Small-Molecule Uncoupling Protein Mimics: Synthetic Anion Receptors as Fatty Acid-Activated Proton Transporters

Uncoupling proteins (UCPs) regulate energy expenditure in living cells by inducing proton leakage across the mitochondrial inner membrane, thereby uncoupling adenosine diphosphate phosphorylation from nutrient oxidation. The proton transport activity of UCP1 and UCP2 requires activation by fatty acids. We report here the first examples of synthetic neutral anion receptors performing this biologically important fatty acid-activated function in phospholipid bilayers. We have shown that a tripodal thiourea possesses poor H⁺/OH^- transport activity without fatty acids, but in the presence of long-chain fatty acids is "switched on" as a proton transporter with an activity close to that of a commonly used protonophore. The fatty acid-enhanced proton transport was also observed for other hydrogen and halogen bond-based synthetic anion transporters. We propose that these compounds induce proton permeability by catalyzing transbilayer movement ("flip-flop") of anionic forms of fatty acids, so allowing the fatty acids to complete a proton transport cycle. Several lines of evidence have been provided to support such a fatty acid cycling mechanism. Our findings open up new applications of anion receptor chemistry and provide important clues for understanding biological activities of synthetic anion transporters and potentially the uncoupling mechanism of naturally occurring membrane proteins.