Glycoprotein
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''Not to be confused with [[peptidoglycan]].''
[[Image:Glicoprotein.svg|thumb|right|N-linked protein glycosylation (N-glycosylation of N-glycans) at Asn residues (Asn-x-Ser/Thr motifs) in glycoproteins.<ref> Ruddock & Molinari (2006) Journal of Cell Science 119, 4373-4380</ref>]]
'''Glycoproteins''' are proteins that contain [[oligosaccharide]] chains ([[glycans]]) covalently attached to their [[Peptide|polypeptide]] side-chains. The carbohydrate is attached to the protein in a [[cotranslational modification|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:
* In [[Glycosylation#N-linked glycosylation|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 [[Glycosylation#O-linked glycosylation|O-glycosylation]], the addition of sugar chains can happen on the hydroxyl oxygen on the side chain of [[hydroxylysine]], [[hydroxyproline]], [[serine]], or [[threonine]].
==Monosaccharides ==
[[Image:Glykoproteine Zucker.svg|thumb|The eight sugars contained in glycoproteins.]]
Monosaccharides commonly found in eukaryotic glycoproteins include:<ref>Robert K. Murray, Daryl K. Granner & Victor W. Rodwell: "Harper's Illustrated Biochemistry 27th Ed.", p. 526, McGraw-Hill, 2006</ref>
{| class="wikitable"
|+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]]
| [[Deoxy sugar|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 [[mucin]]s, 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 [[mammal]]s.{{Fact|date=December 2007}} Examples of glycoproteins in the [[immune system]] are:
* molecules such as [[antibody|antibodies]] (immunoglobulins), which interact directly with [[antigen]]s
* molecules of the ''[[major histocompatibility complex]]'' (or MHC), which are expressed on the surface of cells and interact with [[T cell]]s as part of the adaptive immune response.
Other examples of glycoproteins include:
* [[glycoprotein IIb/IIIa]], an integrin found on [[platelet|platelets]] that is required for normal platelet aggregation and adherence to the [[endothelium]].
* 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==
[[Hormone]]s that are glycoproteins include:
* [[Follicle-stimulating hormone]]
* [[Luteinizing hormone]]
* [[Thyroid-stimulating hormone]]
* [[human chorionic gonadotropin]]
* [[Alpha-fetoprotein]]
* [[Erythropoietin|Erythropoietin (EPO)]]
==Functions==
{| class="wikitable"
|+ Some functions served by glycoproteins<ref>Ibid., p. 524</ref>
|-
! Function
! Glycoproteins
|-
| Structural molecule
| [[Collagen]]s
|-
| Lubricant and protective agent
| [[Mucin]]s
|-
| Transport molecule
| [[Transferrin]], [[ceruloplasmin]]
|-
| Immunologic molecule
| [[Antibody|Immunoglobins]], [[histocompatibility]] antigens
|-
| Hormone
| [[Human chorionic gonadotropin|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
| [[Lectin]]s, [[selectin]]s (cell adhesion lectins), antibodies
|-
| [[Receptor]]
| Various proteins involved in hormone and drug action
|-
| Affect folding of certain proteins
| [[Calnexin]], [[calreticulin]]
|-
| Regulation of development
| [[Notch signaling|Notch]] and its analogs, key proteins in development
|-
| [[Hemostasis]] (and [[thrombosis]])
| Specific glycoproteins on the surface membranes of [[platelet]]s
|-
|}
== Analysis ==
A variety of methods used in detection, purification, and structural analysis of glycoproteins are<ref>Ibid., p. 525</ref><ref>Anne Dell, Howard R Morris: "Glycoprotein structure determination by mass spectrometry", Science 291(5512), 2351-2356 (2001), Review</ref>
{| class="wikitable"
|+Some important methods used to study glycoproteins
|-
! Method
! Use
|-
| [[Periodic acid-Schiff stain]]
| Detects glycoproteins as pink bands after [[Electrophoresis|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 [[Endoglycosidase|endo-]] or [[exoglycosidase]] or [[phospholipase]]s
| 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 [[Complementary DNA|cDNA]] sequencing
| Determination of amino acid sequence.
|-
|}
==References==
{{reflist}}
== See also ==
* [[Proteoglycan]]
* [[Glycocalyx]]
* [[Gp120]]
* [[Gp41]]
== External links ==
* [http://www.ecosci.jp/chem10/weekmol101j_e.html Structure of Glycoprotein and Carbohydrate Chain] - Home Page for Learning Environmental Chemistry
* [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?indexed=google&rid=stryer.section.1531 Biochemistry 5thE 11.3. Carbohydrates Can Be Attached to Proteins to Form Glycoproteins]
* [http://www.sciencemag.org/feature/data/carbohydrates.dtl#glycoproteins Carbohydrate Chemistry and Glycobiology: A Web Tour] SPECIAL WEB SUPPLEMENT Science 23 March 2001 Vol 291, Issue 5512, Pages 2263-2502
* {{MeshName|Glycoproteins}}
{{Glycoproteins}}
{{Glycoprotein metabolism enzymes}}
{{Glycoproteinoses}}
[[Category:Glycoproteins]]
[[Category:carbohydrates]]
[[ar:بروتين سكري]]
[[ca:Glicoproteïna]]
[[cs:Glykoproteiny]]
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[[it:Glicoproteina]]
[[he:גליקופרוטאין]]
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[[ms:Glikoprotein]]
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