuronic-acid-containing repeating disaccharide of chondroitin, N-acetylchondrosine, has recently been identified as a component of thyroglobulin (Spiro, 1977); however, since the disaccharide is present as a single unit, thyroglobulin may not be considered a proteoglycan.