Cytochrome c/cardiolipin relations in mitochondria: a kiss of death

doi:10.1016/j.freeradbiomed.2009.03.004

Free Radical Biology and Medicine

Volume 46, Issue 11, 1 June 2009, Pages 1439-1453

https://doi.org/10.1016/j.freeradbiomed.2009.03.004 Get rights and content

Abstract

Recently, phospholipid peroxidation products gained a reputation as key regulatory molecules and participants in oxidative signaling pathways. During apoptosis, a mitochondria-specific phospholipid, cardiolipin (CL), interacts with cytochrome c (cyt c) to form a peroxidase complex that catalyzes CL oxidation; this process plays a pivotal role in the mitochondrial stage of the execution of the cell death program. This review is focused on redox mechanisms and essential structural features of cyt c’s conversion into a CL-specific peroxidase that represent an interesting and maybe still unique example of a functionally significant ligand change in hemoproteins. Furthermore, specific characteristics of CL in mitochondria—its asymmetric transmembrane distribution and mechanisms of collapse, the regulation of its synthesis, remodeling, and fatty acid composition—are given significant consideration. Finally, new concepts in drug discovery based on the design of mitochondria-targeted inhibitors of cyt c/CL peroxidase and CL peroxidation with antiapoptotic effects are presented.

Introduction

As a triplet biradical with two parallel spins, molecular oxygen readily interacts with other radicals—e.g., lipid alkyl radicals, thiyl radicals—but it has a very poor reactivity toward molecules with fully paired electrons (nonradicals). As kids, everyone was amazed by a famous experiment in chemistry class in which the teacher burned a small strip of iron in an atmosphere of oxygen. Radicals generated by the high temperature of the flame and combustion facilitated the oxidation of the iron. Remarkably, iron is vital to the functions of diverse enzymes for which it catalyzes reactions with oxygen; however, the chemistry of life does not burn our body. On the contrary, aerobically living cells have developed a safe and sophisticated machinery to activate oxygen and catalyze slow and well-controlled oxidation (but not combustion) processes. Yet, oxygen radicals are continuously produced in our body via a univalent reduction of molecular oxygen. Whereas all one-electron products of oxygen reduction are called “reactive oxygen species,” only one of them—the hydroxyl radical HO (a three-electron reduction intermediate of oxygen)—is notorious for its remarkably high and calamitously indiscriminative reactivity toward most biomolecules.

It is a common belief that strict control and elimination of superoxide and hydrogen peroxide (H₂O₂) are protective mechanisms preventing cell damage and death [1]. Recently, however, superoxide radicals and hydrogen peroxide gained a reputation as regulatory molecules and participants in oxidative signaling pathways. Superoxide dismutases (SODs)¹—in the mitochondrial matrix and intermembrane space, in the cytosol, and in extracellular compartments—convert superoxide radicals into H₂O₂. Thus, SODs may act as important regulators and sources of H₂O₂. An important process through which cells utilize H₂O₂ for signaling purposes is the peroxidase catalytic cycle of hemoproteins¹. Whereas activation of H₂O₂ by peroxidases is usually effectively controlled by the participating protein moieties, it is still a high-risk endeavor; changes in the redox environment, protein structure, or genotoxic events may lead to unregulated activation of H₂O₂ and the production of hydroxyl radicals. In this review, we focus on cytochrome c (cyt c)—a well-known hemoprotein electron transporter in mitochondria—to illustrate possible mechanisms and consequences stemming from peroxidase activation of this protein by physiologically relevant anionic phospholipids.

Section snippets

Multiple functions of cytochrome c in cells

Over the past 2 decades, we witnessed the collapse of an old dogma of biochemistry: one gene → one protein → one function. Discoveries of new functions of cyt c are one of the stunning hallmarks of this paradigm shift. In addition to its well-established role as an electron shuttle between respiratory complexes III and IV in mitochondria, the antioxidant role of cyt c has been linked to its propensity to catalyze the oxidation of superoxide radicals to molecular oxygen. Thus, cyt c can act as a

Binding modes of cyt c to anionic phospholipids

During the past 3 decades, studies of cyt c revealed several protein binding sites for anionic lipids; at least 30% of the protein surface constitutes so-called A-, C-, and L-candidate binding domains believed to participate in interactions with anionic lipids (see Fig. 1) [24], [25], [26]. Both the penetration of phospholipid acyl chains into the protein globule and the protein integration into the phospholipid bilayer of the membrane were suggested as possible binding modes. Interactions of

Peroxidase function of cyt c/CL complexes in apoptosis

Multiple functions of cyt c—in mitochondrial electron transport, peroxidase oxidation of CL, interactions with Apaf-1 in the cytosol—raise a question about regulation and switching mechanisms involved in its diverse pathways. One of these mechanisms is a marked negative shift of cyt c’s redox potential upon its interaction with CL, thus precluding its operation as an electron acceptor from mitochondrial complex III or from superoxide radicals (see above). Another important regulatory mechanism

Inhibition of CL peroxidation as a new approach to antiapoptotic drug discovery

The discovery of the specific oxygenase activity of cyt c toward CL peroxidation and its essential role in the execution of the apoptotic program indicates possible directions for an effective regulation of apoptosis. Prevention of CL peroxidation may be important because it can be accomplished in mitochondria before the release of proapoptotic factors into the cytosol, i.e., before the “point of no return” associated with the activation of the caspase cascades [20]. To achieve a substantial

Concluding remarks

The organization of native cyt c favors its most common function as an electron shuttle between complexes III and IV of mitochondria. Hexa-coordination of heme-iron, utilization of not readily oxidizable Met80 as the distal ligand, lack of Arg and His residues in close proximity to heme, remote location of electron-accepting Trp or Tyr residues—all of these features decrease the occurrence of peroxidase functions in native cyt c. However, the binding of cyt c to anionic phospholipids unfolds

Acknowledgments

This work was supported by NIH Grants NIAID U19AI068021, HL70755, HD057587, NS061817, DAMD 17-01-2-637, and R03TW007320; by the Pennsylvania Department of Health, SAP 4100027294; and by the Human Frontier Science Program.

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Review ArticleCytochrome c/cardiolipin relations in mitochondria: a kiss of death

Abstract

Introduction

Section snippets

Multiple functions of cytochrome c in cells

Binding modes of cyt c to anionic phospholipids

Peroxidase function of cyt c/CL complexes in apoptosis

Inhibition of CL peroxidation as a new approach to antiapoptotic drug discovery

Concluding remarks

Acknowledgments

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Review Article
Cytochrome c/cardiolipin relations in mitochondria: a kiss of death