Sphingophospholipids are a distinct subclass of complex phospholipids, primarily represented by sphingomyelins (SM). They are characterized by a ceramide backbone—composed of sphingosine or a related sphingoid base N-acylated with a fatty acid through an amide linkage—and esterified at the C1 position with a phosphocholine or phosphoethanolamine headgroup.
Membrane Organization and Biological Role
Unlike glycerol-based glycerophospholipids such as phosphatidic acid or phosphatidylglycerol, sphingophospholipids contain a rigid sphingoid motif capable of extensive hydrogen bonding. This structural rigidity promotes tight lipid packing and results in relatively high transition temperatures (Tm ~37°C). They constitute approximately 10–20% of the outer leaflet of plasma membranes and are particularly abundant in myelin sheaths, where they contribute to lipid raft formation together with cholesterol.
Molecular Structure
The ceramide core typically consists of sphingosine (d18:1, 4-sphingenine), an 18-carbon chain bearing a Δ4-trans double bond, hydroxyl groups at positions 1 and 3, and an amide-linked fatty acid (commonly C16:0 to C24:0). The attachment of a phosphocholine headgroup (-PO4CH2CH2N+(CH3)3) yields sphingomyelin, while ceramide phosphoethanolamine (CPE) is a rarer variant mainly found in insects.
Sphingomyelins display a cylindrical molecular geometry with a headgroup area of ~70 Ų. Variations in acyl chain length and saturation can promote chain mismatch, supporting the formation of microdomains. Additional derivatives include lyso-sphingomyelin (lysoSM), which lacks the N-acyl chain.
Biophysical Properties
Sphingophospholipids exhibit high gel-phase stability, with transition temperatures (Tm 30–40°C) exceeding those of many glycerophospholipids such as lecithin. Their low hydration and strong intermolecular hydrogen bonding—mediated by the amide group and trans-unsaturated sphingoid backbone—favor phase separation into ordered liquid-ordered (Lo) domains, a hallmark of lipid raft preference.
Transbilayer movement (flip-flop) is rare under physiological conditions and typically requires specialized enzymes such as scramblases.
