Glycosylation

Glycosylation (see also chemical glycosylation) is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor). In biology, glycosylation mainly refers in particular to the enzymatic process that attaches glycans to proteins, or other organic molecules. This enzymatic process produces one of the fundamental biopolymers found in cells (along with DNA, RNA, and proteins). Glycosylation is a form of co-translational and post-translational modification. Glycans serve a variety of structural and functional roles in membrane and secreted proteins.[1] The majority of proteins synthesized in the rough endoplasmic reticulum undergo glycosylation. It is an enzyme-directed site-specific process, as opposed to the non-enzymatic chemical reaction of glycation. Glycosylation is also present in the cytoplasm and nucleus as the O-GlcNAc modification. Aglycosylation is a feature of engineered antibodies to bypass glycosylation.[2][3] Five classes of glycans are produced:

  • N-linked glycans attached to a nitrogen of asparagine or arginine side-chains. N-linked glycosylation requires participation of a special lipid called dolichol phosphate.
  • O-linked glycans attached to the hydroxyl oxygen of serine, threonine, tyrosine, hydroxylysine, or hydroxyproline side-chains, or to oxygens on lipids such as ceramide
  • phosphoglycans linked through the phosphate of a phosphoserine;
  • C-linked glycans, a rare form of glycosylation where a sugar is added to a carbon on a tryptophan side-chain
  • glypiation, which is the addition of a GPI anchor that links proteins to lipids through glycan linkages.

Purpose

Glycosylation is the process by which a carbohydrate is covalently attached to a target macromolecule, typically proteins and lipids. This modification serves various functions.[4] For instance, some proteins do not fold correctly unless they are glycosylated.[1] In other cases, proteins are not stable unless they contain oligosaccharides linked at the amide nitrogen of certain asparagine residues. The influence of glycosylation on the folding and stability of glycoprotein is twofold. Firstly, the highly soluble glycans may have a direct physicochemical stabilisation effect. Secondly, N-linked glycans mediate a critical quality control check point in glycoprotein folding in the endoplasmic reticulum.[5] Glycosylation also plays a role in cell-to-cell adhesion (a mechanism employed by cells of the immune system) via sugar-binding proteins called lectins, which recognize specific carbohydrate moieties.[1] Glycosylation is an important parameter in the optimization of many glycoprotein-based drugs such as monoclonal antibodies.[5] Glycosylation also underpins the ABO blood group system. It is the presence or absence of glycosyltransferases which dictates which blood group antigens are presented and hence what antibody specificities are exhibited. This immunological role may well have driven the diversification of glycan heterogeneity and creates a barrier to zoonotic transmission of viruses.[6] In addition, glycosylation is often used by viruses to shield the underlying viral protein from immune recognition. A significant example is the dense glycan shield of the envelope spike of the human immunodeficiency virus.[7]

Overall, glycosylation needs to be understood by the likely evolutionary selection pressures that have shaped it. In one model, diversification can be considered purely as a result of endogenous functionality (such as cell trafficking). However, it is more likely that diversification is driven by evasion of pathogen infection mechanism (e.g. Helicobacter attachment to terminal saccharide residues) and that diversity within the multicellular organism is then exploited endogenously.

Glycoprotein diversity

Glycosylation increases diversity in the proteome, because almost every aspect of glycosylation can be modified, including:

  • Glycosidic bond—the site of glycan linkage
  • Glycan composition—the types of sugars that are linked to a given protein
  • Glycan structure—can be unbranched or branched chains of sugars
  • Glycan length—can be short- or long-chain oligosaccharides

Mechanisms

There are various mechanisms for glycosylation, although most share several common features:[1]

  • Glycosylation, unlike glycation, is an enzymatic process. Indeed, glycosylation is thought to be the most complex post-translational modification, because of the large number of enzymatic steps involved.[8]
  • The donor molecule is often an activated nucleotide sugar.
  • The process is non-templated (unlike DNA transcription or protein translation); instead, the cell relies on segregating enzymes into different cellular compartments (e.g., endoplasmic reticulum, cisternae in Golgi apparatus). Therefore, glycosylation is a site-specific modification.

Types

N-linked glycosylation

N-linked glycosylation is a very prevalent form of glycosylation and is important for the folding of many eukaryotic glycoproteins and for cell–cell and cell–extracellular matrix attachment. The N-linked glycosylation process occurs in eukaryotes in the lumen of the endoplasmic reticulum and widely in archaea, but very rarely in bacteria. In addition to their function in protein folding and cellular attachment, the N-linked glycans of a protein can modulate a protein's function, in some cases acting as an on/off switch.[9]

O-linked glycosylation

O-linked glycosylation is a form of glycosylation that occurs in eukaryotes in the Golgi apparatus,[10] but also occurs in archaea and bacteria.

Phosphoserine glycosylation

Xylose, fucose, mannose, and GlcNAc phosphoserine glycans have been reported in the literature. Fucose and GlcNAc have been found only in Dictyostelium discoideum, mannose in Leishmania mexicana, and xylose in Trypanosoma cruzi. Mannose has recently been reported in a vertebrate, the mouse, Mus musculus, on the cell-surface laminin receptor alpha dystroglycan4. It has been suggested this rare finding may be linked to the fact that alpha dystroglycan is highly conserved from lower vertebrates to mammals.[11]

C-mannosylation

A mannose sugar is added to the first tryptophan residue in the sequence W–X–X–W (W indicates tryptophan; X is any amino acid). Thrombospondins are one of the proteins most commonly modified in this way. C-mannosylation is unusual because the sugar is linked to a carbon rather than a reactive atom such as nitrogen or oxygen. In 2011, Recently, the first crystal structure of a protein containing this type of glycosylation was determined—that of human complement component 8.[12]

Formation of GPI anchors (glypiation)

Glypiation is a special form of glycosylation that features the formation of a GPI anchor. In this kind of glycosylation a protein is attached to a lipid anchor, via a glycan chain. (See also prenylation.)

Chemical glycosylation

Glycosylation can also be effected using the tools of synthetic organic chemistry. Unlike the biochemical processes, synthetic glycochemistry relies heavily on protecting groups[13] (e.g. the 4,6-O-benzylidene) in order to achieve desired regioselectivity. The other challenge of chemical glycosylation is the stereoselectivity that each glycosidic linkage has two stereo-outcomes, α/β or cis/trans. Generally, the α- or cis-glycoside is more challenging to synthesis.[14] New methods have been developed based on solvent participation or the formation of bicyclic sulfonium ions as chiral-auxiliary groups.[15]

Deglycosylation

There are different enzymes to remove the glycans from the proteins or remove some part of the sugar chain.

  • α2-3,6,8,9-Neuraminidase (from Arthrobacter ureafaciens): cleaves all non-reducing terminal branched and unbranched sialic acids.
  • β1,4-Galactosidase (from Streptococcus pneumoniae): releases only β1,4-linked, nonreducing terminal galactose from complex carbohydrates and glycoproteins.
  • β-N-Acetylglucosaminidase (from Streptococcus pneumoniae): cleaves all non-reducing terminal β-linked N-acetylglucosamine residues from complex carbohydrates and glycoproteins.
  • endo-α-N-Acetylgalactosaminidase (O-glycosidase from Streptococcus pneumoniae): removes O-glycosylation. This enzyme cleaves serine- or threonine-linked unsubstituted Galβ1,3GalNAc
  • PNGase F: cleaves asparagine-linked oligosaccharides unless α1,3-core fucosylated.

Clinical

Over 40 disorders of glycosylation have been reported in humans.[16] These can be divided into four groups: disorders of protein N-glycosylation, disorders of protein O-glycosylation, disorders of lipid glycosylation and disorders of other glycosylation pathways and of multiple glycosylation pathways. No effective treatment is known for any of these disorders. 80% of these affect the nervous system.

Effects on therapeutic efficacy

It has been reported that mammalian glycosylation can improve the therapeutic efficacy of biotherapeutics. For example, therapeutic efficacy of recombinant human interferon gamma, expressed in HEK 293 platform, was improved against drug-resistant ovarian cancer cell lines.[17]

See also
  • Advanced glycation endproduct
  • Chemical glycosylation
  • Fucosylation
  • Glycation
  • Glycorandomization

References
  1. Varki A, ed. (2009). Essentials of Glycobiology (2nd ed.). Cold Spring Harbor Laboratories Press. ISBN 978-0-87969-770-9.
  2. Jung ST, Kang TH, Kelton W, Georgiou G (December 2011). "Bypassing glycosylation: engineering aglycosylated full-length IgG antibodies for human therapy". Current Opinion in Biotechnology. 22 (6): 858–67. doi:10.1016/j.copbio.2011.03.002. PMID 21420850.
  3. "Transgenic plants of Nicotiana tabacum L. express aglycosylated monoclonal antibody with antitumor activity". Biotecnologia Aplicada. 2013.
  4. Drickamer K, Taylor ME (2006). Introduction to Glycobiology (2nd ed.). Oxford University Press, USA. ISBN 978-0-19-928278-4.
  5. Dalziel M, Crispin M, Scanlan CN, Zitzmann N, Dwek RA (January 2014). "Emerging principles for the therapeutic exploitation of glycosylation". Science. 343 (6166): 1235681. doi:10.1126/science.1235681. PMID 24385630.
  6. Crispin M, Harvey DJ, Bitto D, Bonomelli C, Edgeworth M, Scrivens JH, Huiskonen JT, Bowden TA (March 2014). "Structural plasticity of the Semliki Forest virus glycome upon interspecies transmission". Journal of Proteome Research. 13 (3): 1702–12. doi:10.1021/pr401162k. PMC 4428802. PMID 24467287.
  7. 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: 63–9. doi:10.1016/j.coviro.2015.02.002. PMC 4827424. PMID 25747313.
  8. Walsh C (2006). Posttranslational Modification of Proteins: Expanding Nature's Inventory. Roberts and Co. Publishers, Englewood, CO. ISBN 978-0974707730.
  9. 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 (6): 1–13. doi:10.1016/j.jaut.2014.12.002. PMC 4340844. PMID 25578468.
  10. Flynne WG (2008). Biotechnology and Bioengineering. Nova Publishers. pp. 45ff. ISBN 978-1-60456-067-1. Retrieved 13 November 2010.
  11. Yoshida-Moriguchi T, Yu L, Stalnaker SH, Davis S, Kunz S, Madson M, Oldstone MB, Schachter H, Wells L, Campbell KP (January 2010). "O-Mannosyl phosphorylation of alpha-dystroglycan is required for laminin binding". Science. 327 (5961): 88–92. doi:10.1126/science.1180512. PMC 2978000. PMID 20044576.
  12. Lovelace LL, Cooper CL, Sodetz JM, Lebioda L (2011). "Structure of human C8 protein provides mechanistic insight into membrane pore formation by complement". J Biol Chem. 286: 17585–92. doi:10.1074/jbc.M111.219766. PMC 3093833. PMID 21454577.
  13. Crich D (August 2010). "Mechanism of a chemical glycosylation reaction". Accounts of Chemical Research. 43 (8): 1144–53. doi:10.1021/ar100035r. PMID 20496888.
  14. Nigudkar SS, Demchenko AV (May 2015). "cis-Glycosylation as the driving force of progress in synthetic carbohydrate chemistry". Chemical Science. 6 (5): 2687–2704. doi:10.1039/c5sc00280j. PMC 4465199. PMID 26078847.
  15. Fang T, Gu Y, Huang W, Boons GJ (March 2016). "Mechanism of Glycosylation of Anomeric Sulfonium Ions". Journal of the American Chemical Society. 138 (9): 3002–11. doi:10.1021/jacs.5b08436. PMC 5078750. PMID 26878147.
  16. Jaeken J (2013). Congenital disorders of glycosylation. Handbook of Clinical Neurology. 113. pp. 1737–43. doi:10.1016/B978-0-444-59565-2.00044-7. ISBN 9780444595652. PMID 23622397.
  17. Razaghi A, Villacrés C, Jung V, Mashkour N, Butler M, Owens L, Heimann K (October 2017). "Improved therapeutic efficacy of mammalian expressed-recombinant interferon gamma against ovarian cancer cells". Experimental Cell Research. 359 (1): 20–29. doi:10.1016/j.yexcr.2017.08.014. PMID 28803068.

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