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Glycoprotein
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== Examples == The unique interaction between the oligosaccharide chains have different applications. First, it aids in quality control by identifying misfolded proteins.<ref name="Lehninger_2013" /> The oligosaccharide chains also change the solubility and polarity of the proteins that they are bonded to.<ref name="Lehninger_2013" /> For example, if the oligosaccharide chains are negatively charged, with enough density around the protein, they can repulse proteolytic enzymes away from the bonded protein.<ref name="Lehninger_2013" /> The diversity in interactions lends itself to different types of glycoproteins with different structures and functions.<ref name="Gamblin_2009" /> 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 when 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.{{Citation needed|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. * sialyl Lewis X antigen on the surface of leukocytes. H antigen of the ABO blood compatibility antigens. Other examples of glycoproteins include: * gonadotropins (luteinizing hormone and follicle-stimulating hormone) * [[glycoprotein IIb/IIIa]], an integrin found on [[platelet]]s 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]]. * Glycoprotein-41 ([[gp41]]) and glycoprotein-120 ([[gp120]]) are HIV viral coat proteins. Soluble glycoproteins often show a high [[viscosity]], for example, in [[egg white]] and [[blood plasma]]. * [[Miraculin]], is a glycoprotein extracted from ''[[Synsepalum dulcificum]]'' a [[berry]] which alters human tongue receptors to recognize sour foods as sweet.<ref name="jbc-263-23-11536">{{cite journal | vauthors = Theerasilp S, Kurihara Y | title = Complete purification and characterization of the taste-modifying protein, miraculin, from miracle fruit | journal = The Journal of Biological Chemistry | volume = 263 | issue = 23 | pages = 11536β11539 | date = August 1988 | doi = 10.1016/S0021-9258(18)37991-2 | pmid = 3403544 | doi-access = free }}</ref> [[Variable surface glycoprotein]]s allow the sleeping sickness ''Trypanosoma'' parasite to escape the immune response of the host. The viral spike of the human immunodeficiency virus is heavily glycosylated.<ref>{{cite journal |vauthors=Pritchard LK, Vasiljevic S, Ozorowski G, Seabright GE, Cupo A, Ringe R, Kim HJ, Sanders RW, Doores KJ, Burton DR, Wilson IA, Ward AB, Moore JP, Crispin M |display-authors=6 |title=Structural Constraints Determine the Glycosylation of HIV-1 Envelope Trimers |journal=Cell Reports |volume=11 |issue=10 |pages=1604β1613 |date=June 2015 |pmid=26051934 |pmc=4555872 |doi=10.1016/j.celrep.2015.05.017}}</ref> Approximately half the mass of the spike is glycosylation and the glycans act to limit antibody recognition as the glycans are assembled by the host cell and so are largely 'self'. Over time, some patients can evolve antibodies to recognise the HIV glycans and almost all so-called 'broadly neutralising antibodies (bnAbs) recognise some glycans. This is possible mainly because the unusually high density of glycans hinders normal glycan maturation and they are therefore trapped in the premature, high-mannose, state.<ref>{{cite journal |vauthors=Pritchard LK, Spencer DI, Royle L, Bonomelli C, Seabright GE, Behrens AJ, Kulp DW, Menis S, Krumm SA, Dunlop DC, Crispin DJ, Bowden TA, Scanlan CN, Ward AB, Schief WR, Doores KJ, Crispin M |display-authors=6 |title=Glycan clustering stabilizes the mannose patch of HIV-1 and preserves vulnerability to broadly neutralizing antibodies |journal=Nature Communications |volume=6 |pages=7479 |date=June 2015 |pmid=26105115 |pmc=4500839 |doi=10.1038/ncomms8479 |bibcode=2015NatCo...6.7479P}}</ref><ref>{{cite journal |vauthors=Behrens AJ, Vasiljevic S, Pritchard LK, Harvey DJ, Andev RS, Krumm SA, Struwe WB, Cupo A, Kumar A, Zitzmann N, Seabright GE, Kramer HB, Spencer DI, Royle L, Lee JH, Klasse PJ, Burton DR, Wilson IA, Ward AB, Sanders RW, Moore JP, Doores KJ, Crispin M |display-authors=6 |title=Composition and Antigenic Effects of Individual Glycan Sites of a Trimeric HIV-1 Envelope Glycoprotein |journal=Cell Reports |volume=14 |issue=11 |pages=2695β2706 |date=March 2016 |pmid=26972002 |pmc=4805854 |doi=10.1016/j.celrep.2016.02.058}}</ref> This provides a window for immune recognition. In addition, as these glycans are much less variable than the underlying protein, they have emerged as promising targets for vaccine design.<ref>{{cite journal |vauthors=Crispin M, Doores KJ |title=Targeting host-derived glycans on enveloped viruses for antibody-based vaccine design |journal=Current Opinion in Virology |volume=11 |pages=63β69 |date=April 2015 |pmid=25747313 |pmc=4827424 |doi=10.1016/j.coviro.2015.02.002 |series=Viral pathogenesis β’ Preventive and therapeutic vaccines |author-link2=Katie Doores}}</ref> [[P-glycoprotein]]s are critical for antitumor research due to its ability block the effects of antitumor drugs.<ref name="Lehninger_2013" /><ref name="Ambudkar_2003">{{cite journal | vauthors = Ambudkar SV, Kimchi-Sarfaty C, Sauna ZE, Gottesman MM | title = P-glycoprotein: from genomics to mechanism | journal = Oncogene | volume = 22 | issue = 47 | pages = 7468β7485 | date = October 2003 | pmid = 14576852 | doi = 10.1038/sj.onc.1206948 | s2cid = 11259597 | doi-access = free }}</ref> P-glycoprotein, or multidrug transporter (MDR1), is a type of ABC transporter that transports compounds out of cells.<ref name="Lehninger_2013" /> This transportation of compounds out of cells includes drugs made to be delivered to the cell, causing a decrease in drug effectiveness.<ref name="Lehninger_2013" /> Therefore, being able to inhibit this behavior would decrease P-glycoprotein interference in drug delivery, making this an important topic in drug discovery.<ref name="Lehninger_2013" /> For example, P-Glycoprotein causes a decrease in anti-cancer drug accumulation within tumor cells, limiting the effectiveness of chemotherapies used to treat cancer.<ref name="Ambudkar_2003" />
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