The interaction of coenzyme Q with phosphatidylethanolamine membranes

Coenzyme Q (CoQ) is a component of the mitochondrial respiratory chain whic h carries out additional membrane functions, such as acting as an antioxida nt. The location of CoQ in the membrane and the interaction with the phosph olipid bilayer is still a subject of debate. The interaction of CoQ in the oxidized (ubiquinone-10) and reduced (ubiquinol-10) state with membrane mod el systems of 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (Ela(2)Gro-P- Etn) has been studied by means of differential scanning calorimetry (DSC), P-31-nuclear magnetic resonance (P-31-NMR) and small angle X-ray diffractio n (SAXD). Ubiquinone-10 did not visibly affect the lamellar gel to lamellar liquid-crystalline phase transition of Ela(2)Gro-P-Etn, but it clearly per turbed the multicomponent lamellar liquid-crystalline to lamellar gel phase transition of the phospholipid. The perturbation of both transitions was m ore effective in the presence of ubiquinol-10. A location of CoQ forming he ad to head aggregates in the center of the Ela(2)Gro-P-Etn bilayer with the polar rings protruding toward the phospholipid acyl chains is suggested. T he formation of such aggregates are compatible with the strong hexagonal H- II phase promotion ability found for CoQ. This ability was evidenced by the shifting of the lamellar to hexagonal H-II phase transition to lower tempe ratures and by the appearance of the characteristic hexagonal H-II P-31-NMR resonance and SAXD pattern at temperatures at which the pure Ela(2)Gro-P-E tn is still organized in extended bilayer structures. The influence of CoQ on the thermotropic properties and phase behavior of Ela(2)Gro-P-Etn is dis cussed in relation to the role of CoQ in the membrane.