General Index
Amphipathic Lipids: Phospholipids
With the name of Amphipathic Lipids we know a class of lipids that have, on the same molecule, a Polar Portion,
that interacts easily with water, and a Hydrophobic Portion from which water and other polar molecules are excluded.
The most important feature of amphipathic lipids is their ability to autostructurate. Interaction of hydrophobic and
polar portions of the molecules lead to supramolecular aggregates like Micelles, Monolayers and Bilayers. The bilayer
structure is the basic motif of all the Biological Membranes.
Chemically, amphipathic lipids are formed from one alcohol (glycerol or sphingosine) esterified to one or two fatty acids, that
constitute the hydrophobic portion of the molecule. The alcohol appear also bound to a polar group, Depending on the nature of this polar group, we
classify the amphipathic lipids as (1) Phospholipids and (2) Glycolipids.
Phospholipids
The general structure of phospholipids can be seen in the molecule of Phosphatidylcholine or Lecithin:
In this structure we see (a) Glycerol:
R1 y R2 are the fatty acid radicals esterified to sn-1 and sn-2
The hydrophobic portion of the molecule is:
Phospholipids differ according to the nature of the second alcohol. Thus we have Phosphatidyl ethanolamine or Cephaline when the second alcohol is Ethanolamine:
The phosphatidic acid can be esterified to the aminoacid Serine, giving Phosphatidylserine:
The second alcohol may also be another molecule of glycerol, giving Phosphatidylglycerol:
Whose free -OH may appear esterified to a second residue of phosphatidic acid, giving Dihosphatidylglycerol or Cardiolipin:
We can also find as a second alcohol myo-Inositol:
in the Phosphatidylinositols or Phosphoinositides:
Very often other -OH groups of inositol appear esterified to phosphate.
Other type of phospholipids are the Plasmalogens:
In these compounds, the acyl group substituting the sn-1 position is linked through a Vinyl ether bond:
instead of the normal ester bond seen in other phospholipids. Note also that the double bond is in trans-.
If the primary alcohol, instead of glycerol, is Sphingosine:
We get the Sphingophospholipids. The amino group of sphingosine is substituted by an acyl group though an amido bond, and the alcohol group by a phosphocholine giving the structure of Sphingomyelin: