Glycoprotein

Article By:

Schwartz, Nancy B. Department of Pediatrics, University of Chicago, Chicago, Illinois.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.757295

A class of conjugated proteins containing one or more saccharides that lack a serial repeat unit (a disaccharide unit that may be repeated up to 25–30 times, characteristic of the glycosamino glycan structures of proteoglycans) and are bound covalently to a protein. The amount of carbohydrate in glycoproteins is highly variable: Immunoglobulin G (IgG) glycoproteins contain about 4% of carbohydrate by weight, glycophorin contains 60%, and human gastric glycoprotein contains 82%. The carbohydrate may be distributed along the entire polypeptide chain or concentrated in defined regions. In human glycophorin A, the carbohydrate is restricted to the extracellular NH₂-terminal half of the polypeptide chain. The carbohydrate or glycan (the terms carbohydrate, glycan, sugar, and saccharide are used interchangeably) components of glycoproteins usually contain less than 12–15 sugar residues. Some consist of one sugar moiety, as in the submaxillary gland glycoprotein (a single N-acetyl-α-d-galactosaminyl residue) and in some mammalian collagens (a single α-d-galactosyl residue). Orthologous glycoproteins from different animal species often have identical primary protein structures, but different carbohydrate components. For example, pancreatic ribonucleases A and B have identical primary structures and similar specificity toward substrates, but differ significantly in their carbohydrate compositions. See also: Carbohydrate; Oligosaccharide; Peptide; Protein

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