Phosphatidylethanolamine

Phosphatidylethanolamine (cephalin, sometimes abbreviated PE) is a lipid found in biological membranes. It is synthesized by the addition of CDP-ethanolamine to diglyceride, releasing CMP. S-adenosyl methionine can subsequently methylate the amine of phosphatidyl ethanolamine to yield phosphatidyl choline.

Cephalin is a phospholipid, which is a lipid derivative. It is not to be confused with the molecule of the same name that is an alkaloid constituent of Ipecac.

Function
Cephalin is found in all living cells, although in human physiology it is found particularly in nervous tissue such as the white matter of brain, nerves, neural tissue, and in spinal cord. Whereas lecithin is the principal phospholipid in animals, cephalin is the principal one in bacteria.

As a polar head group, phosphatidylethanolamine (PE) creates a more viscous lipid membrane compared to phosphatidylcholine (PC). For example, the melting temperature of di-oleoyl-PE is -16C while the melting temperature of di-oleoyl-PC is -20C. If the lipids had two palmitoyl chains, PE would melt at 63C while PC would melt already at 41C (See references in Wan et al. Biochemistry 47 2008). Lower melting temperatures correspond, in a simplistic view, to more fluid membranes.

Chemistry
In the chemical sense, cephalin is phosphatidylethanolamine. Like lecithin, it consists of a combination of glycerol esterified with two fatty acids and phosphoric acid. Whereas the phosphate group is combined with choline in Lecithin, it is combined with the ethanolamine in Cephalin.

The two fatty acids may be the same, or different, and are usually in the 1,2 positions (though can be in the 1,3 positions).