Phospholipid
 |
Two schematic representations of a phospholipid. |
Phospholipids are a class of
lipids formed from four components:
fatty acids, a negatively-charged
phosphate group, nitrogen containing
alcohol and a backbone. Phospholipids with a
glycerol backbone are known as
glycerophospholipids or phosphoglycerides. There is only one type of phospholipid with a
sphingosine backbone;
sphingomyelin. Phospholipids are a major component of all biological
membranes, along with
glycolipids and
cholesterol.
In phosphoglycerides, the
carboxyl group of each fatty acid is esterified to the
hydroxyl groups on carbon-1 and carbon-2. The phosphate group is attached to carbon-3 by an ester link. This molecule, known as a
phosphatidate, is present in small quantities in membranes, but is also a precursor for the other phosphoglycerides.
Phosphatidyl choline
 |
Phosphatidyl-Choline.png |
Phosphatidyl choline is the major component of
lecithin. It is also a source for
choline in the synthesis of acetylcholine in cholinergic neurons.
Phosphatidyl ethanolamine
 |
Phosphatidyl-Ethanolamine.png |
Phosphatidyl ethanolamine is the major component of
cephalin.
Phosphatidyl inositol
 |
Phosphatidyl-Inositol.png |
Phosphatidyl serine
 |
Phosphatidyl-Serine.png |
Diphosphatidyl glycerol (Cardiolipin)
 |
Diphosphatidyl-Glycerol.png |
Synthesis
In phosphoglyceride synthesis,
phosphatidates must be activated first. Phospholipids can be formed from an activated diacylglycerol or an activated alcohol. Phosphatidyl serine and phosphatidyl inositol are formed from a phosphoester linkage between the hydroxyl of an alcohol (
serine or
inositol) and cytidine diphosphodiacylglycerol (CDP-diacylglycerol).
In the synthesis of phospatidyl ethanolamine, the alcohol is phosphorylated by
ATP first, and subsequently reacts with cytidine diphosphate (CDP) to form the activated alcohol. The alcohol then reacts with a diacylglycerol to form the final product.
In mammals, phosphatidyl choline can be synthesized via two separate pathways; a series of reactions similar to phosphatidyl ethanolamine synthesis, and the methylation of phosphatidyl ethanolamine, which is catalyzed by phosphatidyl ethanolamine methyltransferase, an enzyme produced in the liver.
|
Sphingomyelin (Red:Phosphatidyl choline, Blue:Acyl CoA) |
The backbone of sphingomyelin is
sphingosine, an
amino alcohol formed from
palmitate and
serine. The amino terminal is
acylated with a by a long-chain acyl CoA to yield
ceramide. Subsequent substitution of the terminal hydroxyl group by phosphatidyl choline forms sphingomyelin.
Sphingomyelin is also present in all
eukaryotic cell membranes, especially the plasma membrane, and is particularly concentrated in the nervous system because sphingomyelin is a major component of
myelin, the fatty insulation wrapped around nerve cells by
Schwann cells or
oligodendrocytes. Multiple Sclerosis is a disease characterised by deterioration of the myelin sheath, leading to impairment of nervous conduction.
Due to its polar nature, the head of a phospholipid is
hydrophilic (attracted to
water); the nonpolar tails are
hydrophobic (not attracted to water). When placed in water, phospholipids form a
bilayer, where the hydrophobic tails line up against each other, forming a membrane with hydrophilic heads on both sides extending out into the water. This allows it to form
liposomes spontaneously, or small lipid
vesicles, which can then be used to transport materials into living organisms and study diffusion rates into or out of a cell membrane. This membrane is partially
permeable, very flexible, and has
fluid properties, in which embedded proteins and phospholipid molecules are constantly moving laterally across the membrane because of the forces generated by their vibrations. Such movement can be described by the
Fluid Mosaic Model, which describes the membrane as a "mosaic" of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane.
*
Antiphospholipid syndrome*
Biochemistry*
Lipid*
Lipid bilayer (e.g.,
Cell membrane)
# Berg, J.M., J.L. Tymoczko, and L. Stryer,
Biochemistry. 5th ed. 2002, New York: W.H. Freeman. xxxviii, 974, [976] (various pagings)
