Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending extracellularly, the extracellular segments are often glycosylated. Glycoproteins are often important integral membrane proteins, where they play a role in cell-cell interactions. Glycoproteins also occur in the cytosol, but their functions and the pathways producing these modifications in this compartment are less well-understood.
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Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending extracellularly, the extracellular segments are often glycosylated. Glycoproteins are often important integral membrane proteins, where they play a role in cell-cell interactions. Glycoproteins also occur in the cytosol, but their functions and the pathways producing these modifications in this compartment are less well-understood.
N-glycosylation and O-glycosylation
There are two types of glycosylation:
- In N-glycosylation (see on the right), the addition of sugar chains can happen at the amide nitrogen on the side chain of the asparagine.
- In O-glycosylation, the addition of sugar chains can happen on the hydroxyl oxygen on the side chain of hydroxylysine, hydroxyproline, serine, or threonine.
Monosaccharides
Monosaccharides commonly found in eukaryotic glycoproteins include:
The sugar group(s) can assist in protein folding or improve proteins' stability.
Examples
One example of glycoproteins found in the body is 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 white blood cell recognition, especially in mammals.Fact: date=December 2007 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:
- glycoprotein IIb/IIIa, an integrin found on platelets that is required for normal platelet aggregation and adherence to the endothelium.
- components of the zona pellucida, which surrounds the oocyte, and is important for sperm-egg interaction.
- structural glycoproteins, which occur in connective tissue. These help bind together the fibers, cells, and ground substance of connective tissue. They may also help components of the tissue bind to inorganic substances, such as calcium in bone.
- Glycoprotein-41 (gp41) and glycoprotein-120 (gp120) are HIV viral coat proteins.
Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.
Hormones
Hormones that are glycoproteins include:
- Follicle-stimulating hormone
- Luteinizing hormone
- Thyroid-stimulating hormone
- Human chorionic gonadotropin
- Alpha-fetoprotein
- Erythropoietin (EPO)
