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- Crystallization and Electron Paramagnetic Resonance Characte...Crystallization and Electron Paramagnetic Resonance Characterization of the Complex of Photosystem I with its Natural Electron Acceptor Ferredoxin
Biophysical Journal, Volume 83, Issue 4, 1 October 2002, Pages 1760-1773
Petra Fromme, Hervé Bottin, Norbert Krauss and Pierre Sétif
AbstractThe formation of a transient complex between photosystem I and ferredoxin is involved in the process of ferredoxin photoreduction in oxygenic photosynthetic organisms. Reduced ferredoxin is an essential redox intermediate involved in many assimilatory processes and is necessary for the reduction of NADP to NADPH. Single crystals from a complex of photosystem I with ferredoxin were grown using PEG 400 and CaCl as precipitation agents. The crystals diffract x-rays to a resolution of 7–8Å. The space group was determined to be orthorhombic with the unit cell dimensions =194Å, =208Å, and =354Å. The crystals contain photosystem I and ferredoxin in a 1:1 ratio. Electron paramagnetic resonance (EPR) measurements on these crystals are reported, where EPR signals of the three [4Fe-4S] clusters F, F, F, and the [2Fe-2S] cluster of ferredoxin were detected. From the EPR spectra observed at three particular orientations of the crystal in the magnetic field, the full orientation pattern of the -tensor was simulated. This simulation is consistent with the presence of 12 magnetically inequivalent clusters per unit cell with the axis of the PSI trimers oriented at (23°, 72°, 77°) to the unit cell axes.
Abstract | Full Text | PDF (409 kb) - A Kinetic Assessment of the Sequence of Electron Transfer fr...A Kinetic Assessment of the Sequence of Electron Transfer from FX to FA and Further to FB in Photosystem I: The Value of the Equilibrium Constant between FX and FA
Biophysical Journal, Volume 78, Issue 1, 1 January 2000, Pages 363-372
Vladimir P. Shinkarev, Ilya R. Vassiliev and John H. Golbeck
AbstractThe x-ray structure analysis of photosystem I (PS I) crystals at 4-Å resolution (Schubert et al., 1997, 272:741–769) has revealed the distances between the three iron-sulfur clusters, labeled F, F, and F, which function on the acceptor side of PS I. There is a general consensus concerning the assignment of the F cluster, which is bound to the PsaA and PsaB polypeptides that constitute the PS I core heterodimer. However, the correspondence between the acceptors labeled F and F on the electron density map and the F and F clusters defined by electron paramagnetic resonance (EPR) spectroscopy remains controversial. Two recent studies (Diaz-Quintana et al., 1998, 37:3429–3439; Vassiliev et al., 1998, 74:2029–2035) provided evidence that F is the cluster proximal to F, and F is the cluster that donates electrons to ferredoxin. In this work, we provide a kinetic argument to support this assignment by estimating the rates of electron transfer between the iron-sulfur clusters F, F, and F. The experimentally determined kinetics of P700 dark relaxation in PS I complexes (both F and F are present), HgCl-treated PS I complexes (devoid of F), and P700-F cores (devoid of both F and F) from sp. PCC 6301 are compared with the expected dependencies on the rate of electron transfer, based on the x-ray distances between the cofactors. The analysis, which takes into consideration the asymmetrical position of iron-sulfur clusters F and F relative to F, supports the F → F → F → Fd sequence of electron transfer on the acceptor side of PS I. Based on this sequence of electron transfer and on the observed kinetics of P700 reduction and F oxidation, we estimate the equilibrium constant of electron transfer between F and F at room temperature to be ∼47. The value of this equilibrium constant is discussed in the context of the midpoint potentials of F and F, as determined by low-temperature EPR spectroscopy.
Abstract | Full Text | PDF (208 kb) - Modeling of the P700 Charge Recombination Kinetics with Phyl...Modeling of the P700 Charge Recombination Kinetics with Phylloquinone and Plastoquinone-9 in the A1 Site of Photosystem I
Biophysical Journal, Volume 83, Issue 6, 1 December 2002, Pages 2885-2897
Vladimir P. Shinkarev, Boris Zybailov, Ilya R. Vassiliev and John H. Golbeck
AbstractLight activation of photosystem I (PS I) induces electron transfer from the excited primary electron donor P700 (a special pair of chlorophyll /′ molecules) to three iron–sulfur clusters, F, F, and F via acceptors A (a monomeric chlorophyll ) and A (phylloquinone). PS I complexes isolated from and mutants contain plastoquinone-9 rather than phylloquinone in the A site and show altered rates of forward electron transfer from A to [F/F] and altered rates of back electron transfer from [F/F] to P700 (Semenov, A. Y., et al., . . . 275:23429–23438, 2000). To identify the modified electron transfer steps, we studied the kinetics of flash-induced P700 reduction in PS I that contains either an intact set or a subset of iron–sulfur clusters F, F, and F and with the A binding site occupied by phylloquinone or plastoquinone-9. A modeling of the forward and backward electron transfer kinetics in P700–F/F complexes, P700–F cores, and P700–A cores shows that the replacement of phylloquinone by plastoquinone-9 induces a decrease in the free energy gap between A and F/F from ∼−205mV in wild-type PS I to ∼−70mV in PS I. The +135mV increase in the midpoint potential of A explains the acceleration in the rate of P700 dark reduction in PS I, and the resulting uphill electron transfer from A to F in PS I explains the absence of a contribution from F to the reduction of P700. This fully quantitative description of PS I relates electron transfer rates, equilibrium constants, and redox potentials, and can be used to predict changes in these parameters upon substitution of electron transfer cofactors.
Abstract | Full Text | PDF (857 kb) - …more
Copyright © 1998 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 74, Issue 6, 3173-3181, 1 June 1998
doi:10.1016/S0006-3495(98)78023-3
Secondary Pair Charge Recombination in Photosystem I under Strongly Reducing Conditions: Temperature Dependence and Suggested Mechanism
Marc Polm1 and Klaus Brettel
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Address reprint requests to Klaus Brettel, SBE, Bât. 532, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France. Tel.: +33-169089869; Fax: +33-169088717.1 Marc Polm's present address is Laboratory of Molecular Physics, Department of Biomolecular Sciences, Dreijenlaan 3, 6703 HA, Wageningen, The Netherlands.
Abstract
Photoinduced electron transfer in photosystem I (PS I) proceeds from the excited primary electron donor P700 (a chlorophyll a dimer) via the primary acceptor A0 (chlorophyll a) and the secondary acceptor A1 (phylloquinone) to three [4Fe-4S] clusters, FX, FA, and FB. Prereduction of the iron-sulfur clusters blocks electron transfer beyond A1. It has been shown previously that, under such conditions, the secondary pair P700+A1− decays by charge recombination with t1/2 ≈ 250ns at room temperature, forming the P700 triplet state (3P700) with a yield exceeding 85%. This reaction is unusual, as the secondary pair in other photosynthetic reaction centers recombines much slower and forms directly the singlet ground state rather than the triplet state of the primary donor. Here we studied the temperature dependence of secondary pair recombination in PS I from the cyanobacterium Synechococcus sp. PCC6803, which had been illuminated in the presence of dithionite at pH 10 to reduce all three iron-sulfur clusters. The reaction P700+A1− → 3P700 was monitored by flash absorption spectroscopy. With decreasing temperature, the recombination slowed down and the yield of 3P700 decreased. In the range between 303K and 240K, the recombination rates could be described by the Arrhenius law with an activation energy of ∼170 meV. Below 240K, the temperature dependence became much weaker, and recombination to the singlet ground state became the dominating process. To explain the fast activated recombination to the P700 triplet state, we suggest a mechanism involving efficient singlet to triplet spin evolution in the secondary pair, thermally activated repopulation of the more closely spaced primary pair P700+A0− in a triplet spin configuration, and subsequent fast recombination (intrinsic rate on the order of 109 s−1) forming 3P700.
