Reaction center quinone-type

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

Fig. 2. Time-resolved fluorescence decays of whole cells of Rb. capsulatus. In each sample, the reaction center quinones west reduced with sodium dithionite. Excitation was at 870 nm, detection at 900 nm. For each decay, the instrument response function is also shown. Top decay wild type reaction center operon. Middle decay chimera 15 (yery similar decays w e observed for 4 and 32 as well). Bottom decay chimera 5 (vay similar decays. , -. .. wereobserv for and 2aswell).

Many of the contributors to this volume have addressed the reactions of quinone methides with DNA nucleophiles. The 13C-labeled methide center has the potential of identifying the type and number of such adducts using 13C-NMR. An obvious... 

We see, at first, that the reaction enthalpy for quinone abstraction reactions with the C—H bond of alkylperoxyl radicals is higher than with the O—H bond of the hydroperoxyl radical. The second important factor is different triplet repulsions in these two types of abstraction reactions. Indeed, the reaction with R02 proceeds via TS of the C H O type. Such reaction is characterized by the high thermally neutral activation energies Eeo = 62.9 kJ mol-1. The value of Ee0 for the reaction involving the O H O TS reaction center is much lower (27.3 kJ mol-1). With the rate constants have a very low value, the reaction Q + R02 cannot influence the oxidative chain termination in comparison with the interaction of two R02 radicals. Indeed, the rate constant for the latter is 105—107 L mol-1 s-1 and, in these cases, the inequality (2k6v )1/2 2k[Q] always holds. The reason for such high Ee0 values and, hence,... 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19-48a) is a large protein complex containing four polypeptide subunits and 13 cofactors two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome. 

The Fe-S Reaction Center (Type I Reaction Center) Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and involves additional enzymatic reactions (Fig. 19-47b). Excitation causes an electron to move from the reaction center to the cytochrome bei complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. 

Bacteria have a single reaction center in purple bacteria, it is of the pheophytin-quinone type, and in green sulfur bacteria, the Fe-S type. 

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. 

From QA., an electron moves to the second quinone bound to the reaction center (QB in fig. 15.13). In the meantime, a cytochrome replaces the electron that was removed from P870, preparing the reaction center to operate again. In R. viridis, the electron donor is the bound cytochrome with four hemes shown at the top of figure 15.1 la. In other species, it often is a soluble c-type cytochrome with a single heme, resembling mitochondrial cytochrome c. 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

The observation of a photosynthetic reaction center in green sulfur bacteria dates back to 1963.39 Green sulfur bacteria RCs are of the type I or the Fe-S-type (photosystem I). Here the electron acceptor is not the quinine instead, chlorophyll molecules (BChl 663, 81 -OII-Chi a, or Chi a) serve as primary electron acceptors, and three Fe4S4 centers (ferredoxins) serve as secondary acceptors. A quinone molecule may or may not serve as an intermediate carrier between the primary electron acceptor (Chi) and the secondary acceptor (Fe-S centers).40 The process sequence leading to the energy conversion in RCI is shown in Figure 21. 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

The L and M subunits form the structural and functional core of the bacterial photosynthetic reaction center (see Figure 19.9). Each of these homologous subunits contains five transmembrane helices. The H subunit, which has only one transmembrane helix, lies on the cytoplasmic side of the membrane. The cytochrome subunit, which contains four c-type hemes, lies on the opposite periplasmic side. Four bacteriochlorophyll b (BChl-Z>) molecules, two bacteriopheophytin b (BPh) molecules, two quinones (Q and Qg), and a ferrous ion are associated with the L and M subunits. 

C-P-Q Triad Molecules. As discussed above, the natural reaction center has solved the problem of energy loss due to rapid charge recombination by employing a multistep electron transfer strategy. The same strategy may be applied to the porphyrin-quinone type systems. As we pointed out in 1982 [28], this requires the addition of a secondary electron donor or acceptor moiety. This strategy came to fruition in 1983 when we reported the synthesis of carotenoid-porphyrin-quinone (C-P-Q) triad 2 [29, 30]. This molecule features porphyrin and quinone moieties similar to those found in 1, but a... 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

The crystal structures of Rb. sphaeroides reaction centers from the carotenoidless strain R-26 and from the wild-type strain 2.4.1. were initially determined at 2.8 A and 3.0 A resolution, respectively. The structure of the Rb. sphaeroides R-26 reaction center is shown by the model in Fig. 8. Major structural features, such as a hydrophobic core containing 11 transmembrane helices from the L-, M- and H-subunits, the approximate rotational symmetry of the L- and M-subunits, the position and orientation of cofactors, etc., are all apparently conserved. All cofactors are associated with the LM-complex. Note that the helices and their connecting segments are designated with upper- and lower-case letters, respectively, and the meaning of the underlining is the same as that for the Rp. viridis model in fig. 7. The dot in the circle slightly below the center represents the nonheme iron atom, which is approximately equidistant from the two quinone molecules. 

Photochemical activity in the ubiquinone-depleted reaction centers may also be restored by readdition of other types of quinones. Reaction centers reconstituted with quinones other than ubiquinone have a different recombination times between P870 and Q in the dark, and also a different spectral widths for the broad EPR signal. Such differences are reasonable, as these parameters are expected to be sensitive to the nature of the acceptor quinone molecule. 

We first examine briefly the binding domains of the primary (Qa) and secondary (Qb) quinone-acceptor molecules in the bacterial reaction centers, and see how one can rationalize their functional relationship. In Rp. viridis, the two quinones are different, Qa being menaquinone-7 and Qb ubiquinone-10. In this case, electron transfer from Qa to Qb could be accounted for by the difference in redox potential of the two types of quinones. In Rb. sphaeroides, however, both Qa and Qb are ubiquinone-10 and the condition for an exothermic electron transfer from Qa to Qb might be satisfied if the environments of the two quinone molecules were sufficiently different. Such differences in local structural features might also account for differences in other properties of the two quinone molecules. 

In recognition of the work carried out by various workers on the identification of phylloquinone as a potential electron carrier in the PS-I reaction center, Thumauer and coworkers extended the finding of a quinone-type molecule as an intermediary electron carrier in photosystem I to a more direct study of role played by the quinone molecule. Rustandi, Snyder, Feezel, Michalski, Norris, Thumauer and Biggins removed the endogenous phylloquinone from the CPI particle by organic-solvent extraction and then reconstituted the quinone-depleted sample with either protonated or deuterated phylloquinone and examined the ESP-EPR spectra, as shown in Fig. 2 (B). 

Bidirectional PCET is also featured on the reduction side of the photosynthetic apparatus. In the bacterial photosynthetic reaction center, two sequential photo-induced ET reactions from the P680 excited state to a quinone molecule (Qg) are coupled to the uptake of two protons to form the hydroquinone [213-215]. This diffuses into the inter-membrane quinone pool and is re-oxidized at the Qq binding site of the cytochrome bcj and coupled to translocation of the protons across the membrane, thereby driving ATP production. These PCET reactions are best described by a Type D mechanism because the PCET of Qg appears to involve specifically engineered PT coordinates among amino acid residues [215]. In this case PT ultimately takes place to and from the bulk solvent. Coupling remains tight in... 

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