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glycoprotein

 
Dictionary: gly·co·pro·tein   (glī'kō-prō'tēn', -tē-ĭn) pronunciation
n.

Any of a group of conjugated proteins having a carbohydrate as the nonprotein component.


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Sci-Tech Encyclopedia: Glycoprotein
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A compound in which carbohydrate (sugar) is covalently linked to protein. The carbohydrate may be in the form of monosaccharides, disaccharides, oligosaccharides, or polysaccharides, and is sometimes referred to as glycan. The sugar may be linked to sulfate or phosphate groups. In different glycoproteins, 100–200 glycan units may be present. Therefore, the carbohydratecontent of these compounds varies markedly, from 1% (as in the collagens), to 60%(in certain mucins), to >99% (in glycogen). See also Collagen; Glycogen.

Glycoproteins are ubiquitous in nature, although they are relatively rare in bacteria. They occur in cells, in both soluble and membrane-bound forms, as well as in the intercellular matrix and in extracellular fluids, and include numerous biologically active macromolecules. A number of glycoproteins are produced industrially by genetic engineering techniques for use as drugs; among them are erythropoietin, interferons, colony stimulating factors, and blood-clotting factors. See also Genetic engineering.

In most glycoproteins, the carbohydrate is linked to the polypeptide backbone by either N- or O-glycosidic bonds. A different kind of bond is found in glycoproteins that are anchored in cell membranes by a special carbohydrate-containing compound, glycosylphosphatidylinositol, which is attached to the C-terminal amino acid of the protein. A single glycoprotein may contain more than one type of carbohydrate-peptide linkage. N-linked units are typically found in plasma glycoproteins, in ovalbumin, in many enzymes (for example, the ribonucleases), and in immunoglobulins. O-linked units are found in mucins; collagens; and proteoglycans (typical constituents of connective tissues), including chondroitin sulfates, dermatan sulfate, and heparin. See also Albumin; Carbohydrate; Enzyme; Immunoglobulin; Monosaccharide; Oligosaccharide; Polysaccharide; Protein.

Within any organism, all molecules of a particular protein are identical. In contrast, a variety of structurally distinct carbohydrate units are found not only at different attachment sites of a glycoprotein but even at each single attachment site—a phenomenon known as microheterogeneity. For instance, ovalbumin contains one glycosylated amino acid, but over a dozen different oligosaccharides have been identified at that site, even in a preparation isolated from a single egg of a purebred hen.


Food and Nutrition: glycoproteins
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Proteins conjugated with carbohydrates such as uronic acids, polymerized glucosamine-mannose, etc., including mucins and mucoids; found in the vitreous humour of the eye, cornea, cartilage, and gastric mucosa. See also mucoproteins.

Dental Dictionary: glycoprotein
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n

Any of the large group of conjugated proteins in which the nonprotein substance is a carbohydrate. These include the mucins, the mucoids, and the chondroproteins.

 
Columbia Encyclopedia: glycoprotein
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glycoprotein (glī'kōprō'tēn), organic compound composed of both a protein and a carbohydrate joined together in covalent chemical linkage. These structures occur in many life forms; they are prevalent and important in mammalian tissues. The attached carbohydrate may have several effects: it may help the protein to fold in the proper geometry, stabilize the protein, affect physical properties such as solubility or viscosity, helps it to orient correctly in a membrane, or make it recognizable to another biochemical or cell (see immunity). Many proteins released by cells to the blood and other fluids are glycoproteins. One set of glycoproteins also carry the blood group determinants. The carbohydrate portion of a glycoprotein is usually a small sugar or no more than 8 to 10 individual monosaccharide units. Combinations of up to seven of the many different sugar molecules known to occur in nature comprise the saccharide portions of mammalian glycoproteins: glucose, glucosamine, galactose, galactosamine, mannose, fucose, and sialic acid (a derivative of glucosamine). The linkage between the oligosaccharide and the protein occurs by formation of a chemical bond to only one of four protein amino acids: asparagine, hydroxylysine, serine, or threonine. Solutions of glycoproteins usually exhibit high viscosity, an observation explaining the highly viscous character of egg white, which is composed largely of the glycoprotein ovalbumin. Salivary mucus contains the glycoprotein called mucin. Among other glycoproteins, one particularly interesting example is isolated from certain antarctic fishes who survive near-freezing water temperatures as a result of freezing-point depression of their blood serum by a globular glycoprotein. This molecule is a remarkably effective freezing point depressant.


Veterinary Dictionary: glycoprotein
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Any of a class of conjugated proteins consisting of a compound of protein with a carbohydrate group.

  • g. deficiency — an inherited disorder in dogs in which there is defective phagocytic function. Affected dogs have a marked, persistent neutrophilia and are susceptible to infections.
  • alpha-2HS g. — important in bone resorption.
Wikipedia: Glycoprotein
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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 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 glycosylation:

Monosaccharides

The eight sugars contained 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:

  • molecules such as antibodies (immunoglobulins), which interact directly with antigens
  • molecules of the major histocompatibility complex (or MHC), which are expressed on the surface of cells and interact with T cells as part of the adaptive immune response.

Other examples of glycoproteins include:

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[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, eg, alkaline phosphatase
Cell attachment-recognition site Various proteins involved in cell-cell (eg, sperm-oocyte), virus-cell, bacterium-cell, and hormone cell interactions
Antifreeze 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 To purify glycoproteins or glycopeptides that bind the particular lectin used.
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.

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

See also

External links


 
 

 

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Food and Nutrition. A Dictionary of Food and Nutrition. Copyright © 1995, 2003, 2005 by A. E. Bender and D. A. Bender. All rights reserved.  Read more
Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/ Read more
Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Glycoprotein" Read more