Mitochondrial electron-transfer chain

Atovaquone, a hydroxynaphthoquinone, selectively inhibits the respiratory chain of protozoan mitochondria at the cytochrome bcl complex (complex III) by mimicking the natural substrate, ubiquinone. Inhibition of cytochrome bcl disrupts the mitochondrial electron transfer chain and leads to a breakdown of the mitochondrial membrane potential. Atovaquone is effective against all parasite stages in humans, including the liver stages. 

Cyt c is one of most important and extensively studied electron-transfer proteins, partly because of its high solubility in water compared with other redox-active proteins. In vivo, cyt c transfers an electron from complex III to complex IV, membrane-bound components of the mitochondrial electron-transfer chain. The electrochemical interrogation of cyt c has, however, been hindered because the redox-active heme center is... 

Once an enzyme-catalysed reaction has occurred the product is released and its engagement with the next enzyme in the sequence is a somewhat random event. Only rarely is the product from one reaction passed directly onto the next enzyme in the sequence. In such cases, enzymes which catalyse consecutive reactions, are physically associated or aggregated with each other to form what is called a multi enzyme complex (MEC). An example of this arrangement is evident in the biosynthesis of saturated fatty acids (described in Section 6.30). Another example of an organized arrangement is one in which the individual enzyme proteins are bound to membrane, as for example with the ATP-generating mitochondrial electron transfer chain (ETC) mechanism. Intermediate substrates (or electrons in the case of the ETC) are passed directly from one immobilized protein to the next in sequence. 

It is noteworthy that except for the Rieske center in Complex III, Complexes I and 11 are home to all the iron-sulfur clusters in the mitochondrial electron transfer chain and consequently most of the iron-containing carriers in the entire sequence. Hibbs subsequently showed that CAM-injured cells lose a substantial portion of their total intracellular iron (Hibbs et al., 1984) [later studies specifically identified loss of mitochondrial iron (Wharton et al., 1988)] and Drapier and Hibbs (1986) showed that the activity of another iron-sulfur-containing enzyme, aconitase, is also lost. In early 1987 Hibbs reported that the cytostatic actions of CAMs requires the presence of only one component in culture medium, L-arginine (Hibbs et al., 1987b). Thus, the stage was set for the discovery of a unique reactive species that targets intracellular iron, produced by CAMs. 

As early as 1976 it was demonstrated that NO treatment of iron-sulfur-containing enzymes of the mitochondrial electron transfer chain results in liberation of iron from the clusters (Salerno et al., 1976). Hibbs et al. (1988) and Stuehr and Nathan (1989) demonstrated that treatment of tumor target cells with NO... 

Vanin et al. (1992) have performed a careful EPR study of the treatment of cells from a cultured macrophage line with NO. A 5-min treatment of these cells with low (—20 juM) NO results in the appearance of DNIC signals, but without concomitant decrease in the intensities of the signals from mitochondrial iron-sulfur clusters. These results indicate that under at least some conditions DNIC formation occurs with iron which is not part of the mitochondrial electron transfer chain, as suggested by Drapier et al., (1992) and Bergonia and co-workers... 

TABLE 19-3 The Protein Components of the Mitochondrial Electron-Transfer Chain... 

Tbansmembrane Movement of Reducing Equivalents Under aerobic conditions, extramitochondrial NADH must be oxidized by the mitochondrial electron-transfer chain. Consider a preparation of rat hepatocytes containing mitochondria and all the cytosolic enzymes. If [4-3H]NADH is introduced, radioactivity soon appears in the mitochondrial matrix. However, if [7-14C]NADH is introduced, no radioactivity appears in the matrix. What do these observations reveal about the oxidation of extramitochondrial NADH by the electron-transfer chain ... 

Copper has an essential role in a number of enzymes, notably those involved in the catalysis of electron transfer and in the transport of dioxygen and the catalysis of its reactions. The latter topic is discussed in Section 62.1.12. Hemocyanin, the copper-containing dioxygen carrier, is considered in Section 62.1.12.3.8, while the important role of copper in oxidases is exemplified in cytochrome oxidase, the terminal member of the mitochondrial electron-transfer chain (62.1.12.4), the multicopper blue oxidases such as laccase, ascorbate oxidase and ceruloplasmin (62.1.12.6) and the non-blue oxidases (62.12.7). Copper is also involved in the Cu/Zn-superoxide dismutases (62.1.12.8.1) and a number of hydroxylases, such as tyrosinase (62.1.12.11.2) and dopamine-jS-hydroxylase (62.1.12.11.3). Tyrosinase and hemocyanin have similar binuclear copper centres. 

Since mitochondria are the site of high oxidative metabolism, they are under continual oxidative stress. In fact, it has been estimated that approximately 2 percent of mitochondrial 02 consumption generates ROS. The mitochondrial electron transfer chain is one of the main sources of ROS in aerobic cells, due to electron leakage from energy-transducing sequences leading to the formation of superoxide radicals. 

Under aerobic conditions the TCA cycle allows for the complete oxidation of pyruvate, generating CO2 as a waste product (which we exhale). Although some ATP is generated directly in the TCA cycle, the most important product is NADH NADH is subsequently oxidised by the mitochondrial electron transfer chain, which, through oxidative phosphorylation, is the major provider of the cell s ATP requirement. 

As with chloroplasts, many questions concerning electron flow and the coupled ATP formation in mitochondria remain unanswered. The first part of the mitochondrial electron transfer chain has a number of two-electron carriers (NAD+, FMN, and ubiquinone) that interact with the cytochromes (one-electron carriers). In this regard, the reduction of O2 apparently involves four electrons coming sequentially from the same Cyt a3. Of... 

In addition to hemoproteins, the other major metabolically active pool of intracellular iron is the nonheme iron-sulfur proteins (see Iron-Sulfur Proteins), which function primarily as elecfron carriers, most especially in the mitochondrial electron transfer chain. In vitro treatment with NO has been shown to result in conversion of the iron in these clusters into dinitrosyhron thiol complexes ((RS-)2Fe(NO)2). As described in more detail below, these complexes exhibit a characteristic EPR signal, which has been observed in both NO-producing activated macrophages and their tumor cell targets, suggesting that such complexes may also be formed in vivo. 

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. 

The extraction of energy from organic compounds, carried out by several catabolic pathways (e.g., the citric-acid cycle), involves the oxidation of these compounds to CO2 and H2O with the concomitant production of water-soluble reductants (NADH and succinate). These reductants donate electrons to components of the mitochondrial electron-transfer chain, resulting in the reduction of oxygen to water ... 

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.

At several points in the mitochondrial electron-transfer chain, the release of energy is coupled to the synthesis of ATP from ADP (see Box 14.12), and this provides a means of storing energy in living cells. 

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... 

In the mitochondrial electron-transfer chain, cytochrome c accepts an electron from cytochrome cj and then transfers it to cytochrome c oxidase (equation 28.16). Ultimately, the electron is used in the four-electron reduction of O2 (see... 

What is the mitochondrial electron-transfer chain, and what role do quinones play in the chain ... 

A similar inconsistency exists concerning oxidative phosphorylation in AD. Although activities of enzymes of the mitochondrial electron transfer chain are reported to be normal in AD brain, partial uncoupling of oxidative phosphorylation (electron transfer and phosphorylation of adenosine diphosphate are normally functionally linked) (Sims et al., 1987) and overexpression of cytochrome oxidase subunit-3 gene in cerebral temporal cortices (Alberts et al., 1992) have been reported. In addition, substantial decreases of complex IV activity were detected in platelets from five patients with AD (Parker et al., 1990). 

A primary intracellular target for the biological actions of nitric oxide ( NO) production is intracellular iron (Hibbs et al., 1990 Henry et al., 1993). In activated macrophages and their tumor cell targets, a characteristic pattern of metabolic dysfunction is observed as a result of -NO synthesis, which includes loss of nonheme iron-containing enzyme function, including aconitate hydratase, complexes I and II of the mitochondrial electron transfer chain (Hibbs et al., 1990) as well as the nonheme iron-containing enzyme ribonucleotide reductase (Lepoivre et al., 1991). 

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