Electron transfer cytochrome oxidase

Cytochrome a is usually low-spin and six-coordinate, and will therefore be involved in electron transfer. Cytochrome a3 is high-spin and five-coordinate, and is therefore able to bind small molecules such as 02 and CO in the sixth, axial position when present as Fe11. It appears therefore that cytochrome a3 is the component of cytochrome oxidase that binds dioxygen, and it is possible that at some stage the dioxygen species would bridge the a3 iron and CuB. 

Cytochrome Oxidase (also known as complex IV) is an iron and copper containing enzyme in the electron transport system. It catalyzes the final step in the electron transport process - the transfer of electrons and protons to oxygen, to form water (Figures 15.2,15.3, 15.10). Transfer of electrons through cytochrome oxidase can be blocked by cyanide, azide, and carbon monoxide. 

Cyanide blocks the transfer of electrons from cytochrome oxidase to O2. Therefore, all the respiratory-chain components become reduced and electron transport ceases consequently, oxidative phosphorylation stops. An artificial electron acceptor -with an appropriate redox potential, such as methylene blue, can reoxidize some components of the respiratory chain, reestablish a proton gradient, and thereby restore ATP synthesis. The methylene blue takes the place of cytochrome oxidase as a means of transferring electrons to O2, which remains the terminal electron acceptor. 

Reduced cytochrome c in the membrane is then oxidized by transfer of electrons through cytochrome oxidase present in either the cytoplasm or the mitochondria. This scheme differs from Conway s only in being specific about the role of cytochrome c in the membrane. Two observations supported the proposal the fact that cytochrome c is preponderately in the reduced state as long as secretion continues and that addition of 20 mM ( ) NaSCN causes cytochrome c to shift toward the oxidized state as it inhibits acid secretion. 

Spectral examination of the reaction of reduced cytochrome c oxidase with molecular oxygen has shown the formation of at least three intermediates, designated as Compounds I, II, and III according to the order of their appearance, observed at — 80°C in intact mitochondria (Chance et al, 1975a,b). Compound I is thought to be an active intermediate in the true oxygenated compound in the cytochrome c oxidase reaction sequence. The decay of Compound I is accelerated by some 2 x 10" times in the presence of ferrocytochrome c. Present data suggest that ferrocytochrome c may transfer electrons to cytochrome oxidase in two steps, namely the reduction... 

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. 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

Cytochrome c oxidase contains two, or possibly three, copper atoms referred to as Cua and Cub since they do not fit into the usual classification. The former (possibly a dimer) is situated outside the mitochondrial membrane, whereas the latter is associated with an iron atom within the membrane. Both have electron transfer functions but details are as yet unclear. 

Laccase, 6,699 copper, 6,654 cytochrome oxidases concerted electron transfer, 6,683 fungal... 

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 ... 

Section 18.2). The latest generation of such catalysts (1 in Fig. 18.17) reproduces the key features of the site (i) the proximal imidazole ligation of the heme (ii) the trisi-midazole ligation of distal Cu (iii) the Fe-Cu separation and (iv) the distal phenol covalently attached to one of the imidazoles. As a result, binding of O2 to compound 1 in its reduced (Fe Cu ) state appears to result in rapid reduction of O2 to the level of oxides (—2 oxidation state) without the need for outer-sphere electron transfer steps [Collman et ah, 2007b]. This reactivity is analogous to that of the heme/Cu site of cytochrome c oxidase (see Section 18.2). 

Only three steps of the proposed mechanism (Fig. 18.20) could not be carried out individually under stoichiometric conditions. At pH 7 and the potential-dependent part of the catalytic wave (>150 mV vs. NHE), the —30 mV/pH dependence of the turnover frequency was observed for both Ee/Cu and Cu-free (Fe-only) forms of catalysts 2, and therefore it requires two reversible electron transfer steps prior to the turnover-determining step (TDS) and one proton transfer step either prior to the TDS or as the TDS. Under these conditions, the resting state of the catalyst was determined to be ferric-aqua/Cu which was in a rapid equilibrium with the fully reduced ferrous-aqua/Cu form (the Fe - and potentials were measured to be within < 20 mV of each other, as they are in cytochrome c oxidase, resulting in a two-electron redox equilibrium). This first redox equilibrium is biased toward the catalytically inactive fully oxidized state at potentials >0.1 V, and therefore it controls the molar fraction of the catalytically active metalloporphyrin. The fully reduced ferrous-aqua/Cu form is also in a rapid equilibrium with the catalytically active 5-coordinate ferrous porphyrin. As a result of these two equilibria, at 150 mV (vs. NHE), only <0.1%... 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

Belevich I, Verkhovsky MI, Wikstrom M (2006) Proton-coupled electron transfer drives the proton pump of cytochrome c oxidase. Nature 440 829-832. 

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