Phospholipid

Phospholipids are formed from four components: fatty acids, a negatively charged phosphate group, an alcohol and a backbone. Phospholipids with a glycerol backbone are known as 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.

Phosphoglycerides

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 is the major component of lecithin.

Phosphatidyl ethanolamine

Phosphatidyl ethanolamine is the major component of cephalin.

Phosphatidyl inositol

Phosphatidyl serine

Diphosphatidyl glycerol

Synthesis

In phosphoglyceride synthesis, phosphatidates must first be activated. 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 first phosphorylated by ATP 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 found in the liver.

Sphingomyelin

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 present in all eukaryotic cell membranes, but is mainly present in cells of the nervous system.

Amphiphatic character

Due to its polar nature, the head of a phospholipid is attracted to water (it is hydrophilic). The nonpolar head is not attracted to water and is said to be hydrophobic. When placed in water, phospholipids form a bilayer, where the hydrophobic tails line up against each other, and forms a membrane with hydrophilic heads on both sides extending out into the water. This allows it spontaneously to form liposomes, 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 basically 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 basically free to diffuse laterally through the lipid matrix and migrate over the membrane.

See also

• biochemistry
• lipid
• lipid bilayer
• antiphospholipid syndrome

References

1. Berg, J.M., J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. xxxviii, 974, [976] (various pagings)