Glycoprotein

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


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:

Monosaccharides

The eight sugars contained in glycoproteins.

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 immune cell recognition, especially in mammals. Examples of glycoproteins in the immune system are:

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 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.

See also

References

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

External links

ca:Glicoproteïna

da:Glykoprotein de:Glykoproteinit:Glicoproteina he:גליקופרוטאין mk:Гликопротеин nl:Glycoproteïnesimple:Glycoprotein fi:Glykoproteiini sv:Glykoprotein


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