The Quinone Iron Acceptor Complex (original) (raw)
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Biophysical Journal, 2009
The semiquinone-iron complex of photosystem II was studied using electron spin resonance (ESR) spectroscopy and density functional theory calculations. Two forms of the signal were investigated: 1), the native g~1.9 form; and 2), the g~1.84 form, which is well known in purple bacterial reaction centers and occurs in photosystem II when treated with formate. The g~1.9 form shows low-and high-field edges at g~3.5 and g < 0.8, respectively, and resembles the g~1.84 form in terms of shape and width. Both types of ESR signal were simulated using the theoretical approach used previously for the BRC complex, a spin Hamiltonian formalism in which the semiquinone radical magnetically interacts (J~1 cm À1 ) with the nearby high-spin Fe 2þ . The two forms of ESR signal differ mainly by an axis rotation of the exchange coupling tensor (J) relative to the zero-field tensor (D) and a small increase in the zero-field parameter D (~6 cm À1 ). Density functional theory calculations were conducted on model semiquinone-iron systems to identify the physical nature of these changes. The replacement of formate (or glutamate in the bacterial reaction centers) by bicarbonate did not result in changes in the coupling environment. However, when carbonate (CO 3 2À ) was used instead of bicarbonate, the exchange and zero-field tensors did show changes that matched those obtained from the spectral simulations. This indicates that 1), the doubly charged carbonate ion is responsible for the g~1.9 form of the semiquinone-iron signal; and 2), carbonate, rather than bicarbonate, is the ligand to the iron. SCHEME 1 Acceptor-side electron transfer pathways in PS II and BRC: the two-electron gate (2).
Biochemistry, 1995
Effects of photoinhibition on the iron-quinone electron acceptor complex of oxygen-evolving photosystem 11 have been studied using low-temperature EPR and Mossbauer spectroscopy. Photoinhibition of spinach photosystem I1 membrane particles at 4 "C decreases the EPR signal arising from the interaction of QA-with Fe2+ to 30% in 90 min under our conditions. The free radical EPR signal from QA-induced by cyanide treatment of the iron Biochemistry 33, 9922-99281 declines with the same kinetics as the QA-Fe2+ EPR signal. In contrast, Fe2+ is present in about 70% of the centers after 90 min of photoinhibition, as shown by its EPR-detected interaction with NO and by its Mossbauer absorption. Complete oxidation of this Fe2+ population to Fe3+ by ferricyanide is possible only in the presence of glycolate, which lowers the redox potential of the Fe3+/Fe2+ couple. In a fraction of PSII centers, which reach 30% after 90 min of photoinhibition, the iron cannot be detected. It is concluded that photoinhibition of oxygen-evolving photosystem I1 affects both QA and Fe2+. However, the photoinhibitory impairment of the QA redox functioning precedes the modification of the non-heme iron. cyt b-559, cytochrome b-559; EPR, electron paramagnetic resonance; PQ, plastoquinone; PS 11, photosystem 11; Pheo, pheophytin; QA and QB, the first and second quinone electron acceptors of PSII; P680, the reaction center chlorophyll of PSII; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis; Tyr-Z and Tyr-D, redox-active tyrosine residues of PSII.
Journal of Biological Chemistry, 2007
We report the characterization of the effects of the A249S mutation located within the binding pocket of the primary quinone electron acceptor, Q A , in the D2 subunit of photosystem II in Thermosynechococcus elongatus. This mutation shifts the redox potential of Q A by ϳ؊60 mV. This mutant provides an opportunity to test the hypothesis, proposed earlier from herbicide-induced redox effects, that photoinhibition (light-induced damage of the photosynthetic apparatus) is modulated by the potential of Q A . Thus the influence of the redox potential of Q A on photoinhibition was investigated in vivo and in vitro. Compared with the wild-type, the A249S mutant showed an accelerated photoinhibition and an increase in singlet oxygen production. Measurements of thermoluminescence and of the fluorescence yield decay kinetics indicated that the charge-separated state involving Q A was destabilized in the A249S mutant. These findings support the hypothesis that a decrease in the redox potential of Q A causes an increase in singlet oxygen-mediated photoinhibition by favoring the back-reaction route that involves formation of the reaction center chlorophyll triplet. The kinetics of charge recombination are interpreted in terms of a dynamic structural heterogeneity in photosystem II that results in high and low potential forms of Q A . The effect of the A249S mutation seems to reflect a shift in the structural equilibrium favoring the low potential form. . 2 The abbreviations used are: PSII, photosystem II; P, primary electron donor; 3 P, triplet state of the primary electron donor; Q A , primary quinone acceptor; Q B , secondary quinone acceptor; S 0 -4 , redox states of the charge accumulating part of the water-oxidizing enzyme; Pheo, pheophytin (primary electron acceptor in PSII); BPheo, bacterial pheophytin (primary electron acceptor in bacterial reaction centers); TEMP, 2,2,6,6-tetramethylpiperidine; TEMPO, 2,2,6,6-tetramethylpiperidine-1-oxyl; WTЈ, T. elongatus strain that lacks the second gene coding for the D2 protein (⌬psbD2 strain); OEC, oxygen-evolving complex; Chl, chlorophyll; MES, 4-morpholineethanesulfonic acid; DCMU, 13-(3,4-dichlorophenyl)-1,1-dimethylurea.
Recruitment of a Foreign Quinone into the A1 Site of Photosystem I
2000
Genes encoding enzymes of the biosynthetic pathway leading to phylloquinone, the secondary electron acceptor of photosystem (PS) I, were identified in Synechocystis sp. PCC 6803 by comparison with genes encoding enzymes of the menaquinone biosynthetic pathway in Escherichia coli. Targeted inactivation of the menA and menB genes, which code for phytyl transferase and 1,4-dihydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phylloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotrophically under low light conditions (20 microE m(-2) s(-1)), with doubling times twice that of the wild type, but they are unable to grow under high light conditions (120 microE m(-2) s(-1)). The menA and menB mutants grow photoheterotrophically on media supplemented with glucose under low light conditions, with doubling times similar to that of the wild type, but they are unable to grow under high light conditions unless atrazine is present to inhibit PS II activity. The level of active PS II per cell in the menA and menB mutant strains is identical to that of the wild type, but the level of active PS I is about 50-60% that of the wild type as assayed by low temperature fluorescence, P700 photoactivity, and electron transfer rates. PS I complexes isolated from the menA and menB mutant strains contain the full complement of polypeptides, show photoreduction of F(A) and F(B) at 15 K, and support 82-84% of the wild type rate of electron transfer from cytochrome c(6) to flavodoxin. HPLC analyses show high levels of plastoquinone-9 in PS I complexes from the menA and menB mutants but not from the wild type. We propose that in the absence of phylloquinone, PS I recruits plastoquinone-9 into the A(1) site, where it functions as an efficient cofactor in electron transfer from A(0) to the iron-sulfur clusters.
1D- and 2D-ESEEM Study of the Semiquinone Radical Q A - of Photosystem II
Journal of the American Chemical Society, 1999
The semiquinone radical Q Ahas been studied by Electron Spin-Echo Envelope Modulation (ESEEM) spectroscopy in Photosystem II membranes at various pH values. The observed nuclear modulations have been assigned by the use of two-dimensional Hyperfine Sublevel Correlation Spectroscopy (HYSCORE) and numerical simulations. Two protein 14 N nuclei (N I and N II ) were found to be magnetically coupled with the Q Aspin, and on the basis of 14 N-NQR and 14 N-ESEEM data from the literature, N I is assigned to an amide nitrogen from the protein backbone while N II is assigned to the amino nitrogen, Nδ, of an imidazole. A physical explanation for such couplings is suggested where the coupling occurs through H-bonds from the protein to the carbonyls of the semiquinone. In PSII membranes treated with CN -, only the N I coupling is present above pH 8.5 while both N I and N II couplings are present at lower pH values. In samples treated at high pH to remove the iron, both N I and N II couplings are present throughout the pH range studied but at pH <6 these couplings strengthen. These results are interpreted in terms of a model based on the structure of the bacterial reaction center and involving two determining factors. (1) The nonheme iron, when present, is liganded to the imidazole that H-bonds to one of the Q Acarbonyls. This physical attachment of the imidazole to the iron limits the strength of the H-bond to Q A -. (2) A pH-dependent group on the protein controls the strength of the H-bonds to Q A -. The pK a of this group is influenced by the biochemical treatment used to uncouple the iron, being around pH 7.5 in CN --treated PSII but around pH 6 in high pH-treated PSII. It is proposed that such a pH effect on the H-bond strength exists in untreated PSII and that earlier observations of pH-induced changes in the EPR signal from the semiquinone iron may reflect this change. (1) Klimov, V. V.; Dolan, E.; Shaw, E. R.; Ke, B.
Plant and Cell Physiology
Various benzo-and naphthoquinone derivatives were introduced into the purified photosystem II Dl-D2-cytochrome b 559 reaction center complex, which lacks the intrinsic plastoquinone electron acceptors. Effects of these quinones on the electron transfer reactions in nanoseconds to milliseconds time range were studied at room and cryogenic temperatures. 1) The addition of quinones to the purified photosystem II reaction center complex suppressed the nanosecond charge recombination between oxidized reaction center chlorophyll a (P680 +) and reduced pheophytin a (Ph~), and stabilized P680 + up to millisecond time range at 280 K and at 77 K. 2) In the reaction center complex supplemented with dibromothymoquinone (DBMIB), P68O was almost fully oxidized and cytochrome b 559 was partially reduced by flash excitation. A semiquinone-like signal with a peak around 320 nm was also induced but the shift of pheophytin absorption band (C55O) was not observed. 3) Halogenated quinones, especially DBMIB, were better electron acceptors than unsubstituted or methylated quinones. 4) The affinities of quinones to the reaction center complex were weakly dependent on their molecular structure.
Biochemistry, 2000
The electron transfer between the two quinones Q A and Q B in the bacterial photosynthetic reaction center (bRC) is coupled to a conformational rearrangement. Recently, the X-ray structures of the dark-adapted and light-exposed bRC from Rhodobacter sphaeroides were solved, and the conformational changes were characterized structurally. We computed the reaction free energy for the electron transfer from Q A •to Q B in the X-ray structures of the dark-adapted and light-exposed bRC from Rb. sphaeroides. The computation was done by applying an electrostatic model using the Poisson-Boltzmann equation and Monte Carlo sampling. We accounted for possible protonation changes of titratable groups upon electron transfer. According to our calculations, the reaction energy of the electron transfer from Q A •to Q B is +157 meV for the dark-adapted and-56 meV for the light-exposed X-ray structure; i.e., the electron transfer is energetically uphill for the dark-adapted structure and downhill for the light-exposed structure. A common interpretation of experimental results is that the electron transfer between Q A •and Q B is either gated or at least influenced by a conformational rearrangement: A conformation in which the electron transfer from Q A •to Q B is inactive, identified with the dark-adapted X-ray structure, changes into an electron-transfer active conformation, identified with the light-exposed X-ray structure. This interpretation agrees with our computational results if one assumes that the positive reaction energy for the dark-adapted X-ray structure effectively prevents the electron transfer. We found that the strongly coupled pair of titratable groups Glu-L212 and Asp-L213 binds about one proton in the dark-adapted X-ray structure, where the electron is mainly localized at Q A , and about two protons in the light-exposed structure, where the electron is mainly localized at Q B. This finding agrees with recent experimental and theoretical studies. We compare the present results for the bRC from Rb. sphaeroides to our recent studies on the bRC from Rhodopseudomonas Viridis. We discuss possible mechanisms for the gated electron transfer from Q A •to Q B and relate them to theoretical and experimental results.
Light-induced quinone reduction in photosystem II
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2012
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.
Photosynthesis Research, 1999
A site directed mutant of the Photosystem I reaction center of Chlamydomonas reinhardtii has been described previously. [Hallahan et al. (1995) Photosynth Res 46: 257-264]. The mutation, PsaA: D576L, changes the conserved aspartate residue adjacent to one of the cysteine ligands binding the Fe-S X center to PsaA. The mutation, which prevents photosynthetic growth, was observed to change the EPR spectrum of the Fe-S A/B centers bound to the PsaC subunit. We suggested that changes in binding of PsaC to the PsaA/PsaB reaction center prevented efficient electron transfer. Second site suppressors of the mutation have now been isolated which have recovered the ability to grow photosynthetically. DNA analysis of four suppressor strains showed the original D576L mutation is intact, and that no mutations are present elsewhere within the Fe-S x binding region of either PsaA or PsaB, nor within PsaC or PsaJ. Subsequent genetic analysis has indicated that the suppressor mutation(s) is nuclear encoded. The suppressors retain the altered binding of PsaC, indicating that this change is not the cause of failure to grow photosynthetically. Further analysis showed that the rate of electron transfer from the quinone electron carrier A 1 to Fe-S X is slowed in the mutant (by a factor of approximately two) and restored to wild type rates in the suppressors. ENDOR spectra of A 1 •− in wild-type and mutant preparations are identical, indicating that the electronic structure of the phyllosemiquinone is not changed. The results suggest that the quinone to Fe-S X center electron transfer is sensitive to the structure of the iron-sulfur center, and may be a critical step in the energy conversion process. They also indicate that the structure of the reaction center may be modified as a result of changes in proteins outside the core of the reaction center.