Complexes ubiquinone reactions

In theory, molecular oxygen should be completely reduced in complex IV by four electrons to form water without the formation of intermediates. In practice, occasionally, partial reduction occurs with oxygen being converted to superoxide anion radicals (Chapter 15). Also, the ubiquinone reactions in complexes I and II have an... 

Two light-activated cyclic electron transfer systems have been reincorporated into lipid vesicles in such a way that proton pumping across the membranes may be observed under appropriate conditions. The first of these has been constructed from mammalian cytochrome bc] complex and reaction centres isolated from Rhodopseudomonas sphaeroides (RCbc vesicles), a combination used previously by Packham et al. (1980) for single turnover studies in solution. In order to maintain adequate multiple turnover electron flux under our conditions, it was necessary to add both cytochrome c and ubiquinone-2. In the presence of valinomycin, light activation caused the translocation of four protons outwards across the vesicles for each pair of electrons completing a cycle, although this ratio appeared to fall to two after a significant ApH had built up. 

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

The proton-motive Q-cycle model, put forward by Mitchell (references 80 and 81) and by Trumpower and co-workers, is invoked in the following manner (1) One electron is transferred from ubiquinol (ubiquinol oxidized to ubisemi-quinone see Figure 7.27) to the Rieske [2Fe-2S] center at the Qo site, the site nearest the intermembrane space or p side (2) this electron can leave the bci complex via an attached cytochrome c or be transferred to cytochrome Ci (3) the reactive ubisemiquinone reduces the low-potential heme bL located closer to the membrane s intermembrane (p) side (4) reduced heme bL quickly transfers an electron to high-potential heme bn near the membrane s matrix side and (5) ubiquinone or ubisemiquinone oxidizes the reduced bn at the Qi site nearest the matrix or n side. Proton translocation results from the deprotonation of ubiquinol at the Qo site and protonation of ubisemiquinone at the Qi site. Ubiquinol generated at the Qi site is reoxidized at the Qo site (see Figure 7.27). Additional protons are transported across the membrane from the matrix (see Figure 7.26 illustrating a similar process for cytochrome b(6)f). The overall reaction can be written... 

Reaction centers from photosynthetic organisms are specialized pigment-protein complexes in which photon energy is converted into chemical energy ( ) This is accomplished by a series of rapid electron transfer reactions that produce a spacially-separated oxidized donor and a reduced electron acceptor 2). Reaction centers from the purple photosynthetic bacterium Rhodopseudomonas sphaeroides contain four molecules of bacteriochlorophyll (BChl), two of bac-teriopheophytin (BPh), one tightly-bound or primary ubiquinone (Q), a... 

NADH dehydrogenase (ubiquinone) [EC 1.6.5.3] (also called ubiquinone reductase, type I dehydrogenase, and complex I dehydrogenase) catalyzes the reaction of NADH with ubiquinone to produce NAD and ubiqui-nol. The complex, which uses EAD and iron-sulfur proteins as cofactors, is found in mitochondrial membranes and can be degraded to form NADH dehydrogenase [EC... 

Succinate dehydrogenase (ubiquinone) [EC 1.3.5.1], a multiprotein complex found in the mitochondria, catalyzes the reaction of succinate with ubiquinone to produce fumarate and ubiqumol. The enzyme requires FAD and iron-sulfur groups. It can be degraded to form succinate dehydrogenase [EC 1.3.99.1], a FAD-dependent system that catalyzes the reaction of succinate with an acceptor to produce fumarate and the reduced acceptor, but no longer reacts with ubiquinone. 

This enzyme complex [EC 1.10.2.2], also known as cytochrome bci and complex 111, catalyzes the reaction of ubiquinol (QH2) with two ferricytochrome c to produce ubiquinone (Q) and two ferrocytochrome c. The complex also contains cytochrome h-562, cytochrome h-566, cytochrome Ci, and a two-iron ferredoxin. 

FIGURE 19-9 IMADH ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron-sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons. The detailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19-12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. 

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. 

FIGURE 19-35 Mitochondrial production and disposal of superoxide. Superoxide radical, OJ, is formed in side reactions at Complexes I and III, as the partially reduced ubiquinone radical ( Q—) donates an electron to 02. The reactions shown in blue defend the cell against the damaging effects of superoxide. Reduced glutathione... 

Oxidation-Reduction Reactions The NADH dehydrogenase complex of the mitochondrial respiratory chain promotes the following series of oxidation-reduction reactions, in which Fe3+ and Fez+ represent the iron in iron-sulfur centers, Q is ubiquinone, QH2 is ubiquinol, and E is the enzyme ... 

V). The centers resemble PSII of chloroplasts and have a high midpoint electrode potential E° of 0.46 V. The initial electron acceptor is the Mg2+-free bacteriopheophytin (see Fig. 23-20) whose midpoint potential is -0.7 V. Electrons flow from reduced bacteriopheophytin to menaquinone or ubiquinone or both via a cytochrome bct complex, similar to that of mitochondria, then back to the reaction center P870. This is primarily a cyclic process coupled to ATP synthesis. Needed reducing equivalents can be formed by ATP-driven reverse electron transport involving electrons removed from succinate. Similarly, the purple sulfur bacteria can use electrons from H2S. 

Figure 23-32 Simplified diagram of cyclic electron flow in purple bacteria. Two protons from the cytoplasm bind to QB2 in the reaction center to form QH2 (ubiquinol), which diffuses into the ubiquinone pool. From there it is dehydrogenated by the cytochrome kq complex with expulsion of two protons into the periplasm. A third and possibly a fourth proton may be pumped (green arrows) across the membrane, e.g., via the Q cycle (Fig. 18-9). The protons are returned to the cytoplasm through ATP synthase with formation of ATP. Some electrons may flow to the reaction centers from such reduced substrates as S2 and some electrons may be removed to generate NADPH using reverse electron transport.345...

If the reaction centers of photosystem I and photosystem II are segregated into separate regions of the thylakoid membrane, how can electrons move from photosystem I to photosystem II Evidently the plastoquinone that is reduced in photosystem II can diffuse rapidly in the membrane, just as ubiquinone does in the mitochondrial inner membrane. Plastoquinone thus carries electrons from photosystem II to the cytochrome b6f complex. Plastocyanin acts similarly as a mobile electron carrier from the cytochrome b f complex to the reaction center of photosystem I, just as cytochrome c carries electrons from the mitochondrial cytochrome bct complex to cytochrome oxidase and as a c-type cytochrome provides electrons to the reaction centers of purple bacteria (see fig. 15.13). 

Focusing on the mechanisms of action of BOA into the plant cell, Barnes et al.7 suggested that the chlorotic seedlings observed in the presence of BOA and DIBOA could be the consequence of a benzoxazinone effect on the photophosphorylation and electron transport into the plant metabolism. In this way, Niemeyer et al.28 studied the effects of BOA on energy-linked reactions in mitochondria and reported an inhibition of the electron transfer between flavin and ubiquinone in Complex I, with complete inhibition of electron transport from NADH to oxygen in SMP. They could also detect an inhibition of BOA on ATP synthesis by acting directly on the ATPase complex at the F1 moiety. 

Most anaerobically functioning mitochondria use endogenously produced fumarate as a terminal electron-acceptor (see before) and thus contain a FRD as the final respiratory chain complex (Behm 1991). The reduction of fumarate is the reversal of succinate oxidation, a Krebs cycle reaction catalysed by succinate dehydrogenase (SDH), also known as complex II of the electron-transport chain (Fig. 5.3). The interconversion of succinate and fumarate is readily reversible by FRD and SDH complexes in vitro. However, under standard conditions in the cell, oxidation and reduction reactions preferentially occur when electrons are transferred to an acceptor with a higher standard redox potential therefore, electrons derived from the oxidation of succinate to fumarate (E° = + 30 mV) are transferred by SDH to ubiquinone,... 

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