Glycoprotein

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Overview

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 their polypeptide backbones. Basically, glycoprotein is a biomolecule composed of a protein and a carbohydrate (an oligosaccharide). The carbohydrate is attached to the protein in a cotranslational or posttranslational modification.

In proteins that have segments extending extracellularly, the extracellular segments are often glycosylated.

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 asparagine.

• In O-glycosylation, the addition of sugar chains can happen at hydroxylysine, hydroxyproline, serine, or threonine.

Monosaccharides

The eight sugars contained in glycoproteins.

Monosaccharides commonly found in eukaryotic glycoproteins include^[2]:

The sugar group(s) can assist in protein folding or improve proteins' stability.

Examples

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.
• 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.

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)

Functions

Analysis

A variety of methods used in detection, purification, and structural analysis of glycoproteins are^[4]

See also

• Proteoglycan
• Glycocalix
• Gp120
• Gp41

References

External links

• 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
• MeSH Glycoproteins

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