Proton-coupled back electron transfer

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

II. Intra-Pair Reactions Back electron transfer Proton, atom, or group transfer Coupling... 

This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron transfer from substrates to 02, coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19-33). They translocate protons outward across the plasma membrane as electrons are transferred to 02. Bacteria such as Escherichia coli have F0Fi complexes in their plasma membranes the F portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and P, as protons flow back into the cell through the proton channel of F0. 

Figure 18-5 A current concept of the electron transport chain of mitochondria. Complexes I, III, and IV pass electrons from NADH or NADPH to 02, one NADH or two electrons reducing one O to HzO. This electron transport is coupled to the transfer of about 12 H+ from the mitochondrial matrix to the intermembrane space. These protons flow back into the matrix through ATP synthase (V), four H+ driving the synthesis of one ATP. Succinate, fatty acyl-CoA molecules, and other substrates are oxidized via complex II and similar complexes that reduce ubiquinone Q, the reduced form QH2 carrying electrons to complex III. In some tissues of some organisms, glycerol phosphate is dehydrogenated by a complex that is accessible from the intermembrane space.

Figure 2-3. Catalytic cycle of cytochrome c oxidase. The figure depicts different states of the binuclear haem arCue centre (squares, see the text), and shows the reaction steps where an electron is transferred to the centre from cytochrome c via haem a. If tlie enzyme is deprived of an electron donor, the oxidised state Oh decays into a relaxed fonn O. The latter may be reduced back to state R with uptake of two substrate protons (not shown), but this is not coupled to proton translocation (blue arrows). Note that otherwise each electron transfer into the binuclear site is coupled to proton translocation, and to uptake of a substrate proton (not shown). Thus each blue arrow represents uptake of two protons from the A -side and release of one proton to the P-side of the membrane (see also Fig. 2-1).

M EXPERIMENTAL FIGURE 8-19 Electron transfer from reduced cytochrome c (Cyt c " ) to O2 via the cytochrome c oxidase complex is coupled to proton transport. The oxidase complex is incorporated into liposomes with the binding site for cytochrome c positioned on the outer surface, (a) When O2 and reduced cytochrome c are added, electrons are transferred to O2 to form H2O and protons are transported from the inside to the outside of the vesicles. Valinomycin and are added to the medium to dissipate the voltage gradient generated by the translocation of H, which would otherwise reduce the number of protons moved across the membrane, (b) Monitoring of the medium pH reveals a sharp drop in pH following addition of O2. As the reduced cytochrome c becomes fully oxidized, protons leak back into the vesicles, and the pH of the medium returns to its initial value. Measurements show that two protons are transported per O atom reduced. Two electrons are needed to reduce one O atom, but cytochrome c transfers only one electron thus two molecules of Cyt c are oxidized for each O reduced. [Adapted from B. Reynafarje et al., 1986, J. Biol. Chem. 261 8254.1... 

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