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Proteoglycans

There is another class of complex macro molecules that can contain as much as 95% or more carbohydrate, with properties that may resemble those of polysaccharides more than proteins. To distinguish these compounds from other glycoproteins, they are referred to as proteoglycans.Their carbohydrate' chains are called glycosaminoglycansor by their older name, mucopolysaccharides,especially in reference to the group of storage diseases, mucopolysaccharidosis, which result from an inability to degrade these molecules.

Proteoglycans are high molecular weight polyanionic compounds consisting of many different glycosaminoglycan chains linked covalently to a protein core. Although six distinct classes of glycosaminoglycans are now recognized, certain features are common to all classes. The long unbranched heteropolysaccharide chains are made up largely of disaccharide repeating units, in which one sugar is a hexosamine and the other a uronic acid. Other common constituents of glycosaminoglycans are sulfate groups, linked by ester bonds to certain monosaccharides or by amide bonds to the amino group of glucosamine. However, hyaluronate is not sulfated and is not covalently attached to protein. The carboxyls of uronic acids and sulfate groups contribute to the highly charged polyanionic nature of glycosaminoglycans. Both their electrical charge and macromolecular structure aid in their biological role as lubricants and support elements in connective tissue. Glycosaminoglycans are predominantly components of the extracellular matrix and cell surfaces, and increasingly more dynamic roles in cell adhesion and signaling, as well as a host of other biological activities, have been elucidated for the proteoglycans.

There are 6 types of glucosaminoglycans: hyaluronic acid, chondroitin sulfates A, B, C, D, keratin sulfates and heparines. They differ each from other by nature of uronic acids, hexosamines, by quantity of residues of sulphuric acid chemical lincages between monosaccarides, molecular masses, properties.

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