Glycoprotein

glycoprotein (glī'kōprō`tēn), organic compound composed of both a protein and a carbohydrate joined together in covalent chemical linkage. These structures occur in many life forms; they are prevalent and important in mammalian tissues. The attached carbohydrate may have several effects: it may help the protein to fold in the proper geometry, stabilize the protein, affect physical properties such as solubility or viscosity, helps it to orient correctly in a membrane, or make it recognizable to another biochemical or cell (see immunity). Many proteins released by cells to the blood and other fluids are glycoproteins. One set of glycoproteins also carry the blood group determinants. The carbohydrate portion of a glycoprotein is usually a small sugar or no more than 8 to 10 individual monosaccharide units. Combinations of up to seven of the many different sugar molecules known to occur in nature comprise the saccharide portions of mammalian glycoproteins: glucose, glucosamine, galactose, galactosamine, mannose, fucose, and sialic acid (a derivative of glucosamine). The linkage between the oligosaccharide and the protein occurs by formation of a chemical bond to only one of four protein amino acids: asparagine, hydroxylysine, serine, or threonine. Solutions of glycoproteins usually exhibit high viscosity, an observation explaining the highly viscous character of egg white, which is composed largely of the glycoprotein ovalbumin. Salivary mucus contains the glycoprotein called mucin. Among other glycoproteins, one particularly interesting example is isolated from certain antarctic fishes who survive near-freezing water temperatures as a result of freezing-point depression of their blood serum by a globular glycoprotein. This molecule is a remarkably effective freezing point depressant.

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Glycoprotein

A compound in which carbohydrate (sugar) is covalently linked to protein. The carbohydrate may be in the form of monosaccharides, disaccharides, oligosaccharides, or polysaccharides, and is sometimes referred to as glycan. The sugar may be linked to sulfate or phosphate groups. In different glycoproteins, 100–200 glycan units may be present. Therefore, the carbohydrate content of these compounds varies markedly, from 1% (as in the collagens), to 60% (in certain mucins), to >99% (in glycogen). See Collagen, Glycogen

Glycoproteins are ubiquitous in nature, although they are relatively rare in bacteria. They occur in cells, in both soluble and membrane-bound forms, as well as in the intercellular matrix and in extracellular fluids, and include numerous biologically active macromolecules. A number of glycoproteins are produced industrially by genetic engineering techniques for use as drugs; among them are erythropoietin, interferons, colony stimulating factors, and blood-clotting factors. See Genetic engineering

In most glycoproteins, the carbohydrate is linked to the polypeptide backbone by either N- or O-glycosidic bonds. A different kind of bond is found in glycoproteins that are anchored in cell membranes by a special carbohydrate-containing compound, glycosylphosphatidylinositol, which is attached to the C-terminal amino acid of the protein. A single glycoprotein may contain more than one type of carbohydrate-peptide linkage. N-linked units are typically found in plasma glycoproteins, in ovalbumin, in many enzymes (for example, the ribonucleases), and in immunoglobulins. O-linked units are found in mucins; collagens; and proteoglycans (typical constituents of connective tissues), including chondroitin sulfates, dermatan sulfate, and heparin. See Albumin, Carbohydrate, Enzyme, Immunoglobulin, Monosaccharide, Oligosaccharide, Polysaccharide, Protein

Within any organism, all molecules of a particular protein are identical. In contrast, a variety of structurally distinct carbohydrate units are found not only at different attachment sites of a glycoprotein but even at each single attachment site—a phenomenon known as microheterogeneity. For instance, ovalbumin contains one glycosylated amino acid, but over a dozen different oligosaccharides have been identified at that site, even in a preparation isolated from a single egg of a purebred hen.