Quinones membrane carriers

In stage 2, electrons are transported from the reduced quinone via carriers In the thylakoid membrane until they reach the ultimate electron acceptor, usually NADP", reducing It to NADPH. Electron transport is coupled to movement of protons across the membrane from the stroma to the thylakoid lumen, forming a pH gradient (proton-motive force) across the thylakoid membrane. 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

We next focus on the use of fixed-site cofactors and coenzymes. We note that much of this coenzyme chemistry is now linked to very local two-electron chemistry (H, CH3", CH3CO-, -NH2,0 transfer) in enzymes. Additionally, one-electron changes of coenzymes, quinones, flavins and metal ions especially in membranes are used very much in very fast intermediates of twice the one-electron switches over considerable electron transfer distances. At certain points, the chains of catalysis revert to a two-electron reaction (see Figure 5.2), and the whole complex linkage of diffusion and carriers is part of energy transduction (see also proton transfer and Williams in Further Reading). There is a variety of additional coenzymes which are fixed and which we believe came later in evolution, and there are the very important metal ion cofactors which are separately considered below. 

Over the years, there have been numerous reports of oxidase preparations that contain polypeptide components, additional to those described above. As yet no molecular probes are available for these, and so their true association with the oxidase is unconfirmed. There are many reports in the literature describing the role of ubiquinone as an electron transfer component of the oxidase, but its involvement is controversial. Quinones (ubiquinone-10) have reportedly been detected in some neutrophil membrane preparations, but other reports have shown that neither plasma membranes, specific granules nor most oxidase preparations contain appreciable amounts of quinone, although some is found in either tertiary granules or mitochondria. Still other reports suggest that ubiquinone, flavoprotein and cytochrome b are present in active oxidase preparations. Thus, the role of ubiquinone and other quinones in oxidase activity is in doubt, but the available evidence weighs against their involvement. Indeed, the refinement of the cell-free activation system described above obviates the requirement for any other redox carriers for oxidase function. 

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). 

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. 

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. 

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. 

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].

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. 

In mitochondria there are two types of mechanisms for coupling the electron transport to the movement of protons across the membrane. The first is based on anisotropic reduction and oxidation of a lipid-soluble quinone inside the membrane. The quinone, coenzyme Q, becomes protonated upon reduction and diffuses to an oxidation site on the other side of the membrane where removal of electrons leads to proton release. This is essentially a proton carrier system with the hydroquinone acting as the proton carrier in the lipid phase of the membrane. A further refinement of this system in mitochondria provides for a coenzyme Q redox cycle where the movement of one electron through the chain allows for two protons to cross the... 

Figure 4. Scheme for proton transfer by plastoquinone as a mobile carrier in membrane lipid. Electrons are transferred one by one to a bound plastoquinone A (PQA) which in turn reduces external plastoquinone. When reduced, the anionic plastoquinone takes up protons to become a hydroquinone which is oxidized by the cytochrome bb f complex on the inside of the membrane to release protons. A second quinone, vitamin K, (KQ) is also involved in chloroplast electron transport, but its role in proton movement is not known. 

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. 

Several important groups of plant compounds, including cytokinins, chlorophylls and the quinone-based electron carriers (the plastoquinones and ubiquinones), have terpenoid side chains attached to a non-terpenoid nucleus. These side chains facilitate anchoring to or movement within membranes. In the past decade, proteins have also been found to have terpenoid side chains attached. In fact, all eukaryotic cells appear to contain proteins that have been post-translationally modified by the attachment of C15 and C20 terpenoid side chains via a thioether linkage. 

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

Special electron carriers ferry the electrons from one complex to the next. Electrons are carried from NADH-Q oxidoreductase to Q-cytochrome c oxidoreductase, the second complex ot the chain, by the reduced form of coeti2 ymt2 Q (Q), also known as ubiquinone because it is a ubiquitous quinone in biological systems. Ubiquinone is a hydrophobic quinone that diffuses rapidly within the inner mitochondrial membrane. Cytochrome c. a small soluble protein, shuttles electrons from Q-cytochrome c oxidoreductase to cytochrome c oxidase, the final component in the chain and the one that catalyses the reduction of Oi. Electrons from the FADH generated bv... 

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