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Title: Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides

Abstract

The mechanism of the proton-coupled electron transfer reaction, Q{sub A}{sup -}Q{sub B}{sup -} + H{sup +} {yields} Q{sub A}(Q{sub B}H){sup -} (i.e. k{sup (2)}{sub AB}), was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the Q{sub A} site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the Q{sub B} site. The measured rate constants, k{sup (2)}{sub AB}, increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy). The proton-coupled electron transfer was modeled. The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The best agreement with the experimental data is given by a two-step mechanism in which fast reversible proton transfer is followed by rate limiting electron transfer. For this mechanism the observed free energy dependence for k{sup (2)}{sub AB} can be fitted using reasonable parameters of the Marcus theory. The free energy dependence predicted using a simple model for a concerted reaction also provides a reasonable fit to the data. 75 refs., 9 figs., 2 tabs.

Authors:
; ; ;  [1];  [2]
  1. Univ. of California, San Diego, CA (United States)
  2. Univ. of Manchester (United Kingdom)
Publication Date:
OSTI Identifier:
420820
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the American Chemical Society; Journal Volume: 118; Journal Issue: 38; Other Information: PBD: 25 Sep 1996
Country of Publication:
United States
Language:
English
Subject:
40 CHEMISTRY; 55 BIOLOGY AND MEDICINE, BASIC STUDIES; 99 MATHEMATICS, COMPUTERS, INFORMATION SCIENCE, MANAGEMENT, LAW, MISCELLANEOUS; QUINONES; REDUCTION; PHOTOCHEMICAL REACTIONS; BACTERIA; FREE ENERGY; CHEMICAL REACTION KINETICS; MATHEMATICAL MODELS; PHOTOSYNTHESIS

Citation Formats

Graige, M.S., Paddock, M.L, Feher, G., Okamura, M.Y., and Bruce, J.M. Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides. United States: N. p., 1996. Web. doi:10.1021/ja960056m.
Graige, M.S., Paddock, M.L, Feher, G., Okamura, M.Y., & Bruce, J.M. Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides. United States. doi:10.1021/ja960056m.
Graige, M.S., Paddock, M.L, Feher, G., Okamura, M.Y., and Bruce, J.M. 1996. "Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides". United States. doi:10.1021/ja960056m.
@article{osti_420820,
title = {Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides},
author = {Graige, M.S. and Paddock, M.L and Feher, G. and Okamura, M.Y. and Bruce, J.M.},
abstractNote = {The mechanism of the proton-coupled electron transfer reaction, Q{sub A}{sup -}Q{sub B}{sup -} + H{sup +} {yields} Q{sub A}(Q{sub B}H){sup -} (i.e. k{sup (2)}{sub AB}), was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the Q{sub A} site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the Q{sub B} site. The measured rate constants, k{sup (2)}{sub AB}, increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy). The proton-coupled electron transfer was modeled. The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The best agreement with the experimental data is given by a two-step mechanism in which fast reversible proton transfer is followed by rate limiting electron transfer. For this mechanism the observed free energy dependence for k{sup (2)}{sub AB} can be fitted using reasonable parameters of the Marcus theory. The free energy dependence predicted using a simple model for a concerted reaction also provides a reasonable fit to the data. 75 refs., 9 figs., 2 tabs.},
doi = {10.1021/ja960056m},
journal = {Journal of the American Chemical Society},
number = 38,
volume = 118,
place = {United States},
year = 1996,
month = 9
}
  • No abstract prepared.
  • The pathway of proton transfer in the reaction center (RC) from Rhodobacter sphaeroides was investigated by site-directed mutagenesis. Ser-L223, a putative proton donor that forms a hydrogen bond with the secondary quinone acceptor Q{sub B}, was replaced with Ala and Thr. RCs with Ala-L223 displayed reduced electron transfer and proton uptake rates in the reaction Q{sub A}{sup {minus}}Q{sub B}{sup {minus}} + 2H{sup +} {yields} Q{sub A}Q{sub B}H{sub 2}. The rate constant for this reaction, k{sub AB}{sup (2)}, was found to be reduced {approx}350-fold to 4.0 {plus minus} 0.2 s{sup {minus}1}. Prton uptake measurements using a pH indicator dye showed amore » rapid uptake of 1 H{sup +} per RC followed by a slower uptake of 1 H{sup +} per RC at a rate of 4.1 {plus minus} 0.1 s{sup {minus}1}; native RCs showed a rapid uptake of 2H{sup +} per RC. Evidence is provided that these changes were not due to gross structural changes in the binding site of Q{sub B}. RCs with Thr-L223 showed little reduction in the rats of electron and proton transfer. These results indicate that proton transfer from the hydroxyl group of Ser-L223 or Thr-L223 is required for fast electron and proton transfer associated with the formation of the dihydroquinone QH{sub 2}. In contrast, previous work showed that replacing Glu-L212, another putative proton donor to Q{sub B}, with Gln slowed proton uptake from solution without significantly altering electron transfer. The authors propose a model that involves two distinct proton transfer steps. The first step occurs prior to transfer of the second electron to Q{sub B} and involves proton transfer from Ser-L223. The second step occurs after this electron transfer through a pathway involving Glu-L212.« less
  • We have measured the electrochromic response of the bacteriopheophytin, BPh, and bacteriochlorophyll, BChl, cofactors during the Q{sub A} {sup -}Q{sub B} {yields} Q{sub A}Q{sub B}{sup -} electron transfer in chromatophores of Rhodobacter (Rb.) capsulatus and Rb. sphaeroides. The electrochromic response rises faster in chromatophores and is more clearly biexponential than it is in isolated reaction centers. The chromatophore spectra can be interpreted in terms of a clear kinetic separation between fast electron transfer and slower non-electron transfer events such as proton transfer or protein relaxation. The electrochromic response to electron transfer exhibits rise times of about 4 {micro}s (70%) andmore » 40 {micro}s (30%) in Rb. capsulatus and 4 {micro}s (60%) and 80 {micro}s (40%) in Rb. sphaeroides. The BPh absorption band is shifted to nearly equivalent positions in the Q{sub A}{sup -} and nascent Q{sub B}{sup -} states, indicating that the electrochromic perturbation of BPh absorption from the newly formed Q{sub B}{sup -} state is comparable to that of Q{sub A}{sup -} . Subsequently, partial attenuation of the Q{sub B}{sup -} electrochromism occurs with a time constant on the order of 200 {micro}s. This can be attributed to partial charge compensation by H{sup +} (or other counter ion) movement into the Q{sub B} pocket. Electron transfer events were found to be slower in detergent isolated RCs than in chromatophores, more nearly monoexponential, and overlap H{sup +} transfer, suggesting that a change in rate-limiting step has occurred upon detergent solubilization.« less
  • No abstract prepared.
  • In bacterial photosynthetic reaction centers, the protonation events associated with the different reduction states of the two quinone molecules constitute intrinsic probes of both the electrostatic interactions and the different kinetic events occurring within the protein in response to the light-generated introduction of a charge. The kinetics and stoichiometries of proton uptake on formation of the primary semiquinone Q{sub A}{sup -} and the secondary acceptor Q{sub B}{sup -} after the first and second flashes have been measured, at pH 7.5, in reaction centers from genetically modified strains and from the wild type. The modified strains are mutated at the L212Glumore » and/or at the L213Asp sites near QB; some of them carry additional mutations distant from the quinone sites (M231Arg {yields} Leu, M43Asn {yields} Asp, M5Asn {yields} Asp) that compensate for the loss of L213Asp. Our data show that the mutations perturb the response of the protein system to the formation of a semiquinone, how distant compensatory mutations can restore the normal response, and the activity of a tyrosine residue (M247Ala {yields} Tyr) in increasing and accelerating proton uptake. The data demonstrate a direct correlation between the kinetic events of proton uptake that are observed with the formation of either Q{sub A}{sup -} or Q{sub B}{sup -}, suggesting that the same residues respond to the generation of either semiquinone species. Therefore, the efficiency of transferring the first proton to QB is evident from examination of the pattern of H{sup +}/Q{sub A}{sup -} proton uptake. This delocalized response of the protein complex to the introduction of a charge is coordinated by an interactive network that links the Q{sup -} species, polarizable residues, and numerous water molecules that are located in this region of the reaction center structure. This could be a general property of transmembrane redox proteins that couple electron transfer to proton uptake/release reactions.« less