Cardiolipin Switch in Mitochondria:  Shutting off the Reduction of Cytochrome c and Turning on the Peroxidase Activity

Abstract

Upon interaction with anionic phospholipids, particularly mitochondria-specific cardiolipin (CL), cytochrome c (cyt c) loses its tertiary structure and its peroxidase activity dramatically increases. CL induced peroxidase activity of cyt c has been found to be important for selective CL oxidation in cells undergoing programmed death. During apoptosis, the peroxidase activity and the fraction of CL-bound cyt c markedly increases suggesting that CL may act as a switch to regulate cyt c's mitochondrial functions. Using cyclic voltammetry and equilibrium redox-titrations, we show that the redox potential of cyt c shifts negatively by 350-400 mV upon binding to CL-containing membranes. Consequently, functions of cyt c as an electron transporter and cyt c reduction by Complex III are strongly inhibited. Further, CL/cyt c complexes are not effective in scavenging superoxide anions and are not effectively reduced by ascorbate. Thus, both redox properties and functions of cyt c change upon interaction with CL in the mitochondrial membrane, diminishing cyt c's electron donor/acceptor role and stimulating its peroxidase activity.

Nitration of Solvent-exposed Tyrosine 74 on Cytochrome c Triggers Heme Iron-Methionine 80 Bond Disruption

“…In this regard, cytochrome c association to a pool of the mitochondrial phospholipid cardiolipin (27)(28)(29) and posttranslational modifications such as methionine oxidation and tyrosine nitration (30,31), promotes the transition of cytochrome c to "alternative low spin conformations" (32), which in some cases correspond to state IV.…”

Cardiolipin drives cytochrome c proapoptotic and antiapoptotic actions

“…CL-bound cytc shows a non-native tertiary structure and a disrupted heme-Fe-Met80 distal bond, with Lys79 as the likely sixth axial ligand of the heme-Fe-atom at pH [ 9 (14, 18-22). CL binding to hhcytc induces a drastically reduced midpoint potential, which falls out of the range required for its physiological role in mitochondrial respiration (17). Further, the cleavage of the distal Fe-Met80 bond endows hhcytc with proapoptotic activity, due to the achievement of peroxidase action (15, 23-26, 31, 32), and antiapoptotic functions, increasing the affinity for CO and NO (27,28) and inducing peroxynitrite detoxification properties (29).…”

“…At least 15% of mitochondrial cytc is bound to cardiolipin (CL), an unusual lipid largely confined to the inner mitochondrial membrane (3,4,(13)(14)(15)(16)(17)(18)(19)(20). The interaction with CL is pivotal for switching cytc function(s) from mitochondrial respiration to apoptosis.…”

“…The interaction with CL is pivotal for switching cytc function(s) from mitochondrial respiration to apoptosis. In fact, upon CL binding, cytc has been shown (i) to change its tertiary structure disrupting the hemeFe-Met80 distal bond (the proximal axial ligand being His) and, in some cases, to vary the spin state of the metal (4,14,(18)(19)(20)(21)(22), (ii) to reduce drastically the midpoint potential out of the range required for its role in the respiratory chain (17), (iii) to display peroxidase activity, using CL as a favorable substrate (15,(23)(24)(25)(26), (iv) to bind CO and NO with high affinity (27,28), and (v) to facilitate peroxynitrite scavenging (i.e., conversion to NO 3 2 ) (29). All these effects suggest that CL-bound cytc (CL-cytc) could play either proapoptotic effects, catalyzing lipid peroxidation, and cytc release into the cytoplasm, or antiapoptotic actions, scavenging peroxynitrite (i.e., protecting the mitochondrion from reactive nitrogen and oxygen species), and binding CO and NO (i.e., inhibiting lipid peroxidation and cytc traslocation).…”

“…2) (20). As a whole, CL binding to hhcytc seems to induce a gross conformational change(s) on both the proximal and distal side of the heme, causing the loss of the electron transfer properties of hhcytc and its transformation into a globin-and peroxidaselike heme-protein (15,17,(27)(28)(29).…”