Glycoproteinsareproteinswhich containoligosaccharide(sugar) chainscovalentlyattached toamino acidside-chains. Thecarbohydrateis attached to the protein in acotranslationalorposttranslational modification.This process is known asglycosylation.Secreted extracellular proteinsare often glycosylated.
In proteins that have segments extending extracellularly, the extracellular segments are also often glycosylated. Glycoproteins are also often importantintegral membrane proteins,where they play a role in cell–cell interactions. It is important to distinguish endoplasmic reticulum-based glycosylation of the secretory system from reversible cytosolic-nuclear glycosylation. Glycoproteins of thecytosoland nucleus can be modified through the reversible addition of a single GlcNAc residue that is considered reciprocal to phosphorylation and the functions of these are likely to be an additional regulatory mechanism that controls phosphorylation-based signalling.[2]In contrast, classical secretory glycosylation can be structurally essential. For example, inhibition of asparagine-linked, i.e. N-linked, glycosylation can prevent proper glycoprotein folding and full inhibition can be toxic to an individual cell. In contrast, perturbation of glycan processing (enzymatic removal/addition of carbohydrate residues to the glycan), which occurs in both theendoplasmic reticulumandGolgi apparatus,is dispensable for isolated cells (as evidenced by survival with glycosides inhibitors) but can lead to human disease (congenital disorders of glycosylation) and can be lethal in animal models. It is therefore likely that the fine processing of glycans is important for endogenous functionality, such as cell trafficking, but that this is likely to have been secondary to its role in host-pathogen interactions. A famous example of this latter effect is theABO blood group system.
Though there are different types of glycoproteins, the most common areN-linked andO-linked glycoproteins.[3]These two types of glycoproteins are distinguished by structural differences that give them their names. Glycoproteins vary greatly in composition, making many different compounds such as antibodies or hormones.[4]Due to the wide array of functions within the body, interest in glycoprotein synthesis for medical use has increased.[5]There are now several methods to synthesize glycoproteins, including recombination and glycosylation of proteins.[5]
Glycosylation is also known to occur onnucleocytoplasmicproteins in the form ofO-GlcNAc.[6]
Types of glycosylation
editThere are several types of glycosylation, although the first two are the most common.
- InN-glycosylation,sugars are attached to nitrogen, typically on theamideside-chain ofasparagine.
- InO-glycosylation,sugars are attached to oxygen, typically onserineorthreonine,but also ontyrosineor non-canonical amino acids such ashydroxylysineandhydroxyproline.
- InP-glycosylation,sugars are attached tophosphoruson aphosphoserine.
- InC-glycosylation,sugars are attached directly to carbon, such as in the addition ofmannosetotryptophan.
- InS-glycosylation,a beta-GlcNAcis attached to the sulfur atom of acysteineresidue.[7]
- Inglypiation,aGPIglycolipid is attached to theC-terminusof apolypeptide,serving as a membrane anchor.
- Inglycation,also known as non-enzymatic glycosylation, sugars are covalently bonded to a protein or lipid molecule, without the controlling action of an enzyme, but through aMaillard reaction.
Monosaccharides
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Monosaccharides commonly found in eukaryotic glycoproteins include:[8]: 526
| Sugar | Type | Abbreviation |
|---|---|---|
| β-D-Glucose | Hexose | Glc |
| β-D-Galactose | Hexose | Gal |
| β-D-Mannose | Hexose | Man |
| α-L-Fucose | Deoxyhexose | Fuc |
| N-Acetylgalactosamine | Aminohexose | GalNAc |
| N-Acetylglucosamine | Aminohexose | GlcNAc |
| N-Acetylneuraminic acid | Aminononulosonic acid (Sialic acid) |
NeuNAc |
| Xylose | Pentose | Xyl |
The sugar group(s) can assist inprotein folding,improve proteins' stability and are involved in cell signalling.
Structure
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The critical structural element of all glycoproteins is havingoligosaccharidesbondedcovalentlyto a protein.[4]There are 10 common monosaccharides in mammalianglycansincluding:glucose(Glc),fucose(Fuc),xylose(Xyl),mannose(Man),galactose(Gal), N-acetylglucosamine(GlcNAc),glucuronic acid(GlcA),iduronic acid(IdoA),N-acetylgalactosamine(GalNAc),sialic acid,and 5-N-acetylneuraminic acid(Neu5Ac).[3]These glycans link themselves to specific areas of the proteinamino acidchain.
The two most common linkages in glycoproteins areN-linked andO-linked glycoproteins.[3]AnN-linked glycoprotein has glycan bonds to the nitrogen containing anasparagineamino acid within the protein sequence.[4]AnO-linked glycoprotein has the sugar is bonded to an oxygen atom of aserineorthreonineamino acid in the protein.[4]
Glycoprotein size and composition can vary largely, with carbohydrate composition ranges from 1% to 70% of the total mass of the glycoprotein.[4]Within the cell, they appear in the blood, theextracellular matrix,or on the outer surface of the plasma membrane, and make up a large portion of the proteins secreted by eukaryotic cells.[4]They are very broad in their applications and can function as a variety of chemicals from antibodies to hormones.[4]
Glycomics
editGlycomicsis the study of the carbohydrate components of cells.[4]Though not exclusive to glycoproteins, it can reveal more information about different glycoproteins and their structure.[4]One of the purposes of this field of study is to determine which proteins are glycosylated and where in the amino acid sequence the glycosylation occurs.[4]Historically, mass spectrometry has been used to identify the structure of glycoproteins and characterize the carbohydrate chains attached.[4][10]
Examples
editThe unique interaction between the oligosaccharide chains have different applications. First, it aids in quality control by identifying misfolded proteins.[4]The oligosaccharide chains also change the solubility and polarity of the proteins that they are bonded to.[4]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.[4]The diversity in interactions lends itself to different types of glycoproteins with different structures and functions.[5]
One example of glycoproteins found in the body ismucins,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 toproteolysisby digestive enzymes.
Glycoproteins are important forwhite blood cellrecognition.[citation needed]Examples of glycoproteins in theimmune systemare:
- molecules such asantibodies(immunoglobulins), which interact directly withantigens.
- molecules of themajor histocompatibility complex(or MHC), which are expressed on the surface of cells and interact withT cellsas 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 onplateletsthat is required for normal platelet aggregation and adherence to theendothelium.
- components of thezona pellucida,which surrounds theoocyte,and is important forsperm-egg interaction.
- structural glycoproteins, which occur inconnective tissue.These help bind together the fibers, cells, and ground substance ofconnective tissue.They may also help components of the tissue bind to inorganic substances, such ascalciuminbone.
- Glycoprotein-41 (gp41) and glycoprotein-120 (gp120) are HIV viral coat proteins.
Soluble glycoproteins often show a highviscosity,for example, inegg whiteandblood plasma.
- Miraculin,is a glycoprotein extracted fromSynsepalum dulcificumaberrywhich alters human tongue receptors to recognize sour foods as sweet.[11]
Variable surface glycoproteinsallow the sleeping sicknessTrypanosomaparasite to escape the immune response of the host.
The viral spike of the human immunodeficiency virus is heavily glycosylated.[12]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.[13][14]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.[15]
P-glycoproteinsare critical for antitumor research due to its ability block the effects of antitumor drugs.[4][16]P-glycoprotein, or multidrug transporter (MDR1), is a type of ABC transporter that transports compounds out of cells.[4]This transportation of compounds out of cells includes drugs made to be delivered to the cell, causing a decrease in drug effectiveness.[4]Therefore, being able to inhibit this behavior would decrease P-glycoprotein interference in drug delivery, making this an important topic in drug discovery.[4]For example, P-Glycoprotein causes a decrease in anti-cancer drug accumulation within tumor cells, limiting the effectiveness of chemotherapies used to treat cancer.[16]
Hormones
editHormonesthat are glycoproteins include:
Distinction between glycoproteins andproteoglycans
editQuoting from recommendations for IUPAC:[17]
A glycoprotein is a compound containing carbohydrate (or glycan) covalently linked to protein. The carbohydrate may be in the form of a monosaccharide, disaccharide(s). oligosaccharide(s), polysaccharide(s), or their derivatives (e.g. sulfo- or phospho-substituted). One, a few, or many carbohydrate units may be present.Proteoglycansare a subclass of glycoproteinsin which the carbohydrate units are polysaccharides that contain amino sugars. Such polysaccharides are also known as glycosaminoglycans.
Functions
edit| Function | Glycoproteins |
|---|---|
| Structural molecule | Collagens |
| Lubricant and protective agent | Mucins |
| Transport molecule | Transferrin,ceruloplasmin |
| Immunologic molecule | Immunoglobulins,[18]histocompatibilityantigens |
| Hormone | Human chorionic gonadotropin(HCG), thyroid-stimulating hormone (TSH) |
| Enzyme | Various, e.g., alkalinephosphatase,patatin |
| Cell attachment-recognition site | Various proteins involved in cell–cell (e.g.,sperm–oocyte), virus–cell, bacterium–cell, and hormone–cell interactions |
| Antifreeze protein | 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 | Notchand its analogs, key proteins in development |
| Hemostasis(andthrombosis) | Specific glycoproteins on the surface membranes ofplatelets |
Analysis
editA variety of methods used in detection, purification, and structural analysis of glycoproteins are[8]: 525 [18][10]
| Method | Use |
|---|---|
| Periodic acid-Schiff stain | Detects glycoproteins as pink bands afterelectrophoreticseparation. |
| Incubation of cultured cells with glycoproteins asradioactive decaybands | Leads to detection of a radioactive sugar after electrophoretic separation. |
| Treatment with appropriateendo-orexoglycosidaseorphospholipases | Resultant shifts in electrophoretic migration help distinguish among proteins with N-glycan, O-glycan, or GPI linkages and also between highmannoseand complex N-glycans. |
| Agarose-lectincolumn chromatography,lectin affinity chromatography | To purify glycoproteins or glycopeptides that bind the particular lectin used. |
| Lectinaffinity electrophoresis | Resultant shifts in electrophoretic migration help distinguish and characterizeglycoforms,i.e. variants of a glycoprotein differing in carbohydrate. |
| Compositional analysis following acidhydrolysis | Identifies sugars that the glycoprotein contains and their stoichiometry. |
| Mass spectrometry | Provides information onmolecular mass,composition, sequence, and sometimes branching of a glycan chain. It can also be used for site-specific glycosylation profiling.[18] |
| NMR spectroscopy | To identify specific sugars, their sequence, linkages, and the anomeric nature of glycosidic chain. |
| Multi-angle light scattering | In conjunction with size-exclusion chromatography, UV/Vis absorption and differential refractometry, provides information onmolecular mass,protein-carbohydrate ratio, aggregation state, size, and sometimes branching of a glycan chain. In conjunction with composition-gradient analysis, analyzes self- and hetero-association to determine binding affinity and stoichiometry with proteins or carbohydrates in solution without labeling. |
| Dual Polarisation Interferometry | Measures the mechanisms underlying the biomolecular interactions, including reaction rates, affinities and associatedconformational changes. |
| Methylation(linkage) analysis | To determine linkage between sugars. |
| Amino acidorcDNAsequencing | Determination of amino acid sequence. |
Synthesis
editThe glycosylation of proteins has an array of different applications from influencing cell to cell communication to changing the thermal stability and the folding of proteins.[4][19]Due to the unique abilities of glycoproteins, they can be used in many therapies.[19]By understanding glycoproteins and their synthesis, they can be made to treat cancer,Crohn's Disease,high cholesterol, and more.[3]
The process of glycosylation (binding a carbohydrate to a protein) is apost-translational modification,meaning it happens after the production of the protein.[3]Glycosylation is a process that roughly half of all human proteins undergo and heavily influences the properties and functions of the protein.[3]Within the cell, glycosylation occurs in theendoplasmic reticulum.[3]
Recombination
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There are several techniques for the assembly of glycoproteins. One technique utilizesrecombination.[3]The first consideration for this method is the choice of host, as there are many different factors that can influence the success of glycoprotein recombination such as cost, the host environment, the efficacy of the process, and other considerations.[3]Some examples of host cells include E. coli, yeast, plant cells, insect cells, and mammalian cells.[3]Of these options, mammalian cells are the most common because their use does not face the same challenges that other host cells do such as different glycan structures, shorter half life, and potential unwanted immune responses in humans.[3]Of mammalian cells, the most common cell line used for recombinant glycoprotein production is theChinese hamster ovaryline.[3]However, as technologies develop, the most promising cell lines for recombinant glycoprotein production are human cell lines.[3]
Glycosylation
editThe formation of the link between the glycan and the protein is key element of the synthesis of glycoproteins.[5]The most common method of glycosylation of N-linked glycoproteins is through the reaction between a protected glycan and a protected Asparagine.[5]Similarly, an O-linked glycoprotein can be formed through the addition of aglycosyldonor with a protectedSerineorThreonine.[5]These two methods are examples of natural linkage.[5]However, there are also methods of unnatural linkages.[5]Some methods include ligation and a reaction between a serine-derived sulfamidate and thiohexoses in water.[5]Once this linkage is complete, the amino acid sequence can be expanded upon using solid-phase peptide synthesis.[5]
See also
editNotes and references
edit- ^Ruddock LW, Molinari M (November 2006)."N-glycan processing in ER quality control".Journal of Cell Science.119(Pt 21): 4373–4380.doi:10.1242/jcs.03225.PMID17074831.
- ^Funakoshi Y, Suzuki T (February 2009)."Glycobiology in the cytosol: the bitter side of a sweet world".Biochimica et Biophysica Acta (BBA) - General Subjects.1790(2): 81–94.doi:10.1016/j.bbagen.2008.09.009.PMID18952151.
- ^abcdefghijklmPicanco e Castro V, Swiech SH (2018).Recombinant Glycoprotein Production Methods and Protocols.Springer.ISBN978-1-4939-7312-5.OCLC1005519572.
- ^abcdefghijklmnopqrsNelson DL, Cox MM, Hoskins AA, Lehninger AL (2013).Lehninger Principles of Biochemistry(Sixth ed.). Macmillan Learning.ISBN978-1-319-38149-3.OCLC1249676451.
- ^abcdefghijGamblin DP, Scanlan EM, Davis BG (January 2009). "Glycoprotein synthesis: an update".Chemical Reviews.109(1): 131–163.doi:10.1021/cr078291i.PMID19093879.
- ^Hart GW (27 October 2014)."Three Decades of Research on O-GlcNAcylation - A Major Nutrient Sensor That Regulates Signaling, Transcription and Cellular Metabolism".Frontiers in Endocrinology.5:183.doi:10.3389/fendo.2014.00183.PMC4209869.PMID25386167.
- ^Stepper J, Shastri S, Loo TS, Preston JC, Novak P, Man P, et al. (February 2011). "Cysteine S-glycosylation, a new post-translational modification found in glycopeptide bacteriocins".FEBS Letters.585(4): 645–650.doi:10.1016/j.febslet.2011.01.023.PMID21251913.S2CID29992601.
- ^abcMurray RC, Granner DK, Rodwell VW (2006).Harper's Illustrated Biochemistry(27th ed.). McGraw–Hill.
- ^Glycan classificationArchived27 October 2012 at theWayback MachineSIGMA
- ^abDell A, Morris HR (March 2001). "Glycoprotein structure determination by mass spectrometry".Science.291(5512): 2351–2356.Bibcode:2001Sci...291.2351D.doi:10.1126/science.1058890.PMID11269315.S2CID23936441.
- ^Theerasilp S, Kurihara Y (August 1988)."Complete purification and characterization of the taste-modifying protein, miraculin, from miracle fruit".The Journal of Biological Chemistry.263(23): 11536–11539.doi:10.1016/S0021-9258(18)37991-2.PMID3403544.
- ^Pritchard LK, Vasiljevic S, Ozorowski G, Seabright GE, Cupo A, Ringe R, et al. (June 2015)."Structural Constraints Determine the Glycosylation of HIV-1 Envelope Trimers".Cell Reports.11(10): 1604–1613.doi:10.1016/j.celrep.2015.05.017.PMC4555872.PMID26051934.
- ^Pritchard LK, Spencer DI, Royle L, Bonomelli C, Seabright GE, Behrens AJ, et al. (June 2015)."Glycan clustering stabilizes the mannose patch of HIV-1 and preserves vulnerability to broadly neutralizing antibodies".Nature Communications.6:7479.Bibcode:2015NatCo...6.7479P.doi:10.1038/ncomms8479.PMC4500839.PMID26105115.
- ^Behrens AJ, Vasiljevic S, Pritchard LK, Harvey DJ, Andev RS, Krumm SA, et al. (March 2016)."Composition and Antigenic Effects of Individual Glycan Sites of a Trimeric HIV-1 Envelope Glycoprotein".Cell Reports.14(11): 2695–2706.doi:10.1016/j.celrep.2016.02.058.PMC4805854.PMID26972002.
- ^Crispin M,Doores KJ(April 2015)."Targeting host-derived glycans on enveloped viruses for antibody-based vaccine design".Current Opinion in Virology.Viral pathogenesis • Preventive and therapeutic vaccines.11:63–69.doi:10.1016/j.coviro.2015.02.002.PMC4827424.PMID25747313.
- ^abAmbudkar SV, Kimchi-Sarfaty C, Sauna ZE, Gottesman MM (October 2003)."P-glycoprotein: from genomics to mechanism".Oncogene.22(47): 7468–7485.doi:10.1038/sj.onc.1206948.PMID14576852.S2CID11259597.
- ^"Nomenclature of glycoproteins, glycopeptides and peptidoglycans, Recommendations 1985".qmul.ac.uk.Retrieved16 March2021.
- ^abcMaverakis E, Kim K, Shimoda M, Gershwin ME, Patel F, Wilken R, et al. (February 2015)."Glycans in the immune system and The Altered Glycan Theory of Autoimmunity: a critical review".Journal of Autoimmunity.57(6): 1–13.doi:10.1016/j.jaut.2014.12.002.PMC4340844.PMID25578468.
- ^abDavis BG (February 2002). "Synthesis of glycoproteins".Chemical Reviews.102(2): 579–602.doi:10.1021/cr0004310.PMID11841255.
Further reading
edit- Maverakis E, Kim K, Shimoda M, Gershwin ME, Patel F, Wilken R, Raychaudhuri S, Ruhaak LR, Lebrilla CB (February 2015)."Glycans in the immune system and The Altered Glycan Theory of Autoimmunity: a critical review".Journal of Autoimmunity.57:1–13.doi:10.1016/j.jaut.2014.12.002.PMC4340844.PMID25578468.
- Berg JM, Tymoczko JL, Stryer L (2002)."Carbohydrates Can Be Attached to Proteins to Form Glycoproteins".Biochemistry(5th ed.). New York: W.H. Freeman.ISBN978-0-7167-4684-3.
External links
edit- Glycoproteinsat the U.S. National Library of MedicineMedical Subject Headings(MeSH)
- "Biological Importance of the glycosylation of a protein".BiochemPages.15 August 2015. Archived fromthe originalon 30 November 2020.Retrieved18 August2015.
- "Carbohydrate Chemistry and Glycobiology: A Web Tour".Science.291(5512): 2263–2502. 23 March 2001. Archived fromthe originalon 9 January 2008.
Special Web Supplement
- "Glycan Recognizing Proteins".bioWORLD.
- "Structure of Glycoprotein and Carbohydrate Chain".Home Page for Learning Environmental Chemistry.
