Bioorganic Chemistry of Sphingolipids: Pore Formation and Anion Transport
Harrell, Jr., William A.
Davis, Jeffery T.
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Ceramide is an amphiphilic natural product that plays important roles in multiple cellular processes. Ceramide also is known to self-assemble into transmembrane pores under physiologically relevant concentrations. In order to study the role of ceramide's 1,3-diol functionality in the stabilization of transmembrane pores, ceramide analogs were prepared using protecting groups to block the 1,3-diol unit. Blocking the 1,3-diol with an acetal protecting group led to a drastic decrease in membrane-activity. Surprisingly, blocking the -OH groups of C2-ceramide 2 with simple esters yielded a C2-diacetate 16 analog with increased pore-forming activity. Additionally, a new function of C2-ceramide 2 has been discovered that has important biological implications. Working below concentrations in which it self-assembles into transmembrane pores, C2-ceramide 2 facilitated the transmembrane transport of biologically relevant anions such as Cl<super>-</super> and HCO<sub>3</sub><super>-</super> via an anion exchange mechanism. The 1,3-diol functionality of the C2-ceramide 2 headgroup was found to play an integral role in the binding and transport of anions, as the isopropylidene C2-ceramide 18 analog was unable to facilitate transmembrane anion transport. D-erythro-Sphingosine 3, produced naturally by the metabolism of ceramide, lacks the amide functionality in its hydrophilic head-group. Unlike C2-ceramide 2, sphingosine 3 does not facilitate transmembrane Cl<super>-</super>/HCO<sub>3</sub><super>-</super> exchange. Possible reasons for this failure to facilitate the transmembrane transport of anions are discussed, namely that sphingosine 3 does not bind HCO<sub>3</sub><super>-</super> in a non-covalent manner. Instead, sphingosine 3 forms carbamates in the presence of HCO<sub>3</sub><super>-</super> and CO<sub>2</sub> in a solvent dependent manner.