**2.2 Mitochondrial genome**

*Free Radical Medicine and Biology*

Unless the dismutation of O2

still poorly understood in mechanism [9].

is formed. The O2

O2

reactive metabolites of O2, including superoxide anion (O2

pathological stress, underlying physiological conditions.

**2. Mitochondrial physiology and ROS production**

**2.1 Mitochondrial structure and genome**

oxide (H2O2), formed by one- and two-electron reductions of O2, respectively [7]. In the presence of transition metal ions, the more reactive hydroxyl radical (OH·

Zn-superoxide dismutase (Cu/ZnSOD) in mitochondrial intermembrane space and manganese-dependent superoxide dismutase (MnSOD) in mitochondrial matrix.

DNA damage, protein oxidation and lipid peroxidation in many types of cells. H2O2 is also cell damaging molecule to be degraded to water by catalase [8]. Although the

·<sup>−</sup> generation by respiratory complexes is a well-established phenomenon, it is

Mitochondria, cellular organelles in cells of eukaryotic organisms, have a primarily role in the process of pyruvate breakdown and ATP synthesis, generating water and carbon dioxide (CO2) as the end products via aerobic respiration [21]. Mitochondria turned into driving forces in biological evolution after the symbiotic engulfment of aerobic α-proteobacteria by a precursor of the eukaryotic cells around 2 billion years ago [22, 23]. Although mitochondria have preserved the double-membrane shape and ATP production characters of their ancestors, they have attained numerous additional functions in the cell, dramatically altering their structure and composition [24]. Most part of the bacterial genome was rapidly lost or transferred to the nuclear DNA [25]. Mammalian mitochondrial genome is transmitted solely through the female germ line [26]. Human mitochondrial DNA (mtDNA) is a double-stranded, circular molecule of 16,569 bp and contains 37 genes coding for two ribosomal RNAs (rRNAs), 22 transfer RNAs (tRNAs) and 13 proteins [22]. As the major power plants, mitochondria constantly produce reactive radical oxidants as byproducts during OXPHOS. Thus, in response to the metabolic or environmental stresses, mitochondria have accomplished antioxidant defence system [27]. Mitochondria are also highly dynamic to maintain the functions, which form a tubular network that continually changes by fission and fusion [28]. In this section, we concisely discuss overall mitochondrial biology and the ROS generation.

A mitochondrion comprises four subcompartments, the outer mitochondrial membrane (OMM) and inner mitochondrial membrane (IMM), and the two soluble compartments intermembrane space (IMS) and matrix, which are architecturally and functionally distinct. The OMM encloses the organelle, which has a proteinto-phospholipid ratio similar to that of the plasma membrane [29]. It contains

Mitochondria have been implicated in the regulation of a number of physiological and pathological processes, including proliferation, differentiation, programmed cell death, innate immunity, autophagy, redox signalling, calcium homeostasis, hypoxic stress responses and stem cell reprogramming [10–16]. The mROS production contributes to mitochondrial damage in a range of pathologies, which is also is closely related to redox signalling in the cell [4, 17]. However, accumulating evidences show that mROS are not only deleterious molecules derived from the cellular metabolism but also indispensable participants in diverse cellular signalling and regulations [18–20]. In this chapter, we briefly summarize recent developments in our understanding of the involvement of mROS as signalling mediators in redox biology, rather than

·<sup>−</sup> is rapidly dismutated to H2O2 by two dismutases including Cu/

·<sup>−</sup> is catalyzed into H2O2, the radical oxidant promotes

·<sup>−</sup>) and hydrogen per-

)

**12**

Mitochondria contain their own genetic material (mtDNA), which is maternally inherited without DNA recombination and encodes 37 genes that participate in mitochondrial ATP synthesis. Thirteen genes of them are involved in OXPHOS, and the rest two rRNAs and 22 tRNAs. One human cell has hundreds to thousands of mtDNA copies [39, 40]. mtDNA has high rates of mutation and sequence evolution, and mutant and wild-type mtDNA are present in the cell at different proportions [41, 42]. The mtDNA mutations lead to abnormality in OXPHOS activity and ATP synthesis [43]. The mtDNA is exposed to OXPHOS-derived ROS without conventional histone proteins. Moreover, in the lacking repair mechanisms, mtDNA damage further amplifies during DNA replication [44]. Therefore, the mtDNA is susceptible to mutation

**Figure 1.** *Mitochondrial structure.*

and damage. Therefore, mtDNA mutations and damage cause mitochondrial dysfunction, including ATP synthesis impediment, intracellular calcium level elevation, phospholipase activation and membrane phospholipids decomposition [45, 46].

#### **2.3 Mitochondrial bioenergetics and dynamics**

The most prominent roles of mitochondria are to produce the cellular energy in forms of ATPs via aerobic respiration and to regulate the cellular metabolism. Nutrients such as sugars (mostly glucose), lipids and amino acids are oxidized to primarily produce the energy [47]. Approximately 90% of cellular energy requirements are generated in mitochondria [48]. Glucoses and lipids are glycolysed into pyruvic and fatty acids, respectively, in the cytoplasm. Subsequently, these acids form acetyl coenzyme A (Acetyl CoA) via a series of catabolic reactions and then enter the TCA cycle in the mitochondrial matrix [49]. In the reaction of the TCA cycle, convert three equivalents of nicotinamide adenine dinucleotide (NAD+ ) into three equivalents of reduced NAD+ (NADH), one equivalent of flavin adenine dinucleotide (FAD) into one equivalent of FADH2 and one equivalent each of guanosine diphosphate (GDP) and inorganic phosphate (Pi) into one equivalent of guanosine triphosphate (GTP). The NADH and FADH2 are, in turn, used by the OXPHOS to generate ATPs. Thus, oxidation of nutrients provides electrons to the mitochondrial ETC in the form of NADH and FADH2. The sequential transport of electrons from complex I or II to III and IV extrudes protons from the matrix to the IMS, generating an electrochemical gradient. In this process, the ETC requires two electron carriers: coenzyme Q10 (CoQ10, also known as ubiquinone) and cytochrome c (Cytc) [50]. Along this electrochemical gradient, the protons flow through complex V (ATP synthase) on the IMM to return to the mitochondrial matrix. This reflux alters the conformation of complex V and drives the synthesis of ATP from ADP and Pi [47].

#### **2.4 Mitochondrial dynamics (fission and fusion)**

Mitochondria are highly dynamic and interacting organelles. Mitochondria are able to autonomously integrate (fusion) and divide (fission) by remodelling their morphology and moving along cytoskeletal tracks, in response to their metabolic or pathogenic conditions and cellular environment [29]. The mitochondrial lengths and networks are determined by the balance between fission and fusion rates [51]. Mitochondrial fission and fusion processes are mainly mediated by large guanosine triphosphatases (GTPases) in the dynamin family [51].

Mitochondrial fission requires the dividing of mitochondrial proteins and mtDNA thus that each daughter organelle normally functions without significant loss of soluble proteins from the mitochondrial matrix or intermembrane space [52]. Fission is required for the cellular distribution of mitochondria during cell division and embryonic growth [53]. Exceeded mitochondrial fission, not mutually balanced with fusion, leads to glucose oxidation, MMP decrease and hence the downregulation of ATP production [54]. The fission process is coordinated by a set of components in the cytosol, cytoskeleton, as well as mitochondria. Fission is mediated by a cytosolic dynamin family member, dynamin-related protein 1 (Drp1). Drp1 is recruited from the cytosol to form spirals around mitochondria and, subsequently, constricts the membranes at the fission site to split the mitochondrial cluster [29].

Mitochondrial fusion is mediated by a different set of dynamin-related GTPases. Mitochondrial outer membrane fusion is coordinated with inner membrane fusion. Three large GTPases are essential for mitochondrial fusion [55]. The mitofusins (Mfn1 and Mfn2) are transmembrane GTPases embedded in the OMM [56]. OPA1 is a dynamin-related GTPase associated with the IMM or IMS. Mitofusins and OPA1

**15**

**Figure 2.**

*radical (OH·*

*including NO2*

*protonation of O2*

*· , OH· , CO3 ·*

*generate ONOO<sup>−</sup> toxicity. O2*

*(O2*

(O2

NADP<sup>+</sup>

*Physiological Functions of Mitochondrial Reactive Oxygen Species*

**2.5 Mitochondrial ROS production and antioxidant enzymes**

radicals. Mitochondrial nitric oxide (NO) interacts with O2

physically interact to mechanistically mediate OMM and IMM fusion, respectively [29, 57, 58]. Mitochondrial fusion may increase to maximize the fidelity for

Mitochondria are the major source of ROS generation [9]. In an organism, mitochondria utilize approximately 98% of the total amount of inhaled O2, including 1–2% for ROS generation [59, 60]. Mitochondria actually produce ROS in a number of enzymatic reactions; the vast majority of the free radicals from the mitochondria are formed in the ETC during OXPHOS [61]. In the process of OXPHOS, electron leaks from the ETC combine with O2 molecules to form

lysed by Cu/ZnSOD or Mn SOD to disproportionate into H2O2. Subsequently,

with unsaturated fatty acid of mitochondrial membrane lipids to produce lipid

nitrogen oxide species (RNS) such as peroxynitrite (ONOO<sup>−</sup>), which produce cellular dysfunction by S-nitrosylating proteins [62]. Mammalian cells have multiple enzymes to degrade H2O2, including peroxiredoxins (Prxs), glutathione peroxidases (Gpxs), thioredoxins (Trxs) and catalase. Mitochondrial H2O2 is primarily eliminated by the action of Gpx1, Gpx2 and Gpx4, Prx3 and Prx5 and Trx2 systems, which requires glutathione (GSH) [63–65]. Oxidized GSH (GSSG) is reduced to GSH by glutathione reductase (GR) activity [66]. Similarly, oxidized Trx2 is recycled by Trx reductase (TrxR). These H2O2 scavenging system ultimately depends on reduced nicotinamide adenine dinucleotide phosphate (NADPH) which is regenerated by three mitochondrial matrix-located enzymes:


*Reactions and transformations of mitochondrial ROS. SOD enzymes catalyse the dismutation of superoxide* 

*·<sup>−</sup>), generating hydrogen peroxide (H2O2). The catalase (CAT), glutathione peroxidases (Gpxs) and peroxiredoxins (Prxs) convert H2O2 into water. H2O2 reacts with redox-active iron to generate the hydroxyl* 

*peroxynitrite (ONOO<sup>−</sup>), whose decomposition in turn gives rise to some highly oxidizing intermediates* 

*centres of proteins, leading to enzyme inactivation and concomitant loss of Fe2+ from the enzymes. The* 

*·<sup>−</sup> can form the more reactive hydroperoxyl radical (HO2*

*) through the Fenton reaction. The reaction between O2*

 *as well as, finally, stable NO3*

·<sup>−</sup>, primarily generated in complexes I and III, is cata-

radicals and subsequently react

*·<sup>−</sup> and nitric oxide (NO·*

*<sup>−</sup>. Thus, increased O2*

*·<sup>−</sup> by itself can reduce ferric iron (Fe3+) to ferrous iron (Fe2+) in iron-sulphur* 

*· ).* *) produces* 

 *and* 

*·<sup>−</sup> levels can also reduce NO·*

·<sup>−</sup> can also

·<sup>−</sup> to form reactive

by Fenton reaction. Mitochondrial O2

*DOI: http://dx.doi.org/10.5772/intechopen.88386*

OXPHOS in cellular energy demands [27].

·<sup>−</sup>). Mitochondrial O2

H2O2 can be converted to OH·

bind with protons to form uncharged HOO·

*Free Radical Medicine and Biology*

equivalents of reduced NAD+

**2.3 Mitochondrial bioenergetics and dynamics**

**2.4 Mitochondrial dynamics (fission and fusion)**

triphosphatases (GTPases) in the dynamin family [51].

and damage. Therefore, mtDNA mutations and damage cause mitochondrial dysfunction, including ATP synthesis impediment, intracellular calcium level elevation, phospholipase activation and membrane phospholipids decomposition [45, 46].

The most prominent roles of mitochondria are to produce the cellular energy in forms of ATPs via aerobic respiration and to regulate the cellular metabolism. Nutrients such as sugars (mostly glucose), lipids and amino acids are oxidized to primarily produce the energy [47]. Approximately 90% of cellular energy requirements are generated in mitochondria [48]. Glucoses and lipids are glycolysed into pyruvic and fatty acids, respectively, in the cytoplasm. Subsequently, these acids form acetyl coenzyme A (Acetyl CoA) via a series of catabolic reactions and then enter the TCA cycle in the mitochondrial matrix [49]. In the reaction of the TCA cycle,

(NADH), one equivalent of flavin adenine dinucleo-

) into three

convert three equivalents of nicotinamide adenine dinucleotide (NAD+

tide (FAD) into one equivalent of FADH2 and one equivalent each of guanosine diphosphate (GDP) and inorganic phosphate (Pi) into one equivalent of guanosine triphosphate (GTP). The NADH and FADH2 are, in turn, used by the OXPHOS to generate ATPs. Thus, oxidation of nutrients provides electrons to the mitochondrial ETC in the form of NADH and FADH2. The sequential transport of electrons from complex I or II to III and IV extrudes protons from the matrix to the IMS, generating an electrochemical gradient. In this process, the ETC requires two electron carriers: coenzyme Q10 (CoQ10, also known as ubiquinone) and cytochrome c (Cytc) [50]. Along this electrochemical gradient, the protons flow through complex V (ATP synthase) on the IMM to return to the mitochondrial matrix. This reflux alters the conformation of complex V and drives the synthesis of ATP from ADP and Pi [47].

Mitochondria are highly dynamic and interacting organelles. Mitochondria are able to autonomously integrate (fusion) and divide (fission) by remodelling their morphology and moving along cytoskeletal tracks, in response to their metabolic or pathogenic conditions and cellular environment [29]. The mitochondrial lengths and networks are determined by the balance between fission and fusion rates [51]. Mitochondrial fission and fusion processes are mainly mediated by large guanosine

Mitochondrial fission requires the dividing of mitochondrial proteins and mtDNA thus that each daughter organelle normally functions without significant loss of soluble proteins from the mitochondrial matrix or intermembrane space [52]. Fission is required for the cellular distribution of mitochondria during cell division and embryonic growth [53]. Exceeded mitochondrial fission, not mutually balanced with fusion, leads to glucose oxidation, MMP decrease and hence the downregulation of ATP production [54]. The fission process is coordinated by a set of components in the cytosol, cytoskeleton, as well as mitochondria. Fission is mediated by a cytosolic dynamin family member, dynamin-related protein 1 (Drp1). Drp1 is recruited from the cytosol to form spirals around mitochondria and, subsequently, constricts the membranes at the fission site to split the mitochondrial cluster [29]. Mitochondrial fusion is mediated by a different set of dynamin-related GTPases. Mitochondrial outer membrane fusion is coordinated with inner membrane fusion. Three large GTPases are essential for mitochondrial fusion [55]. The mitofusins (Mfn1 and Mfn2) are transmembrane GTPases embedded in the OMM [56]. OPA1 is a dynamin-related GTPase associated with the IMM or IMS. Mitofusins and OPA1

**14**

physically interact to mechanistically mediate OMM and IMM fusion, respectively [29, 57, 58]. Mitochondrial fusion may increase to maximize the fidelity for OXPHOS in cellular energy demands [27].
