**4. Conclusion**

16 Bioenergetics

viability and infertility. Sperm are particularly susceptible to the undesirable effects of ROS because their high content of polyunsaturated fatty acids present in the plasma membrane and a low concentration of ROS scavenging enzymes in the cytoplasm (Jones et al, 1979, Huttemann et al., 2011; Sharma et al, 1999; Liu et al., 2006). Mammalian germ cells express two types of cytochrome c during their development: the somatic cytochrome c and a testis specific cytochrome c that shares 86,5% identity with the somatic counterpart. During meiosis, the expression of somatic cytochrome c declines and testis cytochrome c becomes the predominant form in sperm. Liu et al, reports that testis cytochrome c is three fold more efficient than the somatic one in the catalysis of H2O2 reduction and is also more resistant to be degraded by the side products of this reaction. In line with the proposal that apoptosis is also an antioxidant protective mechanism, testis cytochrome c exhibited higher apoptotic activity in the well established apoptosis measurement system using *Xenopus* egg extract. Therefore, testis cytochrome c can protect sperm from the damages caused by H2O2 as well as promote the elimination of sperm whose DNA was damaged. Taken together the electron-leak pathway and apoptosis, probably related to a peroxidase activity are the contribution of testis cytochrome c for the biological integrity of sperm produced by

Therefore, a delicate balance controls both antioxidant and prooxidant activities of cytochrome c with repercussions on both bioenergetics and cell death. In this regard, it is noteworthy that cytochrome c import to mitochondria, synthesis and activities underlying life and death fates for cells are regulated by signaling mechanisms and involves thiol redox balance, allosteric regulation and chemical modifications including nitration and phosphorylation. Cytochrome c is a nuclear-coded protein that is imported by mitochondria as apoprotein and, in the intermembrane space, is converted to the holoprotein by the covalent ligation of the heme group to cystein residues 14 and 17, a process catalyzed by the enzyme heme lyase (Dumont et al, 1991). The addition of the heme group confers redox properties for cytochrome c and enables it to participate, as terminal oxidant agent, in the thiol redox cascade involved in the import and assembly of TIMs (transporters of the inner mitochondrial membrane) (Chacinska et al, 2004, Riemer et al., 2011; Allen et al., 2005). The participation of cytochrome in the respiratory chain as electron carrier is also controlled by allosteric and covalent modification mechanisms. ATP has been characterized as a downregulator of the electron-transfer activity of cytochrome c. The mechanism may involve changes of both charge and structure of cytochrome c and is consistent with the adjustment of respiratory chain activity to the energy demand of cell signaled by the ATP/ADP ratio. Recent findings have strongly shown that the well-known mechanism of protein phosphorylation operates also in the control of proteins responsible for the oxidative phosphorylation. The technique of cytochrome c isolation in the presence of nonspecific phosphatase inhibitors enabled the identification of tissue-specific sites of cytochrome c and evidenced the activities of this protein is under the control of this specific cell signaling mechanism mainly operating in higher organisms (Huttemann et al., 2011). Previously, it was demonstrated that redox reaction of cytochrome c with a model aldehyde, diphenylacetaldehyde, is under the control of the protonation of two tyrosine residues (Rinaldi et al, 2004). More recent findings established the phosphorylation of cytochrome c tyrosine residues is involved in the control of the transmembrane potential that in health conditions should not attain the maximal to avoid increase of ROS generation (Yu et al., 2008; Zhao et al., 2010). The consequences of cytochrome c nitration in biological systems

mammalian cells.

The evolutionary acquisition of the O2-dependent aerobic metabolism resulted in a highly more efficient use of the energetic fuels and a cell signaling mechanism based on reactive species. Concerning the ROS, the primary species produced in mitochondria is O2 •- from which both the signaling molecule, H2O2, and the highly deleterious derivative, hydroxyl radical are generated. Therefore, a very efficient antioxidant apparatus was also evolved to assure cell redox balance and repair of oxidative damages. The antioxidant apparatus encompasses enzymes able to decompose reactive species (SOD, catalase) and repair oxidative damages (thioredoxin, glutaredoxine) and free radical trapping (ascorbic acid,

Antioxidant Action of Mobile Electron Carriers of the Respiratory Chain 19

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1083-351X

74-79, ISSN 0006-291X

129), ISSN 0098-2997

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ISSN 0005-2728

ISSN 0006-2952

(545-565), ISSN 08915849

6002

tocopherol, lipoic acid). More recently, cytochrome c was included in the category of antioxidant enzymatic apparatus due to its capacity to oxidize superoxide ion and devolve the electron to the respiratory chain as well as by the capacity to reduce hydrogen peroxide. The electron transport in the respiratory chain can also be considered an antioxidant activity of cytochrome c because it contributes for the fluency of electron transport. The antioxidant activity of CoQ is based on the direct and indirect trapping of free radicals and it is not restricted to mitochondria but exerted in the whole cellular and extra-cellular media. The beneficial antioxidant activity of CoQ has been studied with the aim to develop an antioxidant therapy by the use of CoQ analogous and derivatives. Figure 7 summarizes the antioxidant activity of mobile electron carriers of the respiratory chain.

Fig. 7. Antioxidant activity of mobile electron carriers of the respiratory chain. The aerobic oxidation of biological fuels by using molecular oxygen as final acceptor of electrons in an electron transport chain allowed an efficient mechanism of withdrawing energy from biological fuels concomitant with the generation of reactive species for cell signaling but also able to promote cell damage. The redox cell balance is achieved by prevented the accumulation of reactive species without prejudicing the signaling function. Cytochrome c contributes to the maintenance of the adequate levels of hydrogen peroxide in cells by means of fluency of electron transport in the respiratory chain, oxidation of superoxide ion and reduction of hydrogen peroxide and CoQ by means of direct and indirect trapping of free radicals.

#### **5. Acknowledgments**

Authors are grateful to Fundação de Amparo à Pesquisa do Estado de São Paulo – FAPESP – for the financial support.

## **6. References**

18 Bioenergetics

tocopherol, lipoic acid). More recently, cytochrome c was included in the category of antioxidant enzymatic apparatus due to its capacity to oxidize superoxide ion and devolve the electron to the respiratory chain as well as by the capacity to reduce hydrogen peroxide. The electron transport in the respiratory chain can also be considered an antioxidant activity of cytochrome c because it contributes for the fluency of electron transport. The antioxidant activity of CoQ is based on the direct and indirect trapping of free radicals and it is not restricted to mitochondria but exerted in the whole cellular and extra-cellular media. The beneficial antioxidant activity of CoQ has been studied with the aim to develop an antioxidant therapy by the use of CoQ analogous and derivatives. Figure 7 summarizes the

**-. O2**

**CoQ**

**OH. cytc**

Cell Damage

antioxidant activity of mobile electron carriers of the respiratory chain.

**e-**

**A**

**Biological Fuel Oxidation**

**Energy**

free radicals.

**5. Acknowledgments** 

– for the financial support.

**CoQ**

**ox e-**

**Electron Chain Transport**

able to promote cell damage. The redox cell balance is achieved by prevented the

Fig. 7. Antioxidant activity of mobile electron carriers of the respiratory chain. The aerobic oxidation of biological fuels by using molecular oxygen as final acceptor of electrons in an electron transport chain allowed an efficient mechanism of withdrawing energy from biological fuels concomitant with the generation of reactive species for cell signaling but also

accumulation of reactive species without prejudicing the signaling function. Cytochrome c contributes to the maintenance of the adequate levels of hydrogen peroxide in cells by means of fluency of electron transport in the respiratory chain, oxidation of superoxide ion and reduction of hydrogen peroxide and CoQ by means of direct and indirect trapping of

Authors are grateful to Fundação de Amparo à Pesquisa do Estado de São Paulo – FAPESP

**G < zero**

**Coupled to G > zero processes**

**Ared**

**B B ox red**

> **C ox**

**cytc**

**Cred**

**O2**

**e-**

**e-**

**H2 O**

H+

**Redox Balance**

**H2 O O2 <sup>2</sup>**

Cell Signaling


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*1,3France 2Italy* 

**Mitochondrial Calcium Signalling:** 

 *Department of Neuroscience and Brain Technologies, Genova,* 

*1INSERM U 807, Paris V University, Paris,* 

*3Institute of Cellular and Molecular Pharmacology,* 

*2Istituto Italiano di Tecnologia,* 

Oulès Bénédicte1, Del Prete Dolores2 and Chami Mounia3\*

 *Institute of Molecular Neuromedecine, UMR6097 CNRS/UNSA, Valbonne,* 

Mitochondria are double membrane-bound organelles that not only constitute the "cellular power plants" but also are crucially involved in cell survival, apoptosis, redox control, Ca2+

The mitochondria generate energy by oxidizing hydrogen derived from dietary carbohydrate (TCA: tricarboxylic acid cycle) and lipids (beta-oxidation) with oxygen to generate heat and energy in the form of ATP (Adenosine triphosphate). Energy generation in mitochondria occurs primarily through oxidative phosphorylation (OXPHOS), a process in which electrons are passed along a series of carrier molecules called the electron transport chain (ETC). This chain is composed of four multisubunit assemblies that are embedded in the mitochondrial inner membrane: complex I (NADH:ubiquinone oxidoreductase; EC 1.6.5.3), complex II (succinate:ubiquinone oxidoreductase; EC 1.3.5.1), complex III (ubiquinol:cytochrome-*c* oxidoreductase; EC 1.10.2.2) and complex IV (cytochrome-*c* oxidase; EC1.9.3.1). Complexes I, III and IV actively translocate protons from the matrix into the intermembrane space using energy extracted from electrons passing through the chain. These electrons are liberated from NADH and FADH2, at complexes I and II, respectively, where they are donated to the lipophilic electron carrier coenzyme Q for further transport to complex III. From there, electrons are shuttled to complex IV by cytochrome-*c*. At this complex, electrons are finally used for the reduction of oxygen to water (Hatefi, 1985;

The energy released by the flow of electrons through the ETC and the flux of protons out of the mitochondrial inner membrane creates a capacitance across the mitochondrial inner membrane, the electrochemical gradient (∆P) composed of an electrical potential (Δψ) and a concentration ratio (ΔpH). The potential energy stored in ∆P is coupled to ATP synthesis by complex V (F0/F1-ATP-synthase; EC 3.6.1.34). As protons flow back into mitochondrial

homeostasis and many metabolic and biosynthetic pathways.

**1. Introduction** 

Saraste, 1999) (Figure 1 A).

Corresponding Author

 \*

**Role in Oxidative Phosphorylation Diseases** 

