Sphingomyelin

Sphingomyelin is a type of sphingophospholipid and a major component of animal cell membranes, especially in the myelin sheath surrounding nerve fibers. It consists of a ceramide backbone, formed by a sphingosine molecule amide-linked to a fatty acid, with a phosphocholine head group attached via a phosphodiester bond.

Biosynthesis occurs in the Golgi apparatus, where sphingomyelin synthase transfers a phosphocholine moiety from phosphatidylcholine to ceramide, producing sphingomyelin and diacylglycerol. Sphingomyelin can also be formed through salvage pathways in membranes.

Functions include contributing to membrane structural integrity and the formation of lipid rafts—ordered microdomains that organize signaling molecules. Its high concentration in myelin supports rapid nerve conduction. Metabolically, sphingomyelin serves as a reservoir for bioactive lipids: hydrolysis by sphingomyelinase yields ceramide and phosphocholine. Ceramide acts as a signaling lipid involved in processes such as apoptosis, differentiation, and stress responses; thus, sphingomyelin metabolism links membrane structure to intracellular signaling.

Clinical relevance: Deficiency of acid sphingomyelinase, leading to impaired breakdown of sphingomyelin, causes Niemann-Pick disease types A and B, characterized by lysosomal storage of sphingomyelin and multisystemic symptoms. Abnormal sphingolipid metabolism is also associated with other lysosomal storage disorders and certain neurodegenerative conditions.

Overall, sphingomyelin plays a key role in membrane architecture and lipid signaling, with its metabolism connecting membrane composition to cellular responses.