Biology Reference
In-Depth Information
the best known is Marfan syndrome, the disorder Abraham Lincoln was for many years
believed to have had (see Chapter 6). In addition, it is now believed that all prion proteins
are GPI-anchored. Prions are the infectious agent responsible for a number of incurable brain
or neuronal diseases including 'mad cow disease' and, in humans, Creutzfeldt
Jakob
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Disease.
SUMMARY
In addition to lipids and proteins a thirdmajor component, carbohydrates, exists primarily
on the outer surface of the cell plasma membrane where they can be anchored to lipids
(glycolipids) and proteins (glycoproteins). Carbohydrates are highly water-soluble and so
are never found in a membrane's hydrophobic interior. Their main function is to interact
with the cell's external environment. The amount of carbohydrate on a membrane protein
varies from 0 to >85 weight percent. There are 9 basic sugars commonly found attached to
membranes as either single sugars or short,
15 unit, polysaccharides and are always located
on the extra-cellular membrane leaflet. One sugar, sialic acid, is highly unusual, being a 9-
carbon carboxylic acid that is an anion under physiological conditions. In animals the major
glycolipids are sphingolipids. Sphingolipid (ganglioside) sugar metabolism disorders consti-
tute the approximately 50 fatal lysosomal storage diseases. Glycosylphosphatidylinositol
(GPI)-anchored proteins are a very different type of glycoprotein.
So far we have discussed the major components of membranes - lipids, proteins and carbo-
hydrates. Chapter 8 will assimilate these components into working models for membrane
structure, covering lipid bilayers to lipid rafts.
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References
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[2] Stick RV, Williams S. Carbohydrates: The Essential Molecules of Life. 2nd ed. Elsevier; 2009.
[3] Wang PG, Bertozzi CR, editors. Glycochemistry
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Principles, Synthesis and Applications. Marcel Dekker;
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[4] Fraser-Reid BO, Tatsuta K, Thiem J, et al. editors. Glycoscience
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Chemistry and Chemical Biology. 2nd ed.
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[8] Moss GP. IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN). Nomenclature of Glycolipids,
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