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, serine, or threonine. Possible carbohydrates include glucose, glucosamine, galactose, galactosamine, mannose, fucose, 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:
- molecules such as antibodies (immunoglobulins) which interact directly with antigens
- molecules of the major histocompatibility complex (or MHC), which are expressed on the surface of cells and interact with T-cells as part of the adaptive immune response.