Sphingophospholipids and Glycolipids

SPHINGOPHOSPHOLIPIDS AND GLYCOLIPIDS

  • There are 3 subclasses of sphingolipids. All are derivatives of ceramides, but differing in their head groups. Examples: sphingomyelins, glycolipids, and gangliosides.
  • Sphingomyelins (Sphingophospholipid)
    • The sphingomyelins contain amino alcohol, sphingosine (synthesized from serine and palmitic acid) rather than glycerol.
    • A fatty acid is attached to the amino group of sphingosine by an amide linkage, producing  a  ceramide. A structure also found in glycolipids
    • Ceramide can also serve as a precursor of glycolipids.
      The fatty acids found most frequently in sphingomyelins are palmitic, stearic, lignoceric and nervonic acid.
    • The alcohol group at carbon 1 of sphingosine is esterified to phosphorylcholine, producing sphingomyelin, an important constituent of myelin of nerve fibers, which insulates and protects the nerve fibres of CNS.
    • In Niemann-Pick disease, sphingomyelin accumulates in brain, liver and spleen, which are enlarged causing mental retardation and death occurs in early life.
    • On hydrolysis the sphingomyelin yields a fatty acid, phosphoric acid, choline and a complex amino alcohol sphingosine.

Sphingomyelin

  • Glycolipids
    • The glycolipids contain ceramide and one or more sugars.
    • They do not contain phosphate group. The sugar containing sphingolipids are also called as  “glycosphingolipids”. Eg., Cerebrosides. These are, ceramide attached to single sugar, which may be either a molecule of galactose or glucose. As they contain neutral sugars, they are called as "Neutral Glycosphingolipids".
    • Galactocerebrosides are present in cell membranes of the nervous tissue (particularly white matter of the brain) whereas glucocerebrosides are present in extra-neural tissues and only traces in brain.
    • In humans diseases called Taysach’s disease and Gaucher’s disease are seen. In these the lipids are not broken down properly and accumulate causing severe brain disorder.
    • It is involved in myelin formation, nerve impulse conduction, signal transduction and as receptor component for certain hormones / bacterial toxins. 
    • It is antigenic.
    • A common fatty acid component of cerebroside is cerebronic acid. In addition, it may contain nervonic or lignoceric acid.
    • Psychosin: When fatty acids are cleaved from cerebroside by alkaline hydrolysis with Ba(OH)2, the remaining glycoside sphiingosin is called as psychosin.

Glycolipids

  •  Gangliosides
    • They are also called as "Acidic glycosphingolipids", as they contain sialic acids.
    • These are most complex glycosphingolipids, that contain ceramide and one or more molecules of sialic acid (N- acetyl neuraminic acid). Gangliosides are identified as GM1, GM2, GM3, etc, where G stands for gangliosides and M for mono sialic acid. The subscript number is based on the chromatographic migration.
    • Similarly, GD -D for disialic acid, GT - T-trisialic acid and GQ - Q-quadra (four) sialic acids.

    Gangliosides

    • They are predominantly found in ganglions. They are the components of cell surface membrane. It constitutes 6% of the brain lipid. They act as specific receptors for some pituitary hormones and also involved in cell – cell recognition, growth and differentiation of tissues.
  • Sulfatides
    • Sulfoglycosphingolipids (sulfatides) are cerebrosides that contain a galactosyl residue that has been sulfated and is therefore negatively charged at physiologic pH.
    • Sulfatides are found predominantly in nervous tissue and in other tissues like liver, kidney and testes. Ratio of cerebrosides to sulphatides in brain is normally 3:1.
Last modified: Saturday, 24 December 2011, 5:57 AM