Sphingomyelin (SM) represents the principal phosphosphingolipid in animal cell membranes. It is synthesized from a ceramide backbone N-acylated at C2 of sphingosine (typically C18:1 Δ4-trans) and bears a phosphocholine headgroup at C1. This structure parallels phosphatidylcholine but includes a rigid, hydrogen-bonding sphingoid base that contributes to the stabilization of lipid rafts.
Role in Membrane Organization
Comprising approximately 5–20% of the plasma membrane outer leaflet lipids (with the highest abundance in myelin), sphingomyelin contains predominantly saturated acyl chains (C14–C26, often 16:0/18:0/24:1). Its tight packing with cholesterol distinguishes it from more fluid glycerophospholipids such as phosphatidylcholine (PC) or phosphatidylethanolamine (PE), driving membrane domain segregation critical for cellular signaling processes.
Molecular Structure
Sphingomyelin consists of sphingosine (D-erythro-4-sphingenine: 18 carbons, trans-Δ4 double bond, 3-OH group, and 2-NH-acyl) amide-linked to a fatty acid, combined with a phosphocholine headgroup (-PO4-CH2CH2N+(CH3)3). Representative species include PSM (16:0) and SSM (18:0).
Structurally, SM is quasi-cylindrical (headgroup area ~72 Ų, thickness 38–45 Å) and possesses interfacial hydrogen-bond donors and acceptors (amide and 3-OH groups) absent in diacylglycerols. Chain-length mismatch can induce interdigitation within bilayers.
Biophysical Properties
Sphingomyelin exhibits a relatively high melting temperature (Tm), with PSM around 35°C and SSM near 40°C, providing gel-phase stability close to physiological temperature. Cholesterol condenses SM more strongly than PC, promoting the formation of liquid-ordered (Lo) domains characteristic of lipid rafts.
Small-angle neutron and X-ray scattering (SANS/SAXS) studies confirm bilayer thickness of approximately 50–55 Å and an area per lipid of 38–45 Ų. SM shows low propensity for hexagonal (HII) phase formation, and raft domains typically exclude highly unsaturated acyl chains.
