Which of the following linkages is found in sphingomyelin but not in phosphoglycerides?
Phosphoglycerides use glycerol backbone where fatty acids are linked to sn-1 and sn-2 hydroxyls via O-ester bonds, susceptible to phospholipase A1 and A2 hydrolysis generating lysophospholipids and free fatty acids like arachidonic acid. Sphingomyelin architecture departs: its backbone is sphingosine, containing long-chain amino alcohol with double bond, where fatty acid attaches at amino group at C2 through N-amide bond formed by ceramide synthase in ER, producing ceramide intermediate. This amide bond affords additional hydrogen bond donor and acceptor, raising melting temperature and strengthening interaction with cholesterol's hydroxyl through hydrogen bonding network, favoring liquid-ordered domains resistant to detergent extraction. After ceramide formation, phosphocholine is transferred from phosphatidylcholine to ceramide's C1 hydroxyl by sphingomyelin synthase, creating phosphodiester linkage shared with phosphoglycerides but distinct acyl linkage. Therefore presence of amide bond fundamentally distinguishes sphingomyelin, influencing its metabolic stability, raft partitioning, and role as source for bioactive ceramide upon sphingomyelinase action during stress signaling.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Sphingomyelin Structure, Amide Linkage.