Pool quinones

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

When the fully conserved residue Thr 140, which is packed against the Pro loop, was substituted by Gly, His, or Arg in Rhodobacter capsulatus, the midpoint potential of the Rieske cluster was decreased by 50-100 mV, the cluster interacted with the quinone pool and the bci complex had 10-24% residual activity but the Rieske cluster was rapidly destroyed upon exposure to oxygen (49). In contrast, the residual activity was <5%, the cluster showed no interaction with the quinone pool, and the interaction with the inhibitor stigmatellin... 

Fig. 14. Plot of the g values g,g ) and of the average g value g vs rhombicity (UJ of (a) wild type (open symbol) and variant forms (closed symbols) of the Rieske protein in yeast bci complex where the residues Ser 183 and Tyr 185 forming hydrogen bonds into the cluster have been replaced by site-directed mutagenesis [Denke et al. (35) Merbitz-Zahradnik, T. Link, T. A., manuscript in preparation] and of (b) the Rieske cluster in membranes of Rhodobacter capsulatus in different redox states of the quinone pool and with inhibitors added [data from Ding et al. (79)]. The solid lines represent linear fits to the data points the dashed lines reproduce the fits to the g values of all Rieske and Rieske-type proteins shown in Fig. 13.

Fig. 15. EPR spectra of the Rieske cluster in membranes of Paracoccus denitrificans in different redox states of the quinone pool and with inhibitors added. Q x, ascorbate reduced Qred) reduced with trimethylhydroquinone dissolved in dimethyl sulfoxide +EtOH, reduced with trimethylhydroquinone dissolved in 90% ethanol +Myxo, ascorbate reduced with myxothiazol added + Stigma, ascorbate reduced with stigmatellin added. Only the gy and signals are shown. The dotted line has been drawn at...

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... 

Crofts, A. R., Meinhardt, S. W., Jones, K. R., and Snozzi, M., 1983, The role of the quinone pool in the cyclic electron transfer chain of Rhodopseudomonas sphaeroides. A modified Q-cycle mechanism. Biochim. Biophys. Acta, 723 2029218. 

During electron flow the btc complex functions as an energy transducer that converts a substantial fraction of the energy difference between the quinone pool and Cyt c-554 to potential energy in the form of a transmembrane proton gradient. 

Most photosynthetic eubacteria appear to contain cyclic electron transfer pathways driven by the RCs. Electrons from the secondary acceptor of the RC are transferrred first to a quinone pool and then to the secondary donor (Cyt c) via a Cyt bic complex which stores some of the electron redox energy as potential energy in the form of a transmembrane proton gradient. Evidence for cyclic electron flow in the gram-positive line has not yet been found, but it would be surprising not to find it. 

Those photosynthetic eubacteria with RC-2 centers (filamentous and purple bacteria) reduce NAD" for CO2 fixation by reverse electron flow from the quinone pool, whereas the green sulfur bacteria (RC-1 center) reduce ferredoxin and NAD directly from the secondary acceptor (Fe-S center) of the RC. In both cases an external reductant such as H2S is required. The mechanism of NAD reduction in the gram-positive line has not yet been investigated, but H. chlorum is a het-erotroph rather than an autotroph, and may not need to fix CO2. 

It should be mentioned that electron transfer to the quinone pool, both by PS II and by the reaction centers of purple bacteria, now proceeds via a two-electron gating mechanism after one electron has arrived at the temporarily bound quinone Qb, the semiquinone remains in its unprotonated, negatively charged form and tightly bound to the reaction center only after a second photoreaction does its full reduction, protonation and release as a quinol take place [19]. This procedure may also have played a role in the selection of the dimeric reaction center structure, but its importance most likely has to do with the reactivity of semiqui-nones with molecular oxygen and in that case it probably appeared much later. 

Figure 19.10. Electron Chain in the Photosynthetic Bacterial Reaction Center. The absorption of light by the special pair (P960) results in the rapid transfer of an electron from this site to a bacteriopheophytin (BPh), creating a photoinduced charge separation (steps 1 and 2). (The asterisk on P960 stands for excited state.) The possible return of the electron from the pheophytin to the oxidized special pair is suppressed by the "hole" in the special pair being refilled with an electron from the cytochrome subunit and the electron from the pheophytin being transferred to a quinone (Q ) that is farther away from the special pair (steps 3 and 4). The reduction of a quinone (Qg) on the periplasmic side of the membrane results in the uptake of two protons from the periplasmic space (steps 5 and 6). The reduced quinone can move into the quinone pool in the membrane (step 7).

Members of the Sulfolobaceae (Sulfolobus and Acidianus) are facultative heterotrophs that can oxidize hydrogen sulfide to elemental sulfur and the latter to sulfuric acid. Nothing is known about the enzymology of these processes and to what extent membrane-boimd enzymes are involved. What information there is relates to the electron transport system of Sulfolobus which is relatively simple consisting as it does of dehydrogenases for succinate and NADH, a quinone pool, a complex of b-cytochromes, and several oxidases. 

Complex 111 transfers electrons from the quinone pool dissolved in the membrane to the pool of cytochrome c loosely associated with the cytosolic face of the membrane. This complex also operates with a near equilibrium between AjaH+ and the oxido-reduction span of the electrons. In contrast the final complex of the mitochondrial respiratory chain, cytochrome c oxidase, transferring electrons from cytochrome c to oxygen, operates under non-equilibrium conditions and is strictly irreversible. 

It is possible to introduce or remove electrons at the interfaces between the complexes. Thus, electrons may be added to the quinone pool from Complex II,... 

Fig. 1. Schematic representation of the arrangement of various cofactors in the bacteriai photosynthetic reaction center where Q and Qb are represented by hexagons. A change in redox state of the cofactors is represented by a change of the solid symbols into empty ones or vice versa. A quinone pool is provided to the reaction center as shown in the rightmost frame.

As mentioned in Chapter 5, Qa in native reaction centers is reduced only to the semiquinone form while Qb can undergo two successive one-electron reduction steps to form the quinol (after protonation). In this chapter we discuss the electron-transfer reactions between Qa and Qb, exchange between reduced Qb and the quinone pool, and protonation coupled to electron transfer. 

With a full complement of quinones present in the reaction center [Fig. 3 (C)], the first and second flashes produce Qb which then becomes QbH2, as expected. With the quinone pool present, the fully reduced QbH2 is exchanged with one ofthe ubiquinone molecules in the pool, resulting in the restoration of the Qa Qb state. The same oscillatory absorbance-change pattern with a periodicity of 2 is then produced again by the third and fourth flashes. When monitored at 280 nm, a wavelength at which the quinone loses its absorption when reduced to either the semiquinone or the fully reduced state, the... 

Fig. 3. Absorbance-change patterns due to quinone reaction in the bacterial reaction center of Rb. sphaeroides excited by four consecutive flashes. Quinone compositions examined were (A) only Qa present (B) only Qa and Qb present (C) Qa, Qb and quinone pool all present. Figure adapted from Clayton (1980) Photosynthesis Physical Mechanisms and Chemical Patterns, pp 215, 216. Cambridge University Press.

Fig. 4. Schematic representation of binary oscillation involving electron transfer from QAto Qg in the absence of the quinone pool, as monitored by changes in light-induced, EPR signals, EPR data adapted from Butler, Calvo, Fredkin, Isaacson, Okamura and Feher (1984) The electronic structure ofFe in reaction centers from Rhodospseudomonas sphaeroides. III. EPR measurements of the reduced acceptor complex. Biophysical J. 45 955.

After the doubly-reduced Qb is formed and two protons are picked up from the cytoplasm to form QqH2, the reduced dihydroquinone leaves the reaction center and is replaced by an oxidized ubiquinone from the quinone pool. During the proton-transfer process, a proton gradient is established which provides the driving force for ATP synthesis. 

Fig. 6. Photochemical cycles showing coupling of electron transfer to proton transfer, cytochrome oxidation and quinone exchange in (A) native reaction centers where two Cyt c are oxidized in the cycie, (B) reaction centers where uptake of the first proton is inhibited, and (C) reaction centers where uptake ofthe second proton is inhibited (shading indicates the quinone pool). Figure source (A) Paddock, Rongey, McPherson, Juth, Feher and Okamura (1994) Pathway of proton transfer in bacterial reaction centers role of aspartate-L21Z in proton transfers associated with reduction of quinone to dihydroquinone. Biochemistry 33 734 (B) Okamura and Feher (1992) Proton transfer in reaction centers from photosynthetic bacteria. Annu Rev Biochemistry. 61 868 (C) Feher, Paddock, Rongey and Okamura (1992) Proton transfer pathways in photosynthetic reaction centers studied by site-directed mutagenesis. In A Pullman, J Jortner and B Pullman (eds) Membrane Proteins Structures, Interactions and Models, p 485. Kluwer.

Measurements for testing which route is actually taken for electron and proton transfers by the reduced QbH2 are presented in Fig. 8 (B). Here the normal reaction center with Qio serving as both Qa and Qb is used. About twice as many reduced cytochrome molecules as reaction centers are provided [see table in Fig. 8 (B), top] to assure two turnovers of P870 with the first two flashes. Application of two flashes leads to a net transfer oftwo electrons to Qb (=Qio)- Appropriate quinone pools, one containing Qio and one containing Qo, are provided in the two separate experiments. 

Test of transfer of reducing power to the quinone pool by measuring charge-recombination rate constants... 

---