Reaction centers bacterial photosynthetic, electron

Woodbury, N. W., M. Becker, D. Middendorf, and W. W. Parson, Picosecond kinetics of the initial photochemical electron transfer reaction in bacterial photosynthetic reaction centers. Biochem. 24 7516, 1985. Fast spectrophotometric techniques are used to follow the initial steps in reaction centers purified from photosynthetic bacteria. 

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. 

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.

SH Rongey, ML Paddock, G Feher and MY Okamura (1993) Pathway of proton transform bacterial reaction centers Second-site mutation, Asn-M44- Asp restores electron and proton transfer in reaction centers from photosynthetically deficient Asp-L213-. Asn mutant of Rhodobacter sphaeroldes. Proc Nat Acad Sci, USA 90 1325-1329... 

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

Somewhat surprisingly, the structure of the PSII reaction center, which removes electrons from H2O to form O2, resembles that of the reaction center of photosynthetic purple bacteria, which does not form O2. Like the bacterial reaction center, the PSII reaction center contains two molecules of chlorophyll a (Peso) - as well as two other chlorophylls, two pheophytins, two quinones (Qa and Qb), and... 

Woodbury, N.W., Beeker, M., Middcmdmf, D., Parson, W.W. Picosecond kinetics of the initial photochcanical electron-transfcu reaction in bacterial photosynthetic reaction center. Biochemistry 24, 7516-7521 (1985)... 

In the photosynthetic bacterial reaction center (RQ, the electron transfer reaction proceeds from the primary electron donor P, a dimer of bacteriochlorophyll, via an intermediate acceptor (a bacteriopheophytin molecule) to a primary quinone Qa and then to a secondary quinone Qb- In Rb. sphaeroides RC, both quinones are ubiquinone while in Rps. viridis RC, Qa is a mcnaquinone and Qfi is a ubiquinone. In addition, charge recombination between P+ and QA " or P" and Qb proceeds faster in Rps, viridis ( 1 msec and 100 msec, respectively [1]) than in Rb. sphaeroides ( 100 msec and a few sec, respectively [2]) RCs. Below lOOK, the electron is no longer transferred from Qa to Qb [3],... 

The photophysical and electron transfer properties of bacteriochlorophylls (Bchl) and bacteriopheophytins (Bpheo) found in the reaction centers of photosynthetic bacteria have been directly associated with the mechanism of charge separation which underlies photosynthesis [1]. The appearance of the Bpheo anion (Bpheo ) within 3-5 ps after excitation of the special pair of Bchl (P) is well documented from transient absorption spectroscopy [2-4]. The 200 ps lifetime of Bpheo which is primarily determined by the electron transfer process to a quinone also has been established by picosecond changes in absorption [5,6], Thus, the general kinetic time scale for the primary processes in bacterial photosynthesis has been determined by the transient differences in electronic state properties. 

Further increase in Afi° causes a rate decrease. The inverted effect is now established with certainty. It corresponds to the region in which A,G > X. These ideas were strongly supported by P. L. Dutton, professor at the University of Pennsylvania, Philadelphia. Dutton studied electron-transfer reactions in bacterial photosynthetic centers. The first step in bacterial photosynthesis is a light excitation in bacteriochlorophyll. The excited electron is first transferred to bacteriopheophytin and then to a quinone, and again to another quinone. Electron transfer to the first quinone occurred at a distance of about 1 nm. Dutton and coworkers systematically changed quinones as electron acceptors and thereby also reaction distances. Additional distances were 0.46 and 2.34 nm. A variation of 2 nm (20 A) in the distance between electron donors and acceptors in protein changed the electron-transfer rate by a factor of 10. A linear distance dependence of the maximum electron trans-... 

Boxer S G, Goldstein R A, Lockhart D J, Middendorf T R and Takiff L 1989 Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers J. Rhys. Chem. 93 8280-94... 

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.

Photosynthetic reaction centers from Rhodobacter sphaeroides and bacteri-orhodopsin (BR) from purple membrane (PM) have been used for their unique optoelectronic properties and for their capability of providing light-induced proton and electron pumping. Once assembled they display extremely high thermal and temporal stability... 

The photosynthetic reaction center (RC) of purple nonsulfur bacteria is the core molecular assembly, located in a membrane of the bacteria, that initiates a series of electron transfer reactions subsequent to energy transfer events. The bacterial photosynthetic RCs have been characterized in more detail, both structurally and functionally, than have other transmembrane protein complexes [1-52]. 

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]... 

Interpretation of the Primary Electron Transfer in Bacterial Photosynthetic Reaction Centers... 

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

Bacterial photosynthesis. What is the relationship of the Z scheme of Fig. 23-17 to bacterial photosyntheses In photoheterotrophs, such as the purple Rhodospirillum, organic compounds, e.g., succinate, serve as electron donors in Eq. 23-30. Because they can utilize organic compounds for growth, these bacteria have a relatively low requirement for NADPH or other photochemically generated reductants and a larger need for ATP. Their photosynthetic reaction centers receive electrons via cytochrome c from succinate (E° ... 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

---