Glycoprotein

N-linked protein glycosylation (N-glycosylation of N-glycans) at Asn residues (Asn-x-Ser/Thr motifs) in glycoproteins.^[1]

Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. Secreted extracellular proteins are often glycosylated. In proteins that have segments extending extracellularly, the extracellular segments are also glycosylated. Glycoproteins are often important integral membrane proteins, where they play a role in cell–cell interactions. Glycoproteins are also formed in the cytosol, but their functions and the pathways producing these modifications in this compartment are less understood.^[2]

Types of glycosylation

There are several types of glycosylation, although the first two are the most common.

• In N-glycosylation, sugars are attached to nitrogen, typically on the amide side-chain of asparagine.
• In O-glycosylation, sugars are attached to oxygen, typically on serine or threonine but also on non-canonical amino acids such as hydroxylysine & hydroxyproline.
• In P-glycosylation, sugars are attached to phosphorus on a phosphoserine.
• In C-glycosylation, sugars are attached directly to carbon, such as in the addition of mannose to tryptophan.
• In glypiation, a GPI glycolipid is attached to the C-terminus of a polypeptide, serving as a membrane anchor.

Monosaccharides

Eight sugars commonly found in glycoproteins.

Monosaccharides commonly found in eukaryotic glycoproteins include:^[3]:526

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 when 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. 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 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.

• Miraculin, is a glycoprotein extracted from Synsepalum dulcificum a berry which alters human tongue receptors to recognize sour foods as sweet.^[5]

Variable surface glycoproteins allow the sleeping sickness Trypanosoma parasite to escape the immune response of the host.

Hormones

Hormones that are glycoproteins include:

• Follicle-stimulating hormone
• Luteinizing hormone
• Thyroid-stimulating hormone
• Human chorionic gonadotropin
• Alpha-fetoprotein
• Erythropoietin (EPO)

Functions

Analysis

A variety of methods used in detection, purification, and structural analysis of glycoproteins are^{[3]:525[7][6]}

See also

References

External links

• Glycan Recognizing Proteins
• Structure of Glycoprotein and Carbohydrate Chain – Home Page for Learning Environmental Chemistry
• Biochemistry 5thE 11.3. Carbohydrates Can Be Attached to Proteins to Form Glycoproteins
• Carbohydrate Chemistry and Glycobiology: A Web Tour SPECIAL WeB SUPPLEMENT Science 23 March 2001 Vol 291, Issue 5512, Pages 2263–2502
• Glycoproteins at the US National Library of Medicine Medical Subject Headings (MeSH)
• Biological Importance of the glycosylation of a protein