Mitochondrial respirator chain

The literature on applications of advanced EPR methods for the characterization of paramagnetic sites in proteins is already quite large and growing fast. Instead of giving a review of all of these applications, the potential of these methods will be demonstrated on some of our own research examples. Examples will be shown on protein complexes of a) photosynthetic reaction centres of purple bacteria, b) soluble G-protein-nucleotide complexes with P2T and c) cytochrome c oxidase, a membrane-bound protein of the mitochondrial respiration chain. 

Aminoalkylpyrimidines represent an interesting class of novel inhibitors of complex I in the mitochondrial respiration chain. Broad spectrum activity against ma-... 

The P/O ratio is the number of ATPs made for each O atom consumed by mitochondrial respiration. The P stands for high-energy phosphate equivalents, and the O actually stands for the number of I 02 s that are consumed by the electron transport chain. The full reduction of 02 to 2 H20 takes 4 electrons. Therefore, 2 electrons reduce of an 02. The oxidation of NADH to NAD and the oxidation of FADH2 to FAD are both 2-electron oxidations. O can be read as the transfer of 2 electrons. It s not quite as obscure as it sounds.2... 

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. 

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. 

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. 

Electrochemistry of respiration — The function of the enzymes in the mitochondrial respiratory chain is to transform the energy from the redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Cytochrome oxidase (EC 1.9.3.1, PDB 20CC) is the terminal electron acceptor of the mitochondrial respiratory chain. Its main function is to catalyze the reaction of oxygen reduction to water using electrons from ferrocytochrome c 4H+ + 02 + 4e 2H20. This reaction is exother-... 

Metabolic pathways such as the electron transport chain of mitochondrial respiration and biochemical reduction of oxygen by enzymes of xenobiotic metabolism,... 

The mitochondrial respiratory chain consists of three proton pumps which act in series with respect to the electron flow and in parallel with respect to the proton circuit (Fig. 2.2a). Two limiting states are frequently referred to for isolated mitochondria - State 4 in which the proton current is limited by the inhibition of proton re-entry through the ATP synthase (due to either actual inhibition of the synthase or to the attainment of equilibrium), and State 3 in which there is ready proton re-entry into the matrix and hence brisk respiration. The State 3 condition can be due to an induced proton leak in the membrane or to the maintenance of AG tp below that required to equilibrate with AfiH+ (by either removing ATP, or following the addition of ADP. 

The term respiratory chain may actually be a misnomer in the sense that the respiratory components of the inner mitochondrial membrane probably do not form chain-like physical complexes with one another, at least not with a long life-time. Yet, a chain is still a good collective description of the catalysts of mitochondrial respiration, which transfer reducing equivalents in a very specific and functionally chain-like sequence from hydrogen-donating substrates to dioxygen. 

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