Definitions
glycoprotein [glahy-koh-proh-teen, -tee-in]

glycoprotein

[glahy-koh-proh-teen, -tee-in]
glycoprotein, organic compound composed of both a protein and a carbohydrate joined together in covalent chemical linkage. These structures occur in many life forms; they are prevalent and important in mammalian tissues. The attached carbohydrate may have several effects: it may help the protein to fold in the proper geometry, stabilize the protein, affect physical properties such as solubility or viscosity, helps it to orient correctly in a membrane, or make it recognizable to another biochemical or cell (see immunity). Many proteins released by cells to the blood and other fluids are glycoproteins. One set of glycoproteins also carry the blood group determinants. The carbohydrate portion of a glycoprotein is usually a small sugar or no more than 8 to 10 individual monosaccharide units. Combinations of up to seven of the many different sugar molecules known to occur in nature comprise the saccharide portions of mammalian glycoproteins: glucose, glucosamine, galactose, galactosamine, mannose, fucose, and sialic acid (a derivative of glucosamine). The linkage between the oligosaccharide and the protein occurs by formation of a chemical bond to only one of four protein amino acids: asparagine, hydroxylysine, serine, or threonine. Solutions of glycoproteins usually exhibit high viscosity, an observation explaining the highly viscous character of egg white, which is composed largely of the glycoprotein ovalbumin. Salivary mucus contains the glycoprotein called mucin. Among other glycoproteins, one particularly interesting example is isolated from certain antarctic fishes who survive near-freezing water temperatures as a result of freezing-point depression of their blood serum by a globular glycoprotein. This molecule is a remarkably effective freezing point depressant.
Not to be confused with peptidoglycan.

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.

N-glycosylation and O-glycosylation

There are two types of glycosylation:

Monosaccharides

Monosaccharides commonly found in eukaryotic glycoproteins include:

The principal sugars found in human glycoproteins
Sugar Type Abbreviation
Galactose Hexose Gal
Glucose Hexose Glc
Mannose Hexose Man
N-Acetylneuraminic acid Sialic acid (nine C atoms) NeuAc
Fucose Deoxyhexose Fuc
N-Acetylgalactosamine Aminohexase GalNAc
N-Acetylglucosamine Aminohexase GlcNAc
Xylose Pentose Xyl

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. Examples of glycoproteins in the immune system are:

Other examples of glycoproteins include:

Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.

Hormones

Hormones that are glycoproteins include:

Functions

Some functions served by glycoproteins
Function Glycoproteins
Structural molecule Collagens
Lubricant and protective agent Mucins
Transport molecule Transferrin, ceruloplasmin
Immunologic molecule Immunoglobins, histocompatibility antigens
Hormone Chorionoic gonadotropin, thyroid-stimulating hormone (TSH)
Enzyme Various, eg, alkaline phosphatase
Cell attachment-recognition site Various proteins involved in cell-cell (eg, sperm-oocyte), virus-cell, bacterium-cell, and hormone cell interactions
Antifreeze Certain plasma proteins of coldwater fish
Interact with specific carbohydrates Lectins, selectins (cell adhesion lectins), antibodies
Receptor Various proteins involved in hormone and drug action
Affect folding of certain proteins Calnexin, calreticulin
Regulation of development Notch and its analogs, key proteins in development
Hemostasis (and thrombosis) Specific glycoproteins on the surface membranes of platelets

Analysis

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

Some important methods used to study glycoproteins
Method Use
Periodic acid-Schiff stain Detects glycoproteins as pink bands after electrophoretic separation.
Incubation of cultured cells with glycoproteins as radioactive decay bands Leads to detection of a radioactive sugar after electrophoretic separation.
Treatment with appropriate endo- or exoglycosidase or phospholipases Resultant shifts in electrophoretic migration help distinguish among proteins with N-glycan, O-glycan, or GPI linkages and also between high mannose and complex N-glycans.
Agarose-lectin column chromatography To purify glycoproteins or glycopeptides that bind the particular lectin used.
Compositional analysis following acid hydrolysis Identifies sugars that the glycoprotein contains and their stoichiometry.
Mass spectrometry Provides information on molecular mass, composition, sequence, and sometimes branching of a glycan chain.
NMR spectroscopy To identify specific sugars, their sequence, linkages, and the anomeric nature of glycosidic chain.
Methylation (linkage) analysis To determine linkage between sugars.
Amino acid or cDNA sequencing Determination of amino acid sequence.

References

See also

External links

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