Glycoprotein
A glycoprotein is a macromolecule composed of a protein and a carbohydrate (an oligosaccharide). The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. The addition of sugar chains can happen either at asparagine, and is termed N-glycosylation, or at hydroxylysine, hydroxyproline, serine, or threonine, and is termed O-glycosylation. Monosaccharides commonly found in eukaryotic glycoproteins include glucose, N-acetylglucosamine, galactose, N-acetylgalactosamine, mannose, fucose, xylose and N-acetylneuraminic acid (also known as sialic acid). The sugar group(s) can assist in protein folding or improve proteins' stability.
Glycoproteins are often used in proteins that are at least in part located in extracellular space (that is, outside the cell).
One example of glycoproteins found in the body are mucins, which are secreted in the mucus of the respiratory and digestive tracts. The sugars attached to mucins give them considerable water-holding capacity and also make them resistant to proteolysis by digestive enzymes.
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.
Other examples of glycoproteins include:
- components of the zona pellucida, which surrounds the oocyte, and is important for sperm-egg interaction.
Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.
Hormones that are glycoproteins include:
- Follicle stimulating hormone
- Luteinizing hormone
- Thyroid stimulating hormone
- human chorionic gonadotropin
- Alpha-fetoprotein
- Erythropoietin (EPO)