Complex lipids. Phosphoglycerates, sphingolipids and glycolipids – types, structure, properties, representatives.
Compound (or complex) lipids are lipids that, upon hydrolysis, yield fatty acids (FA), alcohol (either glycerol or sphingosine), and other compounds such as phosphoric acid, amino alcohol, sugar.
Phospholipids. When the lipid contains residues from FA, glycerol and phosphoric acid it is called glycerophospholipid. When besides FA and residue of phosphoric acid, the lipid contains long-chain unsaturated amino alcohol (instead of glycerol), the lipid is called sphingophospholipid.
There are two large groups of glycerophospholipids: phosphatides and plasmalogens. Both are esters of glycerol. Molecules of phosphatides have two ester bonds to fatty acids and one ester bond to phosphoric acid, resulting in phosphatidic acid. This acid has a chiral center (carbon atom 2) which in all natural lipids is with L-configuration related to L-glyceraldehyde in Fisher projection.
Phosphatidylcholine (PC, in older literature also called lecithin), R=C17H35COOH, R’= C17H33COOH, plays an important role in the metabolism of fats in the liver and in the transport of fats from one part of the body to another. It serves as a source of inorganic phosphate for tissue formation and is an excellent emulsifying agent. Egg yolks contain large amounts of PC and are used to emulsify salad oil and vinegar to make mayonnaise.
The removal after selective hydrolysis of one FA from PC gives lysolecithin which is poisonous causing destruction of red blood cells and spasmodic muscle contractions. Poisonous snake venom contains enzymes catalyzing formation of lysolecithin from PC.
Phosphatidylethanolamines are also called cephalins. Like lecithin, cephalin consists of a combination of glycerol esterified with two fatty acids and phosphoric acid. Whereas the phosphate group is esterified with choline in lecithin, it is combined with ethanolamine in cephalin. Cephalin is found in all living cells, although in human physiology it is found particularly in nervous system 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.
Large group phospholipids are the so-called plasmalogens (~10% of lipids in CNS).
They are considered acetal phosphatides. They differ from the ester phosphatides by the presence of an unsaturated ether group instead of an acyl group at the glycerol carbon one. This double bond is a result from dehydration of an acetal formed between the glycerol OH and long-chain aliphatic aldehyde, hence the name acetal phosphatides.