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Not to be confused with peptidoglycan.
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 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
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 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.[citation needed] 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[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][5]
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
- ^ 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
- ^ Anne Dell, Howard R Morris: "Glycoprotein structure determination by mass spectrometry", Science 291(5512), 2351-2356 (2001), Review
See also
External links
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