Glycoprotein

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Not to be confused with peptidoglycan or proteoglycan.
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. Secreted extracellular proteins are often glycosylated. In proteins that have segments extending extracellularly, the extracellular segments are also glycosylated. Glycoproteins are often important integral membrane proteins, where they play a role in cell–cell interactions. Glycoproteins are also formed in the cytosol, but their functions and the pathways producing these modifications in this compartment are less well understood.[2]

N-glycosylation and O-glycosylation[edit]

There are two types of glycosylation:

Monosaccharides[edit]

Eight sugars commonly found in glycoproteins.

Monosaccharides commonly found in eukaryotic glycoproteins include:[3]:526

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[edit]

One example of glycoproteins found in the body is mucins, 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, 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.

Variable surface glycoproteins allow the sleeping sickness Trypanosoma parasite to escape the immune response of the host.

Hormones[edit]

Hormones that are glycoproteins include:

Functions[edit]

Some functions served by glycoproteins[3]:524
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, patatin
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[edit]

A variety of methods used in detection, purification, and structural analysis of glycoproteins are[3]:525[6]

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.
Multi-angle light scattering In conjunction with size-exclusion chromatography, UV/Vis absorption and differential refractometry, provides information on molecular 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 associated conformational changes.
Methylation (linkage) analysis To determine linkage between sugars.
Amino acid or cDNA sequencing Determination of amino acid sequence.

See also[edit]

References[edit]

  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. ^ a b c Robert K. Murray, Daryl K. Granner & Victor W. Rodwell: "Harper's Illustrated Biochemistry 27th Ed.", McGraw–Hill, 2006
  4. ^ Glycan classification SIGMA
  5. ^ Theerasilp S, Kurihara Y (August 1988). "Complete purification and characterization of the taste-modifying protein, miraculin, from miracle fruit". J. Biol. Chem. 263 (23): 11536–9. PMID 3403544. 
  6. ^ Anne Dell, Howard R Morris: "Glycoprotein structure determination by mass spectrometry", Science 291(5512), 2351–2356 (2001), Review

External links[edit]