Sphingomyelin - A unique phospholipid

Sphingomyelins (SMs) are important components of the cellular plasma membranes of eucaryotic cells. SMs together with phosphatidylcholines comprise most of the phospholipids in the external leaflets of these membranes. Natural SMs consist of a polar headgroup (phosphoryl choline) and a long-chain D-erythro (2S,3R) sphingoid base (usually 18 C) with an amide-linked acyl chain. The acyl chain is usually long (16-24 C) and saturated and gives the SM molecules their asymmetric nature. SMs have fairly high (30-45°C) gel to liquid-crystalline phase-transition temperatures (Tm) compared to other naturally occurring phospholipids. SMs are thus potential candidates as molecules that have the ability to introduce lateral heterogeneity in membranes. Lateral domains, also called rafts, are known to exist in biological membranes and SMs have been found in these domains together with glycosphingolipids and cholesterol. SMs also have unique hydrogen bonding properties in the interfacial region. The SM carbonyl can function as a hydrogen bond acceptor, whereas the hydroxyl group at C3 and the amide group at C2 in the sphingosine base can function as hydrogen donors as well. A hydrogen bond has been indicated in the interaction between SMs and cholesterol. These lipids co-localize in the external leaflet of the cellular plasma membranes. Cholesterol appears to interact preferentially with SM in both biological and model membranes. Together, these lipids have also been shown to form detergent-resistant lateral domains. The presence of SM in lateral domains in biological membranes gives this lipid a role in cellular functions such as endocytosis, signal transduction and protein sorting.