Reduction potentials mitochondrial electron-transfer chain

Cytochromes c are widespread in nature. Amblerdivided these electron carriers into three classes on structural grounds. The Class I cytochromes c contain axial His and Met ligands, with the heme located near the N-terminus of the protein. These proteins are globular, as indicated by the ribbon drawing of tuna cytochrome c (Figure 6.7). X-ray structures of Class I cytochromes c from a variety of eukaryotes and prokaryotes clearly show an evolutionarily conserved cytochrome fold, with the edge of the heme solvent-exposed. The reduction potentials of these cytochromes are quite positive (200 to 320 mV). Mammalian cytochrome c, because of its distinctive role in the mitochondrial electron-transfer chain, will be discussed later. 

Before continuing the discussion of specific electron-transfer systems, we take a look at the mitochondrial electron-transfer chain, i.e. the chain of redox reactions that occurs in living cells. This allows us to appreciate how the different systems discussed later fit together. Each system transfers one or more electrons and operates within a small range of reduction potentials as illustrated in Figure 28.12 diagrams 28.16 and 28.17 show the structures of the coenzymes [NAD] and FAD, respectively. 

Fig. 28.12 A schematic representation of part of the mitochondrial electron-transfer chain reduction potentials, E, are measured at physiological pH 7 and are with respect to the standard hydrogen electrode at pH 7. Reduction potentials quoted in this chapter are with respect to the standard hydrogen electrode at pH 7.

Figure 28.12 showed cytochromes to be vital members of the mitochondrial electron-transfer chain they are also essential components in plant chloroplasts for photosynthesis. Cytochromes are haem proteins, and the ability of the iron centre to undergo reversible Fe(III) Fe(II) changes allows them to act as one-electron transfer centres. Many different cytochromes are known, with the reduction potential for the Fe /Fe " " couple being tuned by the surrounding protein environment. Cytochromes belong to various families, e.g. cytochromes a, cytochromes b and cytochromes c, which are denoted according to the substituents on the... 

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

Electron transport chain Present in the mitochondrial membrane, this linear array of redox active electron carriers consists of NADH dehydrogenase, coenzyme Q, cytochrome c reductase, cytochrome c, and cytochrome oxidase as well as ancillary iron sulfur proteins. The electron carriers are arrayed in order of decreasing reduction potential such that the last carrier has the most positive reduction potential and transfers electrons to oxygen. 

The main mechanisms of the appearance of the active radical forms of oxygen (ROS) in the body are usually related to the distortion of the functioning of the electron transport chains (ETC) of the mitochondria or microsomes. The functions of the mitochondrial ETC are a realization of the subsequent oxidation-reduction reactions of the electron transfer from the substrate of oxidation to the oxygen as a final electron acceptor. At the same time, two-electron reduction of to H O takes place, which is why the free radicals (very reactive species with free valence) should not appear. However, it was shown in [19,20] that the normal electron transfer in the mitochondria (two-electron reduction of O ) is inevitably spontaneously intermpted, during which only one-electron reduction of takes place and superoxide ion-radical appears. This radical is not very reactive, but when we discuss the mechanism of the potential harm of O, it is usually referred on the reaction 6.6 ... 

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