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Glycoprotein

Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide. 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.

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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:^[2]

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	GlaNac
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 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 white blood cell recognition, especially in mammals.^{[citation needed]} 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:

Functions

Some functions served by glycoproteins^[3]
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^[4]

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.
Sepharose-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

^ Ruddock & Molinari (2006) Journal of Cell Science 119, 4373-4380
^ Robert K. Murray, Daryl K. Granner & Victor W. Rodwell: "Harper's Illustrated Biochemistry 27th Ed.", p. 526, McGraw-Hill, 2006
^ Ibid., p. 524
^ Ibid., p. 525