A glycoprotein is a macromolecule composed of a protein and a carbohydrate (a sugar). The carbohydrate is usually attached to the protein in a posttranslational modification, at either asparagine, hydroxylysine, hydroxyproline, serine, or threonine. Possible carbohydrates include glucose, glucosamine, galactose, galactosamine , mannose, fructose, and sialic acid.
The sugar group can assist in protein folding or improve its stability. Glycoproteins are often used in proteins that are at least in part located in extracellular space (that is, outside the cell). Glycoproteins are important for immune cell recognition, especially in mammals. Examples of glycoproteins in the immune system are:
Other examples of glycoproteins include: