The interaction of cytochrome c (cyt c) with mitochondrial mimetic vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, and heart cardiolipin (PCPECL) was investigated over the 7.4 -6.2 pH range by means of turbidimetry and photon correlation spectroscopy. In the presence of cyt c, the decrease of pH induced an increase in vesicle turbidity and mean diameter resulting from vesicle fusion as determined by a rapid decrease in the excimer/monomer ratio of 2-(10-(1-pyrene)-decanoyl)-phosphatidylcholine (PyPC). N-acetylated cyt c and protamine, a positively charged protein, increased vesicle turbidity in a pH-independent manner, whereas albumin did not affect PCPECL vesicle turbidity. pH-dependent turbidity kinetics revealed a role for cyt c-ionizable groups with a pK a(app) of ϳ7.0. The carbethoxylation of these groups by diethylpyrocarbonate prevented cyt c-induced vesicle fusion, although cyt c association to vesicles remained unaffected. Matrix-assisted laser desorption ionization time-of-flight analysis revealed that Lys-22, Lys-27, His-33, and Lys-87 cyt c residues were the main targets for carbethoxylation performed at low pH values (<7.5). In fact, these amino acid residues belong to clusters of positively charged amino acids that lower the pK a . Thus, at low pH, protonation of these invariant and highly conserved amino acid residues produced a second positively charged region opposite to the Lys-72 and Lys-73 region in the cyt c structure. These two opposing sites allowed two vesicles to be brought together by the same cyt c molecule for fusion. Therefore, a novel pH-dependent site associating cyt c to mitochondrial mimetic membranes was established in this study.

Cytochrome c exhibits two positively charged sites: site A containing lysine residues with high pKa values and site L containing ionizable groups with pKaobs values around 7.0. This protein feature implies that cytochrome c can participate in the fusion of mitochondria and have its detachment from the inner membrane regulated by cell acidosis and alkalosis. In this study, we demonstrated that both horse and tuna cytochrome c exhibited two types of binding to inner mitochondrial membranes that contributed to respiration: a high-affinity and low-efficiency pH-independent binding (microscopic dissociation constant Ksapp2, approximately 10 nM) and a low-affinity and high-efficiency pH-dependent binding that for horse cytochrome c had a pKa of approximately 6.7. For tuna cytochrome c (Lys22 and His33 replaced with Asn and Trp, respectively), the effect of pH on Ksapp1 was less striking than for the horse heme protein, and both tuna and horse cytochrome c had closed Ksapp1 values at pH 7.2 and 6.2, respectively. Recombinant mutated cytochrome c H26N and H33N also restored the respiration of the cytochrome c-depleted mitoplast in a pH-dependent manner. Consistently, the detachment of cytochrome c from nondepleted mitoplasts was favored by alkalinization, suggesting that site L ionization influences the participation of cytochrome c in the respiratory chain and apoptosis.

Fluorescence quenching of lipid-bound pyrene was used to assess the binding of cytochrome c (cyt c) to liposomes that mimic the inner mitochondrial membrane (IMM) POPC/DOPE/TOCL, with the conditions that it did or did not contain oxidized phosphatidylcholine molecules, i.e., 1-O-hexadecyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC), or a mixture of two hydroperoxide isomers derived from POPC (POPCOX). The binding isotherms reveal two dissociation constants, K(D)(1) and K(D)(2), representing, respectively, the low- and high-affinity states of the membrane. These dissociation constants probably are due to the lipid reorganization promoted by cyt c, as observed in giant unilamellar vesicles that contain fluorescent cardiolipin (CL). The presence of PazePC, which has a nonreactive carboxylic group, increased the K(D)(1) and K(D)(2) values 1.2- and 4.5-fold, respectively. The presence of POPCOX which has a reactive peroxide group, decreased the K(D)(1) value 1.5-fold, increased the K(D)(2) value 10-fold, and significantly reduced the salt-induced detachment of cyt c. MALDI-TOF spectrometry analysis of cyt c incubated with liposomes containing POPCox demonstrated a mass increase corresponding to the formation of nonenal adducts as hydrophobic anchors. Electronic absorption spectroscopy, circular dichroism, and magnetic circular dichroism demonstrated that all of the lipids studied promoted changes in the cyt c coordination sphere. Therefore, in the presence of CL, the oxidation of zwitterionic lipids also promotes changes in the cyt c structure and in the affinity for lipid bilayers.

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