Cytochrome oxidases electron transfer pathways

FIGURE 21.17 The electron transfer pathway for cytochrome oxidase. Cytochrome c binds on the cytosolic side, transferring electrons through the copper and heme centers to reduce O9 on the matrix side of the membrane. 

Another important outcome of the structural analysis is the relative positioning of the metal sites and their distances in order to define plausible electron transfer pathways between electron donors and acceptors. A common pattern starts to emerge (the same applies to cytochrome oxidase (241, 242). Figure 11 gives a pictorial view of the electron transfer pathway ... 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

For reasons discussed in Chapter 20, plants must carry out this reaction even when they do not need NADH for ATP production. To regenerate NAD+ from unneeded NADH, plant mitochondria transfer electrons from NADH directly to ubiquinone and from ubiquinone directly to 02, bypassing Complexes III and IV and their proton pumps. In this process the energy in NADH is dissipated as heat, which can sometimes be of value to the plant (Box 19-1). Unlike cytochrome oxidase (Complex IV), the alternative QH2 oxidase is not inhibited by cyanide. Cyanide-resistant NADH oxidation is therefore the hallmark of this unique plant electron-transfer pathway. 

Poole1305 has reviewed the bacterial cytochrome oxidases, and has drawn attention to features which are not present in the mitochondrial enzyme, and which reflect the metabolic diversity and adaptability of bacteria. These are (1) the synthesis of the oxidases is controlled dramatically by the prevailing environmental conditions (2) some oxidases are multifunctional, and may use electron acceptors other than dioxygen (3) more than one type of oxidase may be present, each terminating a branched electron-transfer pathway. 

When purple non-sulfur bacteria switch from photosynthesis in the light to respiration in the dark, the content of BChl a or is diminished, and the synthesis of cytochrome oxidase is increased [101]. In some bacteria, such as Rb. sphae-roides, two oxidases are formed, Cyt aa [90] and Cyt o. In most others (e.g. R. rubrum) only Cyt o is formed [97]. Work during the past decade has revealed a strong similarity between the electron transfer pathways in purple non-sulfur bacteria and in the mitochondrial inner membrane [92]. 

Mitochondrial cytochrome c is the most widely investigated heme protein with respect to its electrochemical properties. It is active in electron transfer pathways such as the respiratory chain in the mitochondria where it transfers electrons between membrane bmmd C3d ochrome reductase complex III and cytochrome c oxidase. The active site is an iron porphyrin (heme) covalently linked to the protein at Cysl4 and Cysl7 through thioether bonds (heme c). The iron itself lies in the plane of the porphyrin ring, the two axial positions... 

As mentioned above, ammonia is oxidized to nitrous acid via hydroxylamine in N. europaea first ammonia is oxidized to hydroxylamine by the catalysis of ammonia monooxygenase, and hydroxylamine formed is oxidized to nitrous acid by the catalysis of hydroxylamine oxidoreductase. Molecular oxygen is not necessary to the reaction itself of NH2OH — HN02 (Yamanaka and Sakano, 1980) but it is required for the consumption of electrons liberated from the reaction, NH2OH + H20 —> HN02 + 4H+ + 4c. Electrons thus liberated are transferred first to cytochrome c-554, then to cytochrome c-552, and finally oxidized with molecular oxygen by the catalysis of cytochrome c oxidase. Based on the results described above, the electron transfer pathway in the oxidation of ammonia to nitrite or nitrous acid by N. europaea will be presented as shown in Fig. 3.3. 

The inducible arsenite oxidase from the Eubacterium Alcaligenes faecalis (NCIB 8687) has been purified and characterized (22-24). Anderson et al. (24) isolated the enzyme from a sonicate of washed, lysozyme-treated cells that had been harvested in their late exponential growth phase. The sonicate was fractionated by gel filtration through DEAE-sepharose and active fractions concentrated by ultrafiltration. The purified enzyme was found to be monomeric with a molecular mass of 85 kDa. It consisted of two polypeptide chains in an approximate ratio of 70 30. The enzyme stmcture included one molybdenum, five or six iron atoms, and sulfide. Purification of the oxidase also led to recovery of azurin, a blue protein, which was rapidly reduced by arsenite in the presence of catalytic amounts of Aro, and a red protein. The red protein was a c-type cytochrome, which was reduced by arsenite in the presence of catalytic amounts of Aro and azurin. No reduction of the cytochrome occurred in the absence of Aro, but it did occur in the absence of azurin. Denaturation of Aro led to the release of a pterin cofactor characteristic of molybdenum hydroxylases. In intact cells of A. faecalis, the enzyme resides on the outer surface of the inner (plasma) membrane. The cytochrome and azurin may be part of an electron transfer pathway in the periplasm. 

Figure 9.26 (a) Near-equUibiium and non-equilibrium reactions in the electron transfer chain. The electron transfer chain is considered to be the Latter part of the physiological Krebs cycle (see above). The non-equilibrium processes are the Krebs cycle and the terminal reaction cytochrome oxidase. All other reactions are near-equilibrium, including the ATP-generating reactions. These are not shown in the figure, (b) The similarity of electron transfer chain and glycolysis in the position of near-equilibrium/non-equilibrium reactions, in the two pathways. In both cases, non-equilibrium reactions are at the beginning and at the end of the processes (see Chapters 2 and 3 for description of these terms and the means by which such reactions can be identified). 

Various aspects of cytochrome c have been reviewed.637 648 6 1 Cytochrome c is widely distributed, and has the apparently simple role of accepting an electron from cytochrome c, and transferring it to cytochrome oxidase. A major area of discussion is the pathway taken by the electron in its route from the surface of (he protein to the heme and out again. 

The system depends on an electron transport pathway that transfers electrons from NADPH through a flavoprotein (NADPH cytochrome P-450 reductase) to cytochrome P-450 that is the terminal oxidase of the chain (10). The xenobiotic first forms a complex with the oxidized form o cytochrome P-450 which is reduced by an electron passing down the chain from NADPH. The reduced cytochrome P-450/substrate complex then reacts with and activates molecular oxygen to an electrophilic oxene species (an electron deficient species similar to singlet oxygen) that is transferred to the substrate with the concommitant formation of water. Cytochrome P-450 thus acts primarily as an oxene transferase (2). Substrate binding is a relatively nonspecific, passive process that serves to bring the xenobiotic into close association with the active center and provide the opportunity for the oxene transfer to occur. 

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