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Information about Glycoproteins

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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 their polypeptide backbones. Basically, glycoprotein is a biomolecule composed of a protein and a carbohydrate (an oligosaccharide). The carbohydrate is attached to the protein in a cotranslational or posttranslational modification.

In proteins that have segments extending extracellularly, the extracellular segments are often glycosylated.

N-glycosylation and O-glycosylation

There are two types of glycosylation:

Monosaccharides

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The eight sugars contained in glycoproteins.
Monosaccharides commonly found in eukaryotic glycoproteins include[2]:

The principal sugars found in human glycoproteins
Sugar Type Abbreviation
GalactoseHexoseGal
GlucoseHexoseGlc
MannoseHexoseMan
N-Acetylneuraminic acidSialic acid (nine C atoms)NeuAc
FucoseDeoxyhexoseFuc
N-AcetylgalactosamineAminohexaseGalNAc
N-AcetylglucosamineAminohexaseGlaNac
XylosePentoseXyl


The sugar group(s) can assist in protein folding or improve proteins' stability.

Examples

One example of glycoproteins found in the body are 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 immune cell recognition, especially in mammals. Examples of glycoproteins in the immune system are: Other examples of glycoproteins include:
  • 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.
Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.

Hormones

Hormones that are glycoproteins include:

Functions

Some functions served by glycoproteins[3]
Function Glycoproteins
Structural moleculeCollagens
Lubricant and protective agentMucins
Transport moleculeTransferrin, ceruloplasmin
Immunologic moleculeImmunoglobins, histocompatibility antigens
HormoneChorionoic gonadotropin, thyroid-stimulating hormone (TSH)
EnzymeVarious, eg, alkaline phosphatase
Cell attachment-recognition siteVarious proteins involved in cell-cell (eg, sperm-oocyte), virus-cell, bacterium-cell, and hormone cell interactions
AntifreezeCertain plasma proteins of coldwater fish
Interact with specific carbohydratesLectins, selectins (cell adhesion lectins), antibodies
ReceptorVarious proteins involved in hormone and drug action
Affect folding of certain proteinsCalnexin, calreticulin
Regulation of developmentNotch 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[4]

Some important methods used to study glycoproteins
Method Use
Periodic acid-Schiff stainDetects glycoproteins as pink bands after electrophoretic separation.
Incubation of cultured cells with glycoproteins as radioactive bandsLeads to detection of a radioactive sugar after electrophoretic separation.
Treatment with appropriate endo- or exoglycosidase or phospholipasesResultant 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.
Sepharose-lectin column chromatographyTo purify glycoproteins or glycopeptides that bind the particular lectin used.
Compositional analysis following acid hydrolysisIdentifies sugars that the glycoprotein contains and their stoichiometry.
Mass spectrometryProvides information on molecular mass, composition, sequence, and sometimes branching of a glycan chain.
NMR spectroscopyTo identify specific sugars, their sequence, linkages, and the anomeric nature of glycosidic chain.
Methylation (linkage) analysisTo determine linkage between sugars.
Amino acid or cDNA sequencingDetermination of amino acid sequence.

See also

References

1. ^ Ruddock & Molinari (2006) Journal of Cell Science 119, 4373-4380
2. ^ Robert K. Murray, Daryl K. Granner & Victor W. Rodwell: "Harper's Illustrated Biochemistry 27th Ed.", p. 526, McGraw-Hill, 2006
3. ^ Ibid., p. 524
4. ^ Ibid., p. 525

External links

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Protein: glycoproteins
Activin - ADAM protein - Alpha 1-antichymotrypsin - Apolipoprotein H - CD70 - Asialoglycoprotein - Avidin - B-cell activating factor - 4-1BB ligand - Cholesterylester transfer protein - Clusterin - Colony-stimulating factor - Haemopexin - Inhibin - Lactoferrin - Membrane glycoproteins - Mucoprotein - Myelin protein zero - Osteonectin - Protein C - Protein S - Proteoglycan - Serum Amyloid P component - Sialoglycoprotein (CD43, Glycophorin, Glycophorin C) - Thrombopoietin - Thyroglobulin - Thyroxine-binding proteins - Transcortin - Tumor necrosis factor-alpha - Uteroglobin - Vitronectin
An oligosaccharide is a saccharide polymer containing a small number (typically three to ten[1]) of component sugars, also known as simple sugars.

They are generally found either O- or N
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The term glycan refers to a polysaccharide, or oligosaccharide. Glycan may also be used to refer to the carbohydrate portion of a glycoconjugate, such as a glycoprotein, glycolipid, or a proteoglycan. Glycans usually consist solely of O-glycosidic linkages of monosaccharides.
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Peptides (from the Greek πεπτίδια, "small digestibles") are short polymers formed from the linking, in a defined order, of α-amino acids.
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biomolecule is a chemical molecule that naturally occurs in living organisms. Biomolecules consist primarily of carbon and hydrogen, along with nitrogen, oxygen, phosphorus and sulfur. Other elements sometimes are incorporated but are much less common.
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Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues.
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Post-translational modification (PTM) is the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis for many proteins. A protein (also called a polypeptide) is a chain of amino acids.
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For other articles using the abbreviation or acronym asn see ASN.


Asparagine (abbreviated as Asn or N; Asx or B
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Hydroxylysine is an amino acid, C6H14N2O3. It is most widely known as a component of collagen. A standard amino acid with the R' group consisting of an amino terminated butyl hydrocarbon chain, with a hydroxyl group on the carbon atom
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4-Hydroxyproline, or hydroxyproline (C5H9O3N), is an uncommon amino acid, abbreviated as HYP, e.g., in Protein Data Bank.

Structure


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Serine (abbreviated as Ser or S)[1] is an organic compound with the formula HO2CCH(NH2)CH2OH. It is one of the 20 naturally occurring proteinogenic amino acids. Its codons are UCU, UCC, UCA, UCG, AGU and AGC.
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Threonine (abbreviated as Thr or T)[1] is an α-amino acid with the chemical formula HO2CCH(NH2)CH(OH)CH3. Its codons are ACU and ACA. This essential amino acid is classified as polar.
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Galactose (Gal) (also called brain sugar[1]) is a type of sugar which is less sweet than glucose and not very water-soluble. It is considered a nutritive sweetener because it has food energy.

Galactan is a polymer of the sugar galactose.
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In organic chemistry, a hexose is a monosaccharide with six carbon atoms having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses
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Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate.
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Mannose is a sugar monomer of the hexose series of carbohydrates.

Metabolism

Mannose enters the carbohydrate metabolism stream in two steps:
  • First it undergoes phosphorylation to mannose-6-phosphate by hexokinase.

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N-Acetylneuraminic acid (Neu5Ac or NANA) is the predominant sialic acid.

The negative charge of this chemical is responsible for the slippery feel of saliva and mucins coating the body’s organs.
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Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a nine-carbon monosaccharide. It is also the name for the most common member of this group, N-acetylneuraminic acid (Neu5Ac or NANA).
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Fucose is a hexose deoxy sugar with the chemical formula C6H12O5. It is found on N-linked glycans on the mammalian, insect and plant cell surface, and is the fundamental sub-unit of the fucoidan polysaccharide.
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N-Acetylgalactosamine (also called GalNAc, 2-Acetamido-2-deoxy-D-galactopyranose or N-Acetyl-D-galactosamine) is a monosaccharide derivative of galactose.

Function

In humans it is the terminal carbohydrate forming the antigen of blood group A.
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N-Acetylglucosamine (N-Acetyl-D-Glucosamine, or GlcNAc, or NAG) is a monosaccharide derivative of glucose. Chemically it is an amide between glucosamine and acetic acid.
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Xylose or wood sugar is an aldopentose — a monosaccharide containing five carbon atoms and including an aldehyde functional group. It has chemical formula C5H10O5. Xylose is found in the embryos of most edible plants.
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A pentose is a monosaccharide with five carbon atoms.

They either have an aldehyde functional group in position 1 (aldopentoses), or a ketone functional group in position 2 (ketopentoses).
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Protein folding is the physical process by which a polypeptide folds into its characteristic three-dimensional structure.[1] Each protein begins as a polypeptide, translated from a sequence of mRNA as a linear chain of amino acids.
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Mucins are a family of large, heavily glycosylated proteins. Although some mucins are membrane-bound due to the presence of a hydrophobic membrane-spanning domain that favors retention in the plasma membrane, the concentration here is on those mucins that are secreted on mucosal
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Proteolysis is the directed degradation (digestion) of proteins by cellular enzymes called proteases or by intramolecular digestion.

Purposes

Proteolysis is used by the cell for several purposes.
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White blood cells or leukocytes are cells of the immune system which defend the body against both infectious disease and foreign materials. Several different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone
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Mammalia
Linnaeus, 1758

Subclasses & Infraclasses
  • Subclass †Allotheria*
  • Subclass Prototheria
  • Subclass Theria

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immune system is a collection of mechanisms within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own healthy
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Antibodies (also known as immunoglobulins) are proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.
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