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Polysaccharides also occur in covalent combination with proteins and lipids. In the proteoglycans, the protein portion of the molecule represents only a small portion of the total molecular weight.
These macromolecules are also known as mucopolysaccharides.
Among the proteoglycans, one of the most extensively studied is cartilage proteoglycan, which is found extracellularly in cartilage and bone.
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In cartilage proteoglycan (Fig. 5-21), strands of protein, called core protein, extend radially from a long, central hyaluronic acid molecule. Three carbohydrate- bearing regions of each core protein strand may be identified.
The first region contains numerous oligosaccharides, the second contains keratan sulfate chains, and the third contains chondroitin sulfate chains. This complex of protein and polysaccharide gives cartilage its special resilience and tensile properties.
The cell walls of certain bacteria contain peptidoglycans. These are heteropolymers of TV-acetyl- glucosamine and Af-acetylmuramic acid in which short peptides are linked to the polysaccharide backbone. In the glycoproteins, the carbohydrate portion consists of much shorter chains, which are often branched.
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The chains may be few in number or there may be so many as to account for more than half of the total molecular weight. Glycoproteins serve diverse roles in cells and tissues and include certain enzymes, hormones, and antibodies or immunoglubins.
Glycolipids are covalent combinations of carbohydrate and lipid. The carbohydrate portion may be a single monosaccharide or a linear or branched chain. Glycolipids are components of most cell membranes.