Free energy change accompanying electron transfer from P870 to quinone in Rhodopseudomonas sphaeroides chromatophores

Light-induced quinone reduction in photosystem II

The photosystem II core complex is the water:plastoquinone oxidoreductase of oxygenic photosynthesis situated in the thylakoid membrane of cyanobacteria, algae and plants. It catalyzes the light-induced transfer of electrons from water to plastoquinone accompanied by the net transport of protons from the cytoplasm (stroma) to the lumen, the production of molecular oxygen and the release of plastoquinol into the membrane phase. In this review, we outline our present knowledge about the "acceptor side" of the photosystem II core complex covering the reaction center with focus on the primary (Q(A)) and secondary (Q(B)) quinones situated around the non-heme iron with bound (bi)carbonate and a comparison with the reaction center of purple bacteria. Related topics addressed are quinone diffusion channels for plastoquinone/plastoquinol exchange, the newly discovered third quinone Q(C), the relevance of lipids, the interactions of quinones with the still enigmatic cytochrome b559 and the role of Q(A) in photoinhibition and photoprotection mechanisms. This article is part of a Special Issue entitled: Photosystem II.

Energetics of Proton Transfer Pathways in Reaction Centers from Rhodobacter sphaeroides

Electron transfer between the primary and secondary quinones (Q(A), Q(B)) in the bacterial photosynthetic reaction center (bRC) is coupled with proton uptake at Q(B). The protons are conducted from the cytoplasmic side, probably with the participation of two water channels. Mutations of titratable residues like Asp-L213 to Asn (inhibited mutant) or the double mutant Glu-L212 to Ala/Asp-L213 to Ala inhibit these electron transfer-coupled proton uptake events. The inhibition of the proton transfer (PT) process in the single mutant can be restored by a second mutation of Arg-M233 to Cys or Arg-H177 to His (revertant mutant). These revertant mutants shed light on the location of the main proton transfer pathway of wild type bRC. In contrast to the wild type and inhibited mutant bRC, the revertant mutant bRC showed notable proton uptake at Glu-H173 upon formation of the Q(B)- state. In all of these mutants, the pK(a) of Asp-M17 decreased by 1.4-2.4 units with respect to the wild type bRC, whereas a significant pK(a) upshift of up to 5.8 units was observed at Glu-H122, Asp-H170, Glu-H173, and Glu-H230 in the revertant mutants. These residues belonging to the main PT pathway are arranged along water channel P1 localized mainly in subunit H. bRC possesses subunit H, which has no counterpart in photosystem II. Thus, bRC may possess alternative PT pathways involving water channels in subunit H, which becomes active in case the main PT pathway is blocked.

Redox potential of quinones in both electron transfer branches of photosystem I

The redox potentials of the two electron transfer (ET) active quinones in the central part of photosystem I (PSI) were determined by evaluating the electrostatic energies from the solution of the Poisson-Boltzmann equation based on the crystal structure. The calculated redox potentials are -531 mV for A1A and -686 mV for A1B. From these results we conclude the following. (i) Both branches are active with a much faster ET in the B-branch than in the A-branch. (ii) The measured lifetime of 200-290 ns of reduced quinones agrees with the estimate for the A-branch and corroborates with an uphill ET from this quinone to the iron-sulfur cluster as observed in recent kinetic measurements. (iii) The electron paramagnetic resonance spectroscopic data refer to the A-branch quinone where the corresponding ET is uphill in energy. The negative redox potential of A1 in PSI is primarily because of the influence from the negatively charged FX, in contrast to the positive shift on the quinone redox potential in bacterial reaction center and PSII that is attributed to the positively charged non-heme iron atom. The conserved residue Asp-B575 changes its protonation state after quinone reduction. The difference of 155 mV in the quinone redox potentials of the two branches were attributed to the conformation of the backbone with a large contribution from Ser-A692 and Ser-B672 and to the side chain of Asp-B575, whose protonation state couples differently with the formation of the quinone radicals.

Radical-pair energetics and decay mechanisms in reaction centers containing anthraquinones, naphthoquinones or benzoquinones in place of ubiquinone

In reaction centers from Rhodobacter sphaeroides (formerly called Rhodopseudomonas sphaeroides), light causes an electron-transfer reaction that forms the radical pair state (P+I-, or PF) from the initial excited singlet state (P) of a bacteriochlorophyll dimer (P). Subsequent electron transfer to a quinone (Q) produces the state P+Q-. Back electron transfer can regenerate P from P+Q-, giving rise to 'delayed' fluorescence that decays with approximately the same lifetime as P+Q-. The free-energy difference between P+Q- and P can be determined from the initial amplitude of the delayed fluorescence. In the present work, we extracted the native quinone (ubiquinone) from Rps. sphaeroides reaction centers, and replaced it by various anthraquinones, naphthoquinones, and benzoquinones. We found a rough correlation between the halfwave reduction potential (E1/2) of the quinone used for reconstitution (as measured polarographically in dimethylformamide) and the apparent free energy of the state P+Q- relatively to P. As the E1/2 of the quinone becomes more negative, the standard free-energy gap between P+Q- and P decreases. However, the correlation is quantitatively weak. Apparently, the effective midpoint potentials (Em) of the quinones in situ depend subtly on interactions with the protein environment in the reaction center. Using the value of the Em for ubiquinone determined in native reaction centers as a reference, and the standard free energies determined for P+Q- in reaction centers reconstituted with other quinones, the effective Em values of 12 different quinones in situ are estimated. In native reaction centers, or in reaction centers reconstituted with quinones that give a standard free-energy gap of more than about 0.8 eV between P+Q- and P*, charge recombination from P+Q- to the ground state (PQ) occurs almost exclusively by a temperature-insensitive mechanism, presumably electron tunneling. When reaction centers are reconstituted with quinones that give a free-energy gap between P+Q- and P* of less than 0.8 with quinones that give a free-energy gap between P+Q- and P* of less than 0.8 eV, part or all of the decay proceeds through a thermally accessible intermediate. There is a linear relationship between the log of the rate constant for the decay of P+Q- via the intermediate state and the standard free energy of P+Q-. The higher the free energy, the faster the decay. The kinetic and thermodynamic properties of the intermediate appear not to depend strongly on the quinone used for reconstitution, indicating that the intermediate is probably not simply an activated form of P+Q-.(ABSTRACT TRUNCATED AT 400 WORDS)

Nanosecond fluorescence from chromatophores of Rhodopseudomonas sphaeroides and Rhodospirillum rubrum

Single-photon counting techniques were used to measure the fluorescence decay from Rhodopseudomonas sphaeroides and Rhodospirillum rubrum chromatophores after excitation with a 25-ps, 600-nm laser pulse. Electron transfer was blocked beyond the initial radical-pair state (PF) by chemical reduction of the quinone that serves as the next electron acceptor. Under these conditions, the fluorescence decays with multiphasic kinetics and at least three exponential decay components are required to describe the delayed fluorescence. Weak magnetic fields cause a small increase in the decay time of the longest component. The components of the delayed fluorescence are similar to those found previously with isolated reaction centers. We interpret the multi-exponential decay in terms of two small (0.01-0.02 eV) relaxations in the free energy of PF, as suggested previously for reaction centers. From the initial amplitudes of the delayed fluorescence, it is possible to calculate the standard free-energy difference between the earliest resolved form of PF and the excited singlet state of the antenna complexes in R. rubrum strains S1 and G9. The free-energy gap is found to be about 0.10 eV. It also is possible to calculate the standard free-energy difference between PF and the excited singlet state of the reaction center bacteriochlorophyll dimer (P). Values of 0.17 to 0.19 eV were found in both R. rubrum strains and also in Rps. sphaeroides strain 2.4.1. This free-energy gap agrees well with the standard free-energy difference between PF and P determined previously for reaction centers isolated from Rps. sphaeroides strain R26. The temperature dependence of the delayed fluorescence amplitudes between 180 K and 295 K is qualitatively different in isolated reaction centers and chromatophores. However, the temperature dependence of the calculated standard free-energy difference between P* and PF is similar in reaction centers and chromatophores of Rps. sphaeroides. The different temperature dependence of the fluorescence amplitudes in reaction centers and chromatophores arises because the free-energy difference between P* and the excited antenna is dominated by the entropy change associated with delocalization of the excitation in the antenna. We conclude that the state PF is similar in isolated reaction centers and in the intact photosynthetic membrane. Chromatophores from Rps. sphaeroides strain R-26 exhibit an anomalous fluorescence component that could reflect heterogeneity in their antenna.

Nanosecond fluorescence from isolated photosynthetic reaction centers of Rhodopseudomonas sphaeroides

The time-course of fluorescence from reaction centers isolated from Rhodopseudomonas sphaeroides was measured using single-photon counting techniques. When electron transfer is blocked by the reduction of the electron-accepting quinones, reaction centers exhibit a relatively long-lived (delayed) fluorescence due to back reactions that regenerate the excited state (P*) from the transient radical-pair state, PF. The delayed fluorescence can be resolved into three components, with lifetimes of 0.7, 3.2 and 11 ns at 295 K. The slowest component decays with the same time-constant as the absorbance changes due to PF, and it depends on both temperature and magnetic fields in the same way that the absorbance changes do. The time-constants for the two faster components of delayed fluorescence are essentially independent of temperature and magnetic fields. The fluorescence also includes a very fast (prompt) component that is similar in amplitude to that obtained from unreduced reaction centers. The prompt fluorescence presumably is emitted mainly during the period before the initial charge-transfer reaction creates PF from P*. From the amplitudes of the prompt and delayed fluorescence, we calculate an initial standard free-energy difference between P* and PF of about 0.16 eV at 295 K, and 0.05 eV at 80 K, depending somewhat on the properties of the solvent. The multiphasic decay of the delayed fluorescence is interpreted in terms of relaxations in the free energy of PF with time, totalling about 0.05 eV at 295 K, possibly resulting from nuclear movements in the electron-carriers or the protein.