Cytochrome electron flow

While this electron flow takes place, the cytochrome on the periplasmic side donates an electron to the special pair and thereby neutralizes it. Then the entire process occurs again another photon strikes the special pair, and another electron travels the same route from the special pair on the periplasmic side of the membrane to the quinone, Qb, on the cytosolic side, which now carries two extra electrons. This quinone is then released from the reaction center to participate in later stages of photosynthesis. The special pair is again neutralized by an electron from the cytochrome. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

In all protein-protein complexes studied to date in which cytochrome c has been a partner, it has been shown that the ET rates depend strongly on the reaction driving force. It follows that variations in the reorganization energy could control ET rates in these cases [12]. In redox enzymes with two or more active centers, ET between two centers could be turned on by lowering X at roughly constant — AG [1]. Indeed, a proposal has been advanced that this type of mechanism would be an efficient way to gate the electron flow in a redox-linked proton pump such as cytochrome oxidase [75]. 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

FIGURE 19-11 Cytochrome be, complex (Complex III). The complex is a dimer of identical monomers, each with 11 different subunits. (a) Structure of a monomer. The functional core is three subunits cytochrome b (green) with its two hemes (bH and foL, light red) the Rieske iron-sulfur protein (purple) with its 2Fe-2S centers (yellow) and cytochrome ci (blue) with its heme (red) (PDB ID 1BGY). (b) The dimeric functional unit. Cytochrome c, and the Rieske iron-sulfur protein project from the P surface and can interact with cytochrome c (not part of the functional complex) in the intermembrane space. The complex has two distinct binding sites for ubiquinone, QN and QP, which correspond to the sites of inhibition by two drugs that block oxidative phosphorylation. Antimycin A, which blocks electron flow from heme bH to Q, binds at QN, close to heme bH on the N (matrix) side of the membrane. Myxothiazol, which prevents electron flow from... 

Like the homologous Complex III in mitochondria, the cytochrome bci complex of purple bacteria carries electrons from a quinol donor (QH2) to an electron acceptor, using the energy of electron transfer to pump protons across the membrane, producing a proton-motive force. The path of electron flow through this complex is believed to be very similar to that through mitochondrial Complex III, involving a Q cycle (Fig. 19-12) in which protons are consumed on one side of the membrane and released on the other. The ultimate... 

FIGURE 19-55 Dual roles of cytochrome b6f and cytochrome c6 in cyanobacteria. Cyanobacteria use cytochrome bsf, cytochrome c6, and plastoquinone for both oxidative phosphorylation and photophosphorylation. (a) In photophosphorylation, electrons flow (top to bottom) from water to NADP+. (b) In oxidative phosphorylation, electrons flow from NADH to 02. Both processes are accompanied by proton movement across the membrane, accomplished by a Q cycle. 

Structural studies of the reaction center of a purple bacterium have provided information about light-driven electron flow from an excited special pair of chlorophyll molecules, through pheophytin, to quinones. Electrons then pass from quinones through the cytochrome bci complex, and back to the photoreaction center. 

Electron flow through the cytochrome b6f complex drives protons across the plasma membrane, creating a proton-motive force that provides the energy for ATP synthesis by an ATP synthase. 

Using an alternative path of light-induced electron flow, plants can vary the ratio of NADPH to ATP formed in the light this path is called cyclic electron flow to differentiate it from the normally unidirectional or noncyclic electron flow from H20 to NADP+, as discussed thus far. Cyclic electron flow (Fig. 19-49) involves only PSI. Electrons passing from P700 to ferredoxin do not continue to NADP+, but move back through the cytochrome bef complex to plastocyanin. The path of... 

In plants, both the water-splitting reaction and electron flow through the cytochrome baf complex are accompanied by proton pumping across the thylakoid membrane. The proton-motive force thus created drives ATP synthesis by a CF0CF complex similar to the mitochondrial F0Fi complex. 

Correct answer = D. Thirteen of the approximately 100 polypeptides required for oxidative phosphorylation are coded for by mitochondrial DNA, including the electron transport components cytochrome c and coenzyme Q. Oxygen directly oxidizes cytochrome oxidase. Succinate dehydrogenase directly reduces FAD. Cyanide inhibits electron flow, proton pumping, and ATP synthesis. 

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...

We now see that mitochondria contain a variety of molecules—cytochromes, flavins, ubiquinone, and iron-sulfur proteins—all of which can act as electron carriers. To discuss how these carriers cooperate to transport electrons from reduced substrates to 02, it is useful to have a measure of each molecule s tendency to release or accept electrons. The standard redox potential, E°, provides such a measure. Redox potentials are thermodynamic properties that depend on the differences in free energy between the oxidized and reduced forms of a molecule. Like the electric potentials that govern electron flow from one pole of a battery to another, E° values are specified in volts. Because electron-transfer reactions frequently involve protons also, an additional symbol is used to indicate that an E° value applies to a particular pH thus, E° refers to an E° at pH 7. 

The sequence of carriers in the respiratory chain should be generally consistent with the relative E° values of the carriers because, given equal concentrations of reactants and products, electrons flow from a carrier with a more negative E° value to one with a more positive value. The sequence of flavoproteins, UQ, and cytochromes that we presented in equation (4) agrees with this expectation. This proposition is demonstrated by plotting the E° values as a function of the carriers suggested positions in the chain (fig. 14.7). However, the E° values do not dictate the de-... 

The multisubunit complexes of the respiratory chain. Complexes I (NADH dehydrogenase) and II (succinate dehydrogenase) transfer electrons from NADH and succinate to UQ. Complex III (the cytochrome bc complex) transfers electrons from UQH2 to cytochrome c, and complex IV (cytochrome oxidase), from cytochrome c to 02. The arrows represent paths of electron flow. NADH and succinate provide electrons from the matrix side of the inner membrane, and 02 removes electrons on this side. Cytochrome c is reduced and oxidized on the opposite side of the membrane, in the lumen of a crista or in the intermembrane space. 

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