6.1 Chemiosmotic hypothesis

In the chemiosmotic hypothesis [69], the proton gradient is formed by removing H+ from the interior (matrix), while the negative charges remain inside, largely as OHˉ ions; the pH on the outer face of the membrane (intermembrane space) can reach a pH of 5.5, while the pH just at the inner side (matrix) of the same can reach 8.5; this gradient is 3 pH units. Recall that the pH is equal to - log. of [H<sup>+</sup> ], and therefore 3 units of pH mean that the ΔH+ = 1000 between both faces of the membrane, that is to say there are 1000 times more H+ in the intermembrane space than on the side of the membrane that is in contact with the mitochondrial matrix (Figure 1).

#### Figure 1.

The creation of a proton gradient (ΔH<sup>+</sup> ) in the intermembrane space is produced by the chain of electron transport and the synthesis of ATP synthase, which is maintained by the electrons that pass from the reducing equivalents (NADH, FADH2) to the cytochromes along the inner membrane of the mitochondria. ATP synthase uses that gradient to generate ATP. The two processes are associated with the inner membrane of the mitochondria in the mitochondrial crests. Note that the enzymes of the citric acid cycle and β-oxidation are contained in mitochondria, together with the respiratory chain, which collects and transports reducing equivalents, directing them to their final reaction with oxygen to form water, and the machinery for oxidative phosphorylation, the process by which the liberated free energy is trapped as high-energy phosphate. Source: Botham and Mayes [70].

The process begins when carbon substrates enter the tricarboxylic acid cycle through acetyl CoA or anaplerotic reactions. Oxidation of these substrates generates reducing equivalents in the form of NADH and FADH2, which provide electron fluxes through the complexes of the respiratory chain, complex I (NADH dehydrogenase) and complex II (succinate dehydrogenase). The flow of electrons through complexes I and II converges in complex III (ubiquinone-cytochrome c reductase), together with electrons from electron transfer flavoproteins (beta oxidation), although the mobile electron carrier coenzyme Q as second mobile electron carrier transfers electrons to the IV complex (cytochrome c oxidase) where they are finally transferred to oxygen, producing water. A gradient of protons (an electrochemical gradient) through the inner mitochondrial membrane is generated by the action of electron transport through complexes I, III, and IV. The potential energy of this gradient is exploited by the V (ATP synthase) complex to phosphorylate ADP to ATP [71]. It is clear that the maintenance of the mitochondrial membrane potential through the transport of electrons is critical for the proper function of the organelle and, therefore, of the cell and of ascending form of organs and systems.

Infective Endocarditis: Inflammatory Response, Genetic Susceptibility, Oxidative Stress… DOI: http://dx.doi.org/10.5772/intechopen.84908

#### 6.2 Reactive oxygen species

In the process of mitochondrial respiration, the generation of reactive oxygen species (ROS) is generally a cascade of reactions that begins with the production of superoxide O•2. The oxidative stress is defined as an imbalance that favors ROS production on antioxidant defenses; most ROS are products of mitochondrial respiration. Approximately 1–2% of the molecular oxygen consumed during the process of mitochondria respiration is converted to superoxide radicals. Briefly, the reduction of an electron of molecular oxygen produces a relatively stable intermediate, the superoxide anion (O•2); the importance of this is that it serves as the precursor to most ROS.

Therefore, it is very important to take into account the sources that generate it. There is evidence that most of the O•<sup>2</sup> generated by intact mammalian mitochondria in vitro is produced by complex I. The production of superoxide—O•2—is mainly carried out in the inner mitochondrial membrane (IMM) together with complex III [72, 73]. On the other hand, the production of O•<sup>2</sup> is stimulated by the presence of succinate (substrate of complex II) [74]. Ubiquinone as part of the respiratory chain binds complexes I with II and II with III which is also important for the formation of O•<sup>2</sup> by complex III [75]. Oxidation of ubiquinone—Q cycle and unstable semiquinone also generates O•<sup>2</sup> (Figure 2).

The Q cycle couples electron transfer to proton transport in complex III electrons are passed from QH2 to cytochrome c via complex III (Q-cytochrome c oxidoreductase) as described in Figure 2.

$$\text{QH}\_2 + 2\text{cyt}\,\mathcal{c}\_{\text{oxidized}} + 2\text{H}^+ \,\text{matrix} \tag{1}$$

Q þ 2cyt creduced þ 4H<sup>þ</sup> intermembrane space (2)

#### Figure 2.

The flavin adenine dinucleotide (FAD) can be reduced in reactions involving the transfer of two electrons (to form FMNH2 or FADH2), but they can also accept one electron to form the semi Quinone. Electron-transferring flavoprotein (ETF). Fe-S, iron–sulfur proteins (nonheme iron proteins). The Fe-S take part in single-electron transfer reactions in which one Fe atom undergoes oxidoreduction between Fe2+ and Fe3+. Coenzyme Q (Q) (also called ubiquinone) (complex I). Cytochrome c, Q-cytochrome c oxidoreductase (complex III), which passes the electrons on to cytochrome c; and cytochrome c oxidase (complex IV), which completes the chain, passing the electrons to O2 and causing it to be reduced to H2O. Q and cytochrome c are mobile. Q diffuses rapidly within the membrane, while cytochrome c is a soluble protein. Mn-SOD, manganese superoxide dismutase.

Superoxide rapidly dismutates into hydrogen peroxide spontaneously or at a low pH is catalyzed by superoxide dismutase. Other elements in the cascade of ROS generation are small molecules derived from oxygen, like the following: hydroxyl (OH•), peroxyl (RO•2), and alkoxyl (RO•) and certain non-radicals that are oxidizing agents and/or are easily converted to radicals, such as hypochlorous acid (HOCl), ozone (O3), singlet oxygen (½O2), and hydrogen peroxide (H2O2). Nitrogen-containing oxidants, such as nitric oxide (NO), are called reactive nitrogen species (RNS), and the Fenton reaction catalyzed by iron leads to the generation of hydroxyl radical [76, 77]. The dismutation of superoxide anions by superoxide dismutases results in the production of H2O2. The mitochondria contribute 20–30% of the stable cytosolic concentration of H2O2 [78]; the subsequent interaction of H2O2 and O•<sup>2</sup> in a Haber-Weiss reaction, or the cleavage of H2O2 driven by Fe2+- (or Cu2+), can generate the highly reactive hydroxyl radical (OH•).

The Haber-Weiss reaction [79] may occur as a consequence of oxidative stress. The reaction is catalyzed by the iron in oxidation state (III); the first step of the catalytic cycle is produced by the reduction of the ferric cation to ferrous cation:

$$\text{Fe}\_3^+ + \text{\textquotedblleft O}^- \text{\textquotedblright } \text{\textquotedblleft O}^- + \text{\textquotedblleft O}\_2^- \text{\textquotedblright} \tag{3}$$

The second step is a reaction from Fenton:

$$\text{Fe}^{2+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{3+} + \text{OH}^- + \text{\textasciicarpoons}{\text{\textasciicating}} \tag{4}$$

#### 6.3 Superoxide dismutases

Briefly, superoxide dismutases (SOD) are a group of metalloenzymes (containing Fe, Mn, or Cu and Zn) that catalyze the disproportionation of superoxide free radical (2O•) to form hydrogen peroxide and oxygen as shown below:

$$2\bullet \text{O}\_2 + 2\text{H}^+ \Leftrightarrow \text{H}\_2\text{O}\_2 + \text{O}\_2 \tag{5}$$

In some cell types, CuZnSOD is present in the mitochondrial intermembrane space, where it can convert O•<sup>2</sup> to H2O2, thus permitting further diffusion into the cytosol.

Superoxide rapidly dismutates into hydrogen peroxide spontaneously or at a low pH is catalyzed by sequential actions of superoxide dismutase (SOD), and catalase converts superoxide into oxygen and water. Other elements in the cascade of ROS generation are small molecules derived from oxygen, which also include oxygen radicals [80].

Because ROS are biologically damaging, they need to be metabolized to prevent the damage they can cause when interacting with other compounds, for which the cell counts with mechanisms that avoid it like SOD. However, when the formation of ROS increases, they have the capacity to deteriorate mitochondrial function and jeopardize cell survival in different ways, where the mitochondrion seems to be responsible for regulating apoptosis [81]. ROS are a major threat to encode, transfer, and transport electrons and generate ATP by directly damaging mitochondrial DNA (mtDNA) which encodes 13 polypeptides, 12 transfer RNAs (tRNAs), and 2 ribosomal RNAs (rRNAs). All of them are essential in the chain of transport of electrons for the production of ATP, so when interacting with them, oxidative phosphorylation and therefore energy genesis is compromised [67]. ROS, and the release of proapoptotic proteins from the intermembrane space of mitochondria, triggers the activation of cell death.

Infective Endocarditis: Inflammatory Response, Genetic Susceptibility, Oxidative Stress… DOI: http://dx.doi.org/10.5772/intechopen.84908
