Electron bacterial reaction center

Jean J M, Chan C-K and Fleming G R 1988 Electronic energy transfer in photosynthetic bacterial reaction centers Isr. J. Chem. 28 169-75... 

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.

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

The spectroscopy and dynamics of photosynthetic bacterial reaction centers have attracted considerable experimental attention [1-52]. In particular, application of spectroscopic techniques to RCs has revealed the optical features of the molecular systems. For example, the absorption spectra of Rb. Sphaeroides R26 RCs at 77 K and room temperature are shown in Fig. 2 [42]. One can see from Fig. 2 that the absorption spectra present three broad bands in the region of 714—952 nm. These bands have conventionally been assigned to the Qy electronic transitions of the P (870 nm), B (800 nm), and H (870 nm) components of RCs. By considering that the special pair P can be regarded as a dimer of two... 

In the study of the ultrafast dynamics of photosynthetic bacterial reaction centers, we are concerned with the photoinduced electron transfer [72]... 

As pointed out in Ref. [4], no entropy variation appears in the description given by the harmonic model, apart from the weak contribution arising from the frequency shifts of the oscillators. The applications of this model are then a priori restricted to redox reactions in which entropic contributions can be neglected. We shall see in Sect. 3 that the current interpretations of most electron transfer processes which take place in bacterial reaction centers are based on this assumption. 

Some authors have described the time evolution of the system by more general methods than time-dependent perturbation theory. For example, War-shel and co-workers have attempted to calculate the evolution of the function /(r, Q, t) defined by Eq. (3) by a semi-classical method [44, 96] the probability for the system to occupy state v]/, is obtained by considering the fluctuations of the energy gap between and 11, which are induced by the trajectories of all the atoms of the system. These trajectories are generated through molecular dynamics models based on classical equations of motion. This method was in particular applied to simulate the kinetics of the primary electron transfer process in the bacterial reaction center [97]. Mikkelsen and Ratner have recently proposed a very different approach to the electron transfer problem, in which the time evolution of the system is described by a time-dependent statistical density operator [98, 99]. 

It is evident that the preceding considerations do not apply to all biological electron transfer systems. Even in the bacterial reaction center, the transfer between the two quinones Qa Qbj which takes place over 18 A [18], is characterized in Rhodobacter sphaeroides by a large entropic contribution, which has been attributed to the high solvent exposure of Qg [126]. By using the activation energy value reported in Ref. [126], two very different X values may be deduced from Eq. (23) = 0.1 eV and Aj = 2.5 eV. The previous considerations... 

A recent study (Booth PJ, Crystall B, Giorgi LB, Barber J, Klug DR, Porter G (1990) Biochim. Biophys. Acta 1016 141) has shown that the free energy difference of the primary electron transfer is dominated by entropic contributions in photosystem II reaction centers as in bacterial reaction centers (Woodbury NWT, Parson WW (1984) Biochim. Biophys. Acta 767 345), so that the interpretation of the rate temperature dependenee should be revised. 

In Purple Bacterial Reaction Centers, Electrons Move from P870 to Bacteriopheophytin and Then to Quinones... 

The electron acceptors on the reducing side of photosystem II resemble those of purple bacterial reaction centers. The acceptor that removes an electron from P680 is a molecule of pheophytin a. The second and third acceptors are plastoquinones (see fig. 15.10). As in bacterial reaction centers, electrons move one at a time from the first quinone to the second. When the second quinone becomes doubly reduced, it picks up protons from the stromal side of the thylakoid membrane and dissociates from the reaction center. 

Fig. I. Arrangement of the chromophores, electron donors, and electron acceptors in the bacterial reaction center of Rhodobacter sphaeroides [2f], The horizontal lines at the top and bottom of the figure represent the approximate location of the surfaces of the lipid bilayer membrane...

The abundance of cyclic tetrapyrroles in the bacterial reaction center naturally suggests the preparation of triads bearing two porphyrin moieties. A number of such systems have been described. Building upon their earlier work with D-A type systems [19, 66], Sanders, Beddard and coworkers have reported triad 14, which consists of two doubly-linked porphyrins, one of which bears a pyromellitimide acceptor [66]. The pyromellitimide moiety is nearly as good an electron acceptor as benzoquinone (E0 = —0.55 V vs. —0.51 V for benzoquinone). Triad 14,... 

One of the enigmatic problems of photsynthesis is the drastic difference between the rate of photelectron transfer in the active (M) and inactive branches of bacterial reaction centers. The quantum mechanical calculation (Kolbasov and Scherz, 2000) showed that the square of electronic matrix element VA2 for the electron transfer from the excited primary donor, P, to bacteriochlorophyl in the active brunch is larger by three order of magnitude than that in the inactive part Vb2. Therefore, the electron transfer rate in the RC inactive L-brunch should be essentially slower than that in the M-brunch. 

Okamura, M.Y., Paddock, M.L., Graige, M.S., and Feher, G.. (2000) Proton and electron transfer in bacterial reaction centers, Biochim. Biophys. Acta 1458, 148-163. 

Sporlein, S., Zinth, W., Meyer, M., Scheer, H., and Wachtveitl, J. (2000) Primary electron transfer in modified bacterial reaction centers optimization of the first events in photosynthesis, Chem. Phys. Lett. 322(6), 454-464. 

Wachtveitl, J., Huber, H., Feick, R., Rautter, J., Muh, F., and Lubitz, W. (1998) Electron transfer in bacterial reaction centers with an energetically raised primary acceptor ultrafast spectroscopy and endor/triple... 

Chains of redox cofactors for long range electron transfer are clearly the way electrons are transferred over the tens of angstroms dimensions of membranes and their proteins. Once again, purple photosynthetic bacterial reaction centers provide an archetype for understanding electron transfer chain design and behavior. The heme chain in Rps. viridis... 

The redox partners of these proteins have yet to be identified, although it has been shown that auracyanins can donate electrons to the membrane-bound cytochrome c-554, which is the direct electron donor for the photooxidized bacterial reaction center P870+ (McManus et al., 1992). However, whether it is their proper in vivo function remains uncertain. The sulfocyanin gene is in the same operon with the components of the respiratory electron transfer chain and, since Su. acidocaladar-ius completely lack c-type cytochromes, it is implicated as a substrate for the CuA-containing terminal oxidase. Interestingly, the occurrence of... 

Parson, W. W., Chu, Z. T., and Warshel, A., 1998, Reorganization energy of the initial electron-transfer step in photosynthetic bacterial reaction centers Biophysical Journal 74 182nl91. 

Despite lack of sequence homology, the function of the quinone reduction site (Qi site) is similar to that of the secondary quinone-binding site (Qb site) of bacterial reaction centers. Both sites have a conserved histidine residue as quinone ligand and both quinone molecules are reduced to hydroquinone in two consecutive one-electron transfer steps. The midpoint potential for the first step is pH-independent at near neutrality, whereas that for the second reduction varies by 120mV per pH unit (Robertson et al., 1984). This suggests a reaction pathway Q —> Q" QH2, with both protons added concomitantly with the second electron. A stable semi-quinone anion intermediate can be detected by EPR spectroscopy of samples frozen during turnover (Yu et al., 1980 de Vries et al., 1980) or with the redox potential adjusted near the midpoint of ubiquinone (Robertson et al., 1984 Ohnishi and Trumpower, 1980). The semiquinone signal is not observed in the presence of antimycin, which is consistent with the proposal that antimycin inhibits the reaction at the site (Mitchell, 1976 Mitchell, 1975). 

Graige, M. S., Feher, G., and Okamura, M. Y., 1998, Conformational gating of the electron transfer reaction Qa Qb QaQb bacterial reaction centers of Rhodobacter sphaeroides determined by a driving force assay. Proc. Natl. Acad. Sci. USA, 95 11679911684. 

Kirmaier, C., and Holten, D., 1988, Temperature effects on the ground state absorption spectra and electron transfer kinetics of bacterial reaction centers. In NATO ASI Ser., Ser. A, 149 219n228. 

McDowell, L. M., Gaul, D., Kirmaier, C., Holten, D., and Schenck, C. C., 1991, Investigation into the source of electron transfer asymmetry in bacterial reaction centers. Biochemistry, 30 8315fi8322. 

Parson, W. W., 1996, Photosynthetic bacterial reaction centers. In Protein Electron Transfer (D. S. Bendall, ed.) pp. 1259160, BIOS Scientific Publishers Ltd. Oxford, U.K. 

Shuvalov, V. A., 1993, Time and frequency domain study of different electron transfer processes in bacterial reaction centers. In The Photosynthetic Reaction Center, (J. Deisenhofer and J. R. Norris, eds.) Volume 2, 89nl03, Academic Press, San Diego, USA. 

Vos, M. H., Breton, J., and Martin, J. L., 1997, Electronic energy transfer within the hexamer cofactor system of bacterial reaction centers. J. Phys. Chem., 101 9820fi9832. 

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