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.
Monosaccharides commonly found in eukaryotic glycoproteins include:
|N-Acetylneuraminic acid||Sialic acid (nine C atoms)||NeuAc|
The sugar group(s) can assist in protein folding or improve proteins' stability.
Other examples of glycoproteins include:
|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|
|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.|
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Feb 18, 2008; Investigators publish new data in the report 'Reversible sialylation: synthesis of cytidine 5'-monophospho-N-acetylneuraminic...