Glycoprotein

Overview
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
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 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: Other examples of glycoproteins include:
 * 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.
 * 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)

Analysis
A variety of methods used in detection, purification, and structural analysis of glycoproteins are