Quinones as Electron Carriers

Substances undergoing redox reactions (such as quinone-hydroquinone, sulphide-disulphide, metal complexes, redox couples) may serve as electron carriers and allow the coupling of oxidation-reduction processes across membranes (see, for instance, [6.44-6.46]) to cation or anion transport. 

In natural photosynthesis the quinones are widely used as electron carriers. Unfortunately, the low values of cpc in the reaction of quinones with 3Ru(bpy) + make the direct use of these important electron carriers rather inefficient. However, introduction of the electron carrier Rh(bpy) + into the inner volume of the vesicle in addition to photosensitizer Ru(bpy) +, provides much more efficient electron transfer from 3Ru(bpy) + to a quinone embedded into the membrane. This was found for System 25 of Table 1. 

TABLE 7.8 Structural Formulas, Reduction Potentials ( ],)> and Acidity Constants for the Two Quinones and the Iron Porphyrin Used as Electron Carriers"... 

In order to obtain the catalytic coefficient, both balances in organic and in aqueous phase were considered. The enzymatic degradation of anthracene by MnP was considered as pseudo-first order kinetics with an autocatalytic effect due to the presence of the degradation products (Eq. 6.6.7). Quinones, which are the main degradation products of PAHs, can act as electron carriers as described by M6ndez-Paz et al. (2005), thus accelerating the overall degradation. 

Simple quinones or benzoquinones are found commonly in nature. Some quinonoid compounds such as the electron carrier ubiquinone (13), sometimes known as coenzyme Q, play important primary roles in plants. Structurally related plasto-quinones are important as electron carriers in photosynthesis. 

Figure 18.2 Summary of respiratory energy flows. Foods ate converted into the reduced form of nicotinamide adenine dinucleotide (NADH), a strong reductant, which is the most reducing of the respiratory electron carriers (donors). Respiration can he based on a variety of terminal oxidants, such as O2, nitrate, or fumarate. Of those, O2 is the strongest, so that aerobic respiration extracts the largest amount of free energy from a given amount of food. In aerobic respiration, NADH is not oxidized directly by O2 rather, the reaction proceeds through intermediate electron carriers, such as the quinone/quinol couple and cytochrome c. The most efficient respiratory pathway is based on oxidation of ferrocytochrome c (Fe ) with O2 catalyzed by cytochrome c oxidase (CcO). Of the 550 mV difference between the standard potentials of c)Tochrome c and O2, CcO converts 450 mV into proton-motive force (see the text for further details).

There are two kinds of redox interactions, in which ubiquinones can manifest their antioxidant activity the reactions with quinone and hydroquinone forms. It is assumed that the ubiquinone-ubisemiquinone pair (Figure 29.10) is an electron carrier in mitochondrial respiratory chain. There are numerous studies [235] suggesting that superoxide is formed during the one-electron oxidation of ubisemiquinones (Reaction (25)). As this reaction is a reversible one, its direction depends on one-electron reduction potentials of semiquinone and dioxygen. 

The second type of biological electron transfer involves a variety of small molecules, both organic and inorganic. Examples of these are (a) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) as two electron carriers and (b) quinones and flavin mononucleotide (FMN), which may transfer one or two electrons. The structure of NAD and its reduced counterpart NADH are shown in Figure 1.12. 

In such vesicle systems, the electrons are transported through the membrane. Electron carriers such as quinones or alloxazines in the vesicle wall enhance remarkably the rate of photoinduced charge separation. The vesicle system shown in Fig. 6 contains the surfactant Zn-porphyrine complex (ZnC12TPyP) in the wall 23). 

Ubiquinone (also called coenzyme Q) and plasto-quinone (Fig. 10-22d, e) are isoprenoids that function as lipophilic electron carriers in the oxidation-reduction reactions that drive ATP synthesis in mitochondria and chloroplasts, respectively. Both ubiquinone and plasto-quinone can accept either one or two electrons and either one or two protons (see Fig. 19-54). 

Like Complex III of mitochondria, cytochrome b6f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQb in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19-12) in which electrons pass, one at a time, from PQBH2 to cytochrome bs. This cycle results in the pumping of protons across the membrane in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis. 

The vitamin E derivative a-tocopherolquinone (Fig. 15-24) can also serve as an electron carrier, being reversibly reduced to the hydro-quinone form a-tocopherolquinol. Such a function has been proposed for the anaerobic rumen bacterium Butyrovibrio fibrisolvens 497... 

This scheme was supported and refined by examining the effects of specific inhibitors of individual steps in the electron-transport chain. If CO or CN was added in the presence of a reducing substrate and 02, all of the electron carriers became more reduced. This fits the idea that these inhibitors act at the end of the respiratory chain, preventing the transfer of electrons from cytochrome to 02. If amytal (a barbiturate) or rotenone (a plant toxin long used as a fish poison) was added instead, NAD+ and the flavin in NADH dehydrogenase were reduced, but the carriers downstream became oxidized. The antibiotic antimycin caused NAD+, flavins, and the b cytochromes to become more reduced, but cytochromes c, cx, a, and a3 all became more oxidized. The situation here is analogous to the construction of a dam across a stream When the gates are closed, the water level rises upstream from the dam, and falls downstream. The observation that antimycin did not inhibit reduction of UQ showed that the quinone fits into the chain upstream of cytochromes c, t i, a, and a3. 

Fig. 14. Schematic representation of light-driven (2e + 2H+) symport across a membrane via the quinone carrier molecule vitamin Kj and its hydroquinone form proflavine (PF)-sen-sitized photoreduction of methyl-viologen MV2+ in the RED phase, yields the reducing species MV+, with simultaneous oxidative decomposition of EDTA used as electron donor the OX phase contains ferricyanide as electron acceptor [6.49].

Reduction of plastoquinone Qb by QA- and protonation at the acceptor side of PSII. The Qa is tightly bound to the protein, acting as a one electron acceptor. It passes electrons to a second plastoquinone, Qb, which can accept two electrons and two protons and acts as a mobile electron carrier connecting PSII to the next complex of the photosynthetic apparatus (i.e. the cytochrome b(f complex). After two electron-reductions and two protonation events, QbH2 leaves the reaction center and is replaced by an oxidized quinone from the pool in the membrane. 

Like the purple bacterial species mentioned above, Prostheocochloris aestuarii and other members of the Chlorobiaceae subgroup of the green photosynthetic bacteria appear to use a BChl dimer as an initial electron donor, but they evidently use BChl c istead of BPh as an initial electron acceptor [82-85]. The Chlorobiaceae also differ in using iron-sulfur proteins as the next electron carriers, instead of quinones. Their electron acceptor system appears to resemble that found in PS 1 of plants and cyanobacteria more than it does that of other groups of photosynthetic bacteria. 

Among electron carriers used for indirect oxidation reactions, cerium salts [Ce -t- e Ce E° = -t-1.44 V vs. NHE] appear to be of particular interest when a mild oxidation has to be considered. Substituted toluenes and methylaryl compounds are easily functionalized to the corresponding aldehydes in high yields [125-129]. Acidic solutions are required (such as aqueous AcOH, aqueous methane sulfonic acid, or aqueous trifluorosulfonic acid). The conversion of aromatic compounds into quinones may also be conducted by means of electrogenerated ceric ions (see Table 3). Let us stress the example... 

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