pH Dependence of Ferricytochrome <i>c</i> Conformational Transitions during Binding to Cardiolipin Membranes: Evidence for Histidine as the Distal Ligand at Neutral pH

The conformational changes of ferricytochrome <i>c</i> upon binding to cardiolipin-containing small unilamellar vesicles were studied at slightly acidic pH using fluorescence, visible circular dichroism, UV–visible absorption, and resonance Raman spectroscopy. The obtained spectroscopic response data suggest a mode of interaction, which is clearly distinct from the binding process observed at neutral pH. Evidence of a reversible and electrostatic binding mechanism under these conditions is provided through binding inhibition in the presence of 150 mM NaCl. Moreover, UV–visible absorption and resonance Raman spectra reveal that the conformational ensemble of membrane bound cytochrome <i>c</i> is dominated by a mixture of conformers with pentacoordinated and hexacoordinated high-spin heme irons, which contrast with the dominance of low-spin species at neutral pH. While our results confirm the L-site binding proposed by Kawai et al., they point to the protonation of a histidine ligand (H33) as conformational trigger

Autoxidation of Reduced Horse Heart Cytochrome <i>c</i> Catalyzed by Cardiolipin-Containing Membranes

Visible circular dichroism, absorption, and fluorescence spectroscopy were used to probe the binding of horse heart ferrocytochrome <i>c</i> to anionic cardiolipin (CL) head groups on the surface of 1,1′,2,2′-tetraoleoyl cardiolipin (TOCL)/1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine (DOPC) (20%:80%) liposomes in an aerobic environment. We found that ferrocytochrome <i>c</i> undergoes a conformational transition upon binding that leads to complete oxidation of the protein at intermediate and high CL concentrations. At low lipid concentrations, the protein maintains a structure that is only slightly different from its native one, whereas an ensemble of misligated predominantly hexacoordinated low-spin states become increasingly populated at high lipid concentrations. A minor fraction of conformations with either high- or quantum-mixed-spin states were detected at a CL to protein ratio of 200 (the largest one investigated). The population of the non-native state is less pronounced than that found for cytochrome <i>c</i>–CL interactions initiated with oxidized cytochrome <i>c</i>. Under anaerobic conditions, the protein maintains its reduced state but still undergoes some conformational change upon binding to CL head groups on the liposome surface. Our data suggest that CL-containing liposomes function as catalysts by reducing the activation barrier for a Fe²⁺ → O₂ electron transfer. Adding NaCl to the existing cytochrome–liposome mixtures under aerobic conditions inhibits protein autoxidation of ferrocytochrome <i>c</i> and stabilizes the reduced state of the membrane-bound protein