Glycoprotein 164912 225636649 2008-07-14T17:49:17Z 84.63.97.203 ''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]] [[da:Glykoprotein]] [[de:Glykoprotein]] [[es:Glicoproteína]] [[fr:Glycoprotéine]] [[it:Glicoproteina]] [[he:גליקופרוטאין]] [[mk:Гликопротеин]] [[ms:Glikoprotein]] [[nl:Glycoproteïne]] [[ja:糖タンパク質]] [[no:Glykoprotein]] [[oc:Glicoproteïna]] [[pl:Glikoproteiny]] [[pt:Glicoproteína]] [[ru:Гликопротеины]] [[simple:Glycoprotein]] [[fi:Glykoproteiini]] [[sv:Glykoprotein]] [[tr:Glikoprotein]] [[uk:Глікопротеїни]] [[zh:糖蛋白]]