Comparative aspects of glycoprotein structure

Glycoproteins have a wide distribution in nature and serve a vast number of functions. There are glycoprotein enzymes and hormones; glycoproteins are found in blood and secretions, in cell membranes, and in connective tissue. Of all the biologically occurring macromolecules the glycoproteins, which consist of carbohydrate moieties convalently linked to a polypeptide backbone, represent the most diverse group, ranging from substances in which the carbohydrate component represents less than 1% of the total weight to those in which it represents over 80% of the total. The proteoglycans, which are classified separately from other glycoproteins and include the chondroitin sulfates, dermatan sulfates, and heparin primarily carbohydrate in the form of numerous heteropolysaccharide chains attached to a polypeptide chain at closely spaced intervals. The sugars that commonly occur in glycoproteins include galactose, mannose, glucose. N-acetylglucosamine, N-acetylgalactosamine, sialic acids, fucose, and xylose. The proteoglycans also contain various uronic and sulfated amino sugars.