Interaction of the mitochondria-targeted antioxidant MitoQ with phospholipid bilayers and ubiquinone oxidoreductases

Abstract

MitoQ10 is a ubiquinone that accumulates within mitochondria driven by a conjugated lipophilic triphenylphosphonium cation (TPP+). Once there, MitoQ10 is reduced to its active ubiquinol form, which has been used to prevent mitochondrial oxidative damage and to infer the involvement of reactive oxygen species in signaling pathways. Here we show MitoQ10 is effectively reduced by complex II, but is a poor substrate for complex I, complex III, and electron-transferring flavoprotein (ETF):quinone oxidoreductase (ETF-QOR). This differential reactivity could be explained if the bulky TPP+ moiety sterically hindered access of the ubiquinone group to enzyme active sites with a long, narrow access channel. Using a combination of molecular modeling and an uncharged analog of MitoQ 10 with similar sterics (tritylQ10), we infer that the interaction of MitoQ10 with complex I and ETF-QOR, but not complex III, is inhibited by its bulky TPP+ moiety. To explain its lack of reactivity with complex III we show that the TPP+ moiety of MitoQ10 is ineffective at quenching pyrene fluorophors deeply buried within phospholipid bilayers and thus is positioned near the membrane surface. This superficial position of the TPP+ moiety, as well as the low solubility of MitoQ10 in non-polar organic solvents, suggests that the concentration of the entire MitoQ10 molecule in the membrane core is very limited. As overlaying MitoQ10 onto the structure of complex III indicates that MitoQ10 cannot react with complex III without its TPP+ moiety entering the low dielectric of the membrane core, we conclude that the TPP+ moiety does anchor the tethered ubiquinol group out of reach of the active site(s) of complex III, thus explaining its slow oxidation. In contrast the ubiquinone moiety of MitoQ10 is able to quench fluorophors deep within the membrane core, indicating a high concentration of the ubiquinone moiety within the membrane and explaining its good antioxidant efficacy. These findings will facilitate the rational design of future mitochondria-targeted molecules. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.