Ubiquinones centers

Figure 12.13 Photosynthetic pigments are used hy plants and photosynthetic bacteria to capture photons of light and for electron flow from one side of a membrane to the other side. The diagram shows two such pigments that are present in bacterial reaction centers, bacteriochlorophyll (a) and ubiquinone (b). The light-absorbing parts of the molecules are shown in yellow, attached to hydrocarbon "tails" shown in green.

Complex II contains four peptides, the two largest form succinate dehydrogenase, the largest has covalently boiuid flavin adenine dinucleotide (FAD) which reacts with succinate, and the other has three iron-sulphur centers. Smaller subunits anchor the two larger subunits to the membrane and form the UQ binding site. Ubiquinone is the electron acceptor but complex II does not pump protons (see below). 

All these latter centers were seen to titrate at around 150 mV, that is, some 150 mV lower than the traditional centers, and thus form a separate subclass of this type of redox proteins (see Fig. 7). Since similar downshifts were observed for almost all redox components in the mentioned species (for a compilation, see 133), it is generally assumed that the differences between the two groups represent an adaptation to the difference in value of the quinone pool, which is plastoquinone(PQ)/ubiquinone(UQ) E 100 mV) in the traditional species and menaquinone (MK) Em,i---70 mV) in the other... 

Reaction centers of bacteria contain polypeptides, bacteriochlorophylls, bacteriopheo-phytins, two quinines, and nonheme iron atom. In some bacterial species, both the quinones are ubiquinones, whereas in some others one of the quinones is menaquinone [37,39]. Depending on the bacterial species chloroplasts contain plastoquinone and phyl-loquinone. Structures of ubiquinone, menaquinone, and phylloquinone are provided in Figures 7.12 through 7.14, respectively. 

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... 

A third, clearer explanation of the electron transfer, proton translocation cycle is given by Saratse. Each ubiquinol (QH2) molecule can donate two electrons. A hrst QH2 electron is transferred along a high-potential chain to the [2Fe-2S] center of the ISP and then to cytochrome Ci. From the cytochrome Cl site, the electron is delivered to the attached, soluble cytochrome c in the intermembrane space. A second QH2 electron is transferred to the Qi site via the cytochrome b hemes, bL and bn. This is an electrogenic step driven by the potential difference between the two b hemes. This step creates part of the proton-motive force. After two QH2 molecules are oxidized at the Qo site, two electrons have been transferred to the Qi site (where one ubiquinone (Qio) can now be reduced, requiring two protons to be translocated from the matrix space). The net effect is a translocation of two protons for each electron transferred to cytochrome c. Each explanation of the cytochrome bci Q cycle has its merits and its proponents. The reader should consult the literature for updates in this ongoing research area. 

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... 

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. 

Amytal (a barbiturate drug), rotenone (a plant product commonly used as an insecticide), and piericidin A (an antibiotic) inhibit electron flow from the Fe-S centers of Complex I to ubiquinone (Table 19-4) and therefore block the overall process of oxidative phosphorylation. 

Myxothiazol Rotenone Amytal Prevent electron transfer from Fe-S center to ubiquinone... 

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... 

QH2 to the Rieske iron-sulfur protein, binds at QP, near the 2Fe-2S center and heme bL on the P side. The dimeric structure is essential to the function of Complex III. The interface between monomers forms two pockets, each containing a QP site from one monomer and a QN site from the other. The ubiquinone intermediates move within these sheltered pockets. 

Reducing equivalents are then passed through a series of Fe-S centers to ubiquinone, which transfers the electrons to cytochrome b, the first carrier in Complex III. In this complex, electrons take two separate paths through two b-type cytochromes and cytochrome cq to an Fe-S center. The Fe-S center passes electrons, one at a time, through cytochrome c and into... 

Some electrons enter this chain of carriers through alternative paths. Succinate is oxidized by succinate dehydrogenase (Complex II), which contains a flavoprotein that passes electrons through several Fe-S centers to ubiquinone. Electrons derived from the oxidation of fatty acids pass to ubiquinone via the electron-transferring flavoprotein. 

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 ... 

Ubiquinones function as electron transport agents within the inner mitochondrial membranes496 and also within the reaction centers of the photosynthetic membranes of bacteria (Eq. 23-32).484/488/494 The plasto-quinones also function in electron transport within... 

During the 1940s, when it had become clear that formation of ATP in mitochondria was coupled to electron transport, the first attempts to pick the system apart and understand the molecular mechanism began. This effort led to the identification and at least partial characterization of several flavoproteins, iron-sulfur centers, ubiquinones, and cytochromes, most of which have been described in Chapters 15 and 16. It also led to the picture of mitochondrial electron transport shown in Fig. 10-5 and which has been drawn in a modem form in Fig. 18-5. 

The functions of the heme is uncertain. The soluble mammalian succinate dehydrogenase resembles closely that of E. coli and contains three Fe-S centers binuclear SI of E° 0 V, and tetranuclear S2 and S3 of -0.25 to -0.40 and + 0.065 V, respectively. Center S3 appears to operate between the -2 and -1 states of Eq. 16-17 just as does the cluster in the Chromatium high potential iron protein. The function of the very low potential S2 is not certain, but the following sequence of electron transport involving SI and S3 and the bound ubiquinone QD-S66 has been proposed (Eq. 18-4). 

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...

Electron transfer to 02 occurs stepwise, through a series of flavoproteins, cytochromes (heme-proteins), iron-sulfur proteins, and a quinone. Most of the electron carriers are collected in four large complexes, which communicate via two mobile carriers— ubiquinone (UQ) and cytochrome c. Complex I transfers electrons from NADH to UQ, and complex II transfers electrons from succinate to UQ. Both of these complexes contain flavins and numerous iron-sulfur centers. Complex III, which contains three cyto-... 

Reaction centers of purple bacteria typically contain three polypeptides, four molecules of bacteriochlorophyll, two bacteriopheophytins, two quinones, and one nonheme iron atom. In some bacterial species, both quinones are ubiquinone. In others, one of the quinones is menaquinone (vitamin K2), a naphthoquinone that resembles ubiquinone in having a long side chain (fig. 15.10). Reaction centers of some species, such as Rhodopseudomonas viridis, also have a cytochrome subunit with four c-type hemes. 

Although the structures of the plant reaction centers are not yet known in detail, photosystem II reaction centers resemble reaction centers of purple bacteria in several ways. The amino acid sequences of their two major polypeptides are homologous to those of the two polypeptides that hold the pigments in the bacterial reaction center. Also, the reaction centers of photosystem II contain a nonheme iron atom and two molecules of plastoquinone, a quinone that is closely related to ubiquinone (see fig. 15.10), and they contain one or more molecules of pheophytin a and several... 

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). 

---