Respiration Respiratory chain

The rate of respiration of mitochondria can be controlled by the availability of ADP. This is because oxidation and phosphorylation are tightly coupled ie, oxidation cannot proceed via the respiratory chain without concomitant phosphorylation of ADP. Table 12-1 shows the five conditions controlling the rate of respiration in mitochondria. Most cells in the resting state are in state 4, and respiration is controlled by the availability of ADP. When work is performed, ATP is converted to ADP, allowing more respiration to occur, which in turn replenishes the store of ATP. Under certain conditions, the concentration of inorganic phosphate can also affect the rate of functioning of the respiratory chain. As respiration increases (as in exercise). 

Barbiturates such as amobarbital inhibit NAD-hnked dehydrogenases by blocking the transfer from FeS to Q. At sufficient dosage, they are fatal in vivo. Antin cin A and dimercaprol inhibit the respiratory chain between cytochrome b and cytochrome c. The classic poisons H2S, carbon monoxide, and cyanide inhibit cytochrome oxidase and can therefore totally arrest respiration. Malonate is a competitive inhibitor of succinate dehydrogenase. 

The action of uncouplers is to dissociate oxidation in the respiratory chain from phosphorylation. These compounds are toxic in vivo, causing respiration to become uncontrolled, since the rate is no longer limited by the concentration of ADP or Pj. The uncoupler that has been used most frequently is 2,4-dinitrophenol, but other compounds act in a similar manner. The antibiotic oligomycin completely blocks oxidation and phosphorylation by acting on a step in phosphorylation (Figures 12-7 and 12-8). 

Many well-known poisons such as cyanide arrest respiration by inhibition of the respiratory chain. 

As described earlier, superoxide is a well-proven participant in apoptosis, and its role is tightly connected with the release of cytochrome c. It has been proposed that a switch from the normal four-electron reduction of dioxygen through mitochondrial respiratory chain to the one-electron reduction of dioxygen to superoxide can be an initial event in apoptosis development. This proposal was supported by experimental data. Thus, Petrosillo et al. [104] have shown that mitochondrial-produced oxygen radicals induced the dissociation of cytochrome c from bovine heart submitochondrial particles supposedly via cardiolipin peroxidation. Similarly, it has been found [105] that superoxide elicited rapid cytochrome c release in permeabilized HepG2 cells. In contrast, it was also suggested [106] that it is the release of cytochrome c that inhibits mitochondrial respiration and stimulates superoxide production. 

Abnormalities of the respiratoiy chain. These are increasingly identified as the hallmark of mitochondrial diseases or mitochondrial encephalomyopathies [13]. They can be identified on the basis of polarographic studies showing differential impairment in the ability of isolated intact mitochondria to use different substrates. For example, defective respiration with NAD-dependent substrates, such as pyruvate and malate, but normal respiration with FAD-dependent substrates, such as succinate, suggests an isolated defect of complex I (Fig. 42-3). However, defective respiration with both types of substrates in the presence of normal cytochrome c oxidase activity, also termed complex IV, localizes the lesions to complex III (Fig. 42-3). Because frozen muscle is much more commonly available than fresh tissue, electron transport is usually measured through discrete portions of the respiratory chain. Thus, isolated defects of NADH-cytochrome c reductase, or NADH-coenzyme Q (CoQ) reductase suggest a problem within complex I, while a simultaneous defect of NADH and succinate-cytochrome c reductase activities points to a biochemical error in complex III (Fig. 42-3). Isolated defects of complex III can be confirmed by measuring reduced CoQ-cytochrome c reductase activity. 

Electron transfer (ET) is a key reaction in biological processes such as photosynthesis and respiration [1], Photosynthetic and respiratory chain redox proteins contain one or more redox-active prosthetic groups, which may be metal complexes or organic species. Since it is known from crystal structure analyses that the prosthetic groups often are located in the protein interior, it is likely that ET in protein-protein complexes will occur over large molecular distances ( > 10 A) [2-4],... 

It is interesting that a new group of fungicides based on the natural products from the fungus Strobilurus tenacellus also inhibit mitochondrial respiration at the site of complex III (bei-complex) of the respiratory chain (see Chapter 4). Recently synthesised compounds from within this class are showing interesting insecticidal effects. 

ATP and ADP concentrations set the rate of electron transfer through the respiratory chain via a series of interlocking controls on respiration, glycolysis, and the citric acid cycle. 

In this chapter we will look at the processes by which reduced carriers such as NADH and FADH2 are oxidized within cells. Most familiar to us, because it is used in the human body, is aerobic respiration. Hydrogen atoms of NADH, FADH2, and other reduced carriers appear to be transferred through a chain of additional carriers of increasingly positive reduction potential and are finally combined with 02 to form H20. In fact, the hydrogen nuclei move freely as protons (or sometimes as H ions) it is the electrons that are deliberately transferred. For this reason, the chain of carriers is often called the electron transport chain. It is also referred to as the respiratory chain. 

Looking at the other end of the respiratory chain, Otto WarburgC/d noted in 1908 that all aerobic cells contain iron. Moreover, iron-containing charcoal prepared from blood catalyzed nonenzymatic oxidation of many substances, but iron-free charcoal prepared from cane sugar did not. Cyanide was found to inhibit tissue respiration at low concentrations similar to those needed to inhibit nonenzymatic catalysis by iron salts. On the basis of these investigations, Warburg proposed in 1925 that aerobic cells contain an iron-based Atmungsferment (respiration enzyme), which was later called cytochrome oxidase. It was inhibited by carbon monoxide. 

Aerobic respiration can be subdivided into a number of distinct but coupled processes, such as the carbon flow pathways resulting in the production of carbon dioxide and the oxidation of NADH + H+ and FADH2 (flavin adenine dinucleotide) to water via the electron transport systems or the respiratory chain. 

Current estimates are that three protons move into the matrix through the ATP-synthase for each ATP that is synthesized. We see below that one additional proton enters the mitochondrion in connection with the uptake of ADP and Pi and export of ATP, giving a total of four protons per ATP. How does this stoichiometry relate to the P-to-O ratio When mitochondria respire and form ATP at a constant rate, protons must return to the matrix at a rate that just balances the proton efflux driven by the electron-transport reactions. Suppose that 10 protons are pumped out for each pair of electrons that traverse the respiratory chain from NADH to 02, and 4 protons move back in for each ATP molecule that is synthesized. Because the rates of proton efflux and influx must balance, 2.5 molecules of ATP (10/4) should be formed for each pair of electrons that go to 02. The P-to-O ratio thus is given by the ratio of the proton stoichiometries. If oxidation of succinate extrudes six protons per pair of electrons, the P-to-O ratio for this substrate is 6/4, or 1.5. These ratios agree with the measured P-to-O ratios for the two substrates. 

Fumarate is able to serve as an electron acceptor in anaerobic respiration, as it may be reduced reversibly to succinate in a two-electron process. The succinate-fumarate couple may therefore be utilized as an oxidant or reductant in the respiratory chain, and so differs from the other examples given in this section. These two reactions are catalyzed by succinate dehydrogenase and fumarate reductase, which have many similarities in subunit structure. These are shown in Table 29. Although they are different enzymes, the fumarate reductase can substitute for succinate dehydrogenase under certain conditions. The synthesis of succinate dehydrogenase is induced... 

L-selegiline alters the redox state of ubiquinone, suggesting that the flow of electrons is impaired in the respiratory chain. Furthermore, a decrease in ubiquinone levels has been observed, whereas ubiquinol (reduced ubiquinone) concentrations are increased in the striatum. Ubiquinol levels have been shown to augment as a result of impaired mitochondrial respiration. For example, ubiquinol concentrations were demonstrated to increase in tubular kidney cells exposed to complex IV inhibitors and in disease states with defects in respiratory chain components. These results are also consistent with the hypothesis that L-selegiline enhances 02 formation by altering the rate of electron transfer within the respiratory chain leading to increases in SOD activities in the mouse striatum. 

Active fish have a better developed capillary system in the red muscle to supply oxygen to the mitochondria, and a higher haematocrit (Blaxter et al., 1971). The red muscle tissue also contains more cytochromes (respiratory proteins), and exhibits more cytochrome oxidase activity, which is responsible for transferring electrons in die respiratory chain, more efficient respiration control (oxidative phosphorylation and P/O coefficient) and a greater Atkinson charge, which characterizes energy reserve accumulated in adenyl nucleotides ... 

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