The electron transfer rate from BPhA to QA in reaction centers of Rhodobacter sphaeroides R-26: Influence of the H-subunit, the QA and Fe2+ cofactors, and the isoprene tail of QA (original) (raw)
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Biochemistry, 1997
Absorbance difference kinetics were measured on quinone-reduced membrane-bound wild type Rhodobacter sphaeroides reaction centers in the wavelength region from 690 to 1060 nm using 800 nm excitation. Global analysis of the data revealed five lifetimes of 0.18, 1.9, 5.1, and 22 ps and a long-lived component for the processes that underlie the spectral evolution of the system. The 0.18 ps component was ascribed to energy transfer from the excited state of the accessory bacteriochlorophyll (B*) to the primary donor (P*). The 1.9 ps component was associated with a state involving a BChl anion absorbing in the 1020 nm region. This led to the conclusion that primary electron transfer is best described by a model in which the electron is passed from P* to the acceptor bacteriopheophytin (H L) via the monomeric bacteriochlorophyll (B L), with the formation of the radical pair state P + B L-† The investigations were supported by the Life Sciences Foundation (SLW), which is subsidized by the Netherlands Organization for Scientific Research (NWO) and by EC Contracts CT92-0796 and CT93-0278. M.R.J. is a BBSRC Advanced Research Fellow.
Biochemistry, 1997
Absorbance difference kinetics were measured on quinone-reduced membrane-bound wild type Rhodobacter sphaeroides reaction centers in the wavelength region from 690 to 1060 nm using 800 nm excitation. Global analysis of the data revealed five lifetimes of 0.18, 1.9, 5.1, and 22 ps and a long-lived component for the processes that underlie the spectral evolution of the system. The 0.18 ps component was ascribed to energy transfer from the excited state of the accessory bacteriochlorophyll (B*) to the primary donor (P*). The 1.9 ps component was associated with a state involving a BChl anion absorbing in the 1020 nm region. This led to the conclusion that primary electron transfer is best described by a model in which the electron is passed from P* to the acceptor bacteriopheophytin (H L ) via the monomeric bacteriochlorophyll (B L ), with the formation of the radical pair state P + B L -† The investigations were supported by the Life Sciences Foundation (SLW), which is subsidized by the Netherlands Organization for Scientific Research (NWO) and by EC Contracts CT92-0796 and CT93-0278. M.R.J. is a BBSRC Advanced Research Fellow.
1998
The efficiency of energy transfer from the monomeric pigments to the primary donor was determined from 77 K steady-state fluorescence excitation spectra of three mutant reaction centers, YM210L, YM210F and LM160H / FM197H. For all three reaction centers this efficiency was not 100% and ranged between 55 and 70%. For the YM210L mutant it was shown using pump-probe spectroscopy with B band excitation at 798 nm that the excitations which are not transferred to P give rise to efficient charge separation. The results can be interpreted with a model in which excitation of the B absorbance band leads to direct formation of the radical pair state B A + H − A in addition to energy transfer to P. It is also possible that some P + B A − is formed from B *. In previous publications we have demonstrated the operation of such alternative pathways for transmembrane electron transfer in a YM210W mutant reaction center [van Brederode et al. (1996) The Reaction center of Photosynthetic Bacteria, pp 225-238; (1997a,b) Chem Phys Lett 268: 143-149; Biochemistry 36: 6855-6861]. The results presented here demonstrate that these alternative mechanisms are not peculiar to the YM210W reaction center. Abbreviations: RC-reaction center; P-special pair of bacteriochlorophyll molecules; H A-Bpheo present in the active branch of the reaction center; H B-Bpheo present in the inactive branch of the reaction center; B A-Bchl present in the active branch of the reaction center; B B-Bchl present in the inactive branch of the reaction center; Q-quinone
Biochemistry, 1998
Electron transfer from P + Q A-Q B to form P + Q A Q Bwas measured in Rhodobacter sphaeroides R-26 reaction centers (RCs) where the native primary quinone, ubiquinone-10 (UQ A), was replaced by 2-methyl-3-phytyl-1,4-naphthoquinone (MQ A). The native secondary quinone, UQ-10, was retained as UQ B. The difference spectrum of the semiquinone MQ Aminus UQ Babsorption is very similar to that of MQminus UQin solution (398-480 nm). Thus, the absorption change provides a direct monitor of the electron transfer from MQ Ato UQ B. In contrast, when both Q A and Q B are UQ-10 the spectral difference between UQ Aand UQ Barises from electrochromic responses of RC chromophores. Three kinetic processes are seen in the near UV (390-480 nm) and near-IR (740-820 nm). Analysis of the time-correlated spectra support the conclusion that the changes at τ 1 ≈ 3 µs are mostly due to electron transfer, electron transfer and charge compensation are mixed in τ 2 ≈ 80 µs, while little or no electron transfer occurs at 200-600 µs (τ 3) in MQ A UQ B RCs. The 80-µs rate has been previously observed, while the fast component has not. The fast phase represents 60% of the electron-transfer reaction (398 nm). The activation energy for electron transfer is ∆G ≈ 3.5 kcal/mol for both τ 1 and τ 2 between 0 and 30°C. In isolated RCs with UQ A , if there is any fast component, it appears to be faster and less important than in the MQ A reconstituted RCs.
Biochemistry (Moscow), 2005
The effect of molecular oxygen on the temporary stabilization of an electron at the secondary quinone acceptor (Q B ) of photosynthetic reaction centers (RC) of the purple bacterium Rhodobacter sphaeroides was studied in the preceding work [1]. In the absence of exogenous donors of an electron, photoactivation of pigment-pro tein complexes of RC isolated from chromatophore membranes of Rb. sphaeroides induces fast (characteristic time, ~200 psec) transfer of an electron from the pho toactive bacteriochlorophyll dimer (P) to the primary quinone acceptor (Q A ). Then, within about 200 µsec the electron is transferred to Q B . In the RC of Rb. sphaeroides, the two quinone acceptors are molecules of ubiquinone 10. In further dark reactions the electron returns back to oxidized P. Electrostatic stabilization of the electron in quinone acceptors of RC is due to proton displacement in the RC microenvironment. Changes in the charge state of quinone modify the pK value of the protonated amino acid residues of the RC protein located within the vicinity of 15 17 Å [2 4].
Biophysics, 2008
The evolution of the light-induced absorption difference spectrum (380-500 nm) of the reaction centers from photosynthetic purple bacteria Rhodobacter sphaeroides has been examined over 200 µ s. The observed changes are interpreted as the effects of proton movement along the H-bond between the primary quinone acceptor and its protein surroundings. A theoretical analysis of the spectral evolution, considering the proton tunneling kinetics, corroborates this interpretation. The electronic state of the primary quinone is stabilized within tens of microseconds; the process is retarded upon deuteration of the reaction center as well as in 90% glycerol, and accelerated upon nondestructive heating to 40 ° C.
Initial electron-transfer in the reaction center from Rhodobacter sphaeroides
Proceedings of the National Academy of Sciences, 1990
The initial electron transfer steps in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides have been investigated by femtosecond timeresolved spectroscopy. The experimental data taken at various wavelengths demonstrate the existence of at least four intermediate states within the first nanosecond. The difference spectra of the intermediates and transient photodichroism data are fully consistent with a sequential four-step model of the primary electron transfer: Light absorption by the special pair P leads to the state P*. From the excited primary donor P*, the
Steady-state cyclic electron transfer through solubilized Rhodobacter sphaeroides reaction centres
Biophysical Chemistry, 2000
The mechanism, thermodynamics and kinetics of light-induced cyclic electron transfer have been studied in a model energy-transducing system consisting of solubilized Rhodobacter sphaeroides reaction centerrlight harvesting-1 Ž . complexes so-called core complexes , horse heart cytochrome c and a ubiquinone-0rubiquinol-0 pool. An analysis of the steady-state kinetics of cytochrome c reduction by ubiquinol-0, after a light-induced steady-state electron flow had been attained, showed that the rate of this reaction is primarily controlled by the one-electron oxidation of the ubiquinol-anion. Re-reduction of the light-oxidized reaction center primary donor by cytochrome c was measured at different reduction levels of the ubiquinone-0rubiquinol-0 pool. These experiments involved single turnover flash excitation on top of background illumination that elicited steady-state cyclic electron transfer. At low reduction levels of the ubiquinone-0rubiquinol-0 pool, the total cytochrome c concentration had a major control over the rate of reduction of the primary donor. This control was lost at higher reduction levels of the ubiquinonerubiquinol-pool, and possible reasons for this behaviour are discussed. ᮊ B.J. van Rotterdam . 0301-4622r00r$ -see front matter ᮊ 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 3 0 1 -4 6 2 2 0 0 0 0 2 0 6 -4 ( ) B.J.¨an Rotterdam et al. r Biophysical Chemistry 88 2000 137᎐152 138