Glycoprotein

Glycoprotein
N-linked protein glycosylation (N-glycosylation of N-glycans) at Asn residues (Asn-x-Ser/Thr motifs) in glycoproteins.[1]

Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a 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. Glycoproteins also occur in the cytosol, but their functions and the pathways producing these modifications in this compartment are less well-understood.[2]

Contents

N-glycosylation and O-glycosylation

There are two types of glycoproteins:

Monosaccharides

Eight sugars commonly found in glycoproteins.

Monosaccharides commonly found in eukaryotic glycoproteins include:[3]

The principal sugars found in human glycoproteins[4]
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 in protein folding or improve proteins' stability.

Examples

One example of glycoproteins found in the body is mucins, 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 mammals.[citation needed] Examples of glycoproteins in the immune system are:

Other examples of glycoproteins include:

Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.

  • Miraculin, which alters human tongue receptors to recognize sour foods as sweet.

Hormones

Hormones that are glycoproteins include:

Functions

Some functions served by glycoproteins[5]
Function Glycoproteins
Structural molecule Collagens
Lubricant and protective agent Mucins
Transport molecule Transferrin, ceruloplasmin
Immunologic molecule Immunoglobins, histocompatibility antigens
Hormone Human chorionic gonadotropin (HCG), thyroid-stimulating hormone (TSH)
Enzyme Various, e.g., alkaline phosphatase
Cell attachment-recognition site Various proteins involved in cell–cell (e.g., spermoocyte), 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 Notch and its analogs, key proteins in development
Hemostasis (and thrombosis) Specific glycoproteins on the surface membranes of platelets

Analysis

A variety of methods used in detection, purification, and structural analysis of glycoproteins are[6][7]

Some important methods used to study glycoproteins
Method Use
Periodic acid-Schiff stain Detects glycoproteins as pink bands after 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 endo- or exoglycosidase or phospholipases 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, lectin affinity chromatography To purify glycoproteins or glycopeptides that bind the particular lectin used.
Lectin affinity electrophoresis Resultant shifts in electrophoretic migration help distinguish and characterize glycoforms, i.e. variants of a glycoprotein differing in carbohydrate.
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.
Dual Polarisation Interferometry Measures the mechanisms underlying the biomolecular interactions, including reaction rates, affinities and associated conformational changes.
Methylation (linkage) analysis To determine linkage between sugars.
Amino acid or cDNA sequencing Determination of amino acid sequence.

See also

References

  1. ^ Ruddock & Molinari (2006) Journal of Cell Science 119, 4373–4380
  2. ^ Funakoshi Y, Suzuki T (January 2009). "Glycobiology in the cytosol: The bitter side of a sweet world". Biochim. Biophys. Acta 1790 (2): 81–94. doi:10.1016/j.bbagen.2008.09.009. PMID 18952151. 
  3. ^ Robert K. Murray, Daryl K. Granner & Victor W. Rodwell: "Harper's Illustrated Biochemistry 27th Ed.", p. 526, McGraw-Hill, 2006
  4. ^ https://www.sigmaaldrich.com/img/assets/15880/glycan_classification.pdf
  5. ^ Ibid., p. 524
  6. ^ Ibid., p. 525
  7. ^ Anne Dell, Howard R Morris: "Glycoprotein structure determination by mass spectrometry", Science 291(5512), 2351–2356 (2001), Review

External links


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