sphingolipids

sphingolipids

1. composed of:

   • long-chain fatty acid
   • long-chain amino alcohol (sphingosine)
   • a polar head group

2. constitute a ceramide

3. sphingomyelin

   • phosphoryl choline or phosphorylethanolamine derivatives of ceramide
   • formed from ceramide plus phosphorylcholine or phosphorylethanolamine esterfied to the hydroxyl
   • mainly in nerve tissue but also found in blood (lipoproteins)
   • they are abundant in the myelin sheath, a multilayer protective layer for insultation of cells of central nervous system and nerves of the spinal cord.
   • they account for about 25% of total lipid in human myelin
   • multiple sclerosis
     • is a crippling and eventually fatal disease where the myelin sheath is progressively dstroyed by sclerotic plaques that affect the brain and spinal cord. persons affected suffer from weakness, lack of co-ordination and speech and vision problems

4. glycosphingolipids

   • cerebrosides
     • most abundant in myelin sheath of nerves
     • ceramide monosaccharides
       • the monosaccharides moiety is D-glucose or D-galactose. this is linked through C, in a B-linkage to C1 of ceramide (glucocerebroside, galactocerebroside)
     • further addition of monosaccharides afford more complex glycosphingolipids with two, three, four or more sugars in glycosidic linkage
       • e.g. blood group antigens, of which there are over a hundred belonging to fourteen well-defined genetically independent blood group systems (in humans), are oligosaccharides which are protein or ceramide linked through their reducing end.
   • gangliosides and globosides
     • in the ganglion cells of the CNS, plasma membranes of many extraneutral cell types - spleen.
     • these are abundant in nerve endings and form components of specific receptor sites on cell membrane surfaces
     • they form cell specific surface markers unipue to a specific cell type
     • related to cerebrosides in that they are ceramides attached to D-galactose or D-glucose but several other carbohydrates are attached
     • gangliosides differ from other types of glycolipids in the sense that they contain N-acetyl neuraminic acid (NANA or sialic acid)
     • their names are normally abbreviated to G with a subscript M, D or T to indicate whether it contains one, two or three (mono, di or tri) sialic acid residues.
     • a number or letter refers to specific members of the family (eg. Gm1, Go1).
     • gangliosides mediate in many biological process
     • a specific ganglioside located in the plasma membrane of intestinal mucosal cells (Gm1) binds the B subunit of the cholera toxin; whereupon it enters the cell and the A subunit catalyses the activation of the adenylate cyclase complex (by ADP ribosylation of one of the regulatory proteins). the elevated intracellular cAMP levels lead to massive secretion of isotonic fluid by the intestinal epithelial cells (diarrheoa)
     • a specific ganglioside in the plasma membrane binds the influenza virus as a first step in the infection process
     • this class of molecule is also important in the cell-cell recognition process

5. steroids

   • members of this group are derivatives of a fused reduced ring system - perhydrocyclopentanophenanthrene.
   • many compounds of widely differing functions are classified as steroids on the basis of the same general structure: a fused ring system consisting of three six membered rings (the A, B and C rings) and one five membered ring (the D ring)
   • the steroid of most interest is cholesterol.
   • the only hydophilic group is the single hydroxyl group.
     • as a result, the molecule is highly hydrophobic
   • cholesterol is widespread in biological membranes.
   • in spite of its many important biological functions including its role as a precursor of other steroids such as cholate, one of the bile acids which play a role in digestion of fat in the body, cholesterol is best known for its harmful effects on health.
   • it plays a role in the development of atherosclerosis a condition in which lipid block the blood vessels and lead to heart disease

6. terpenes

   • a great number of biological molecules are related to the C5 units derived from the isoprene unit and are referred to as terpenes
   • many of these molecules contain multiples of C5 units derived from the isoprene unit and are referred to as terpenes