| 1. |
- Keable, Stephen M., et al.
(författare)
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Room temperature XFEL crystallography reveals asymmetry in the vicinity of the two phylloquinones in photosystem I
- 2021
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.
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| 2. |
- Bag, Pushan, 1993-, et al.
(författare)
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Flavodiiron-mediated O2 photoreduction at photosystem I acceptor-side provides photoprotection to conifer thylakoids in early spring
- 2023
-
Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 14:1
-
Tidskriftsartikel (refereegranskat)abstract
- Green organisms evolve oxygen (O2) via photosynthesis and consume it by respiration. Generally, net O2 consumption only becomes dominant when photosynthesis is suppressed at night. Here, we show that green thylakoid membranes of Scots pine (Pinus sylvestris L) and Norway spruce (Picea abies) needles display strong O2 consumption even in the presence of light when extremely low temperatures coincide with high solar irradiation during early spring (ES). By employing different electron transport chain inhibitors, we show that this unusual light-induced O2 consumption occurs around photosystem (PS) I and correlates with higher abundance of flavodiiron (Flv) A protein in ES thylakoids. With P700 absorption changes, we demonstrate that electron scavenging from the acceptor-side of PSI via O2 photoreduction is a major alternative pathway in ES. This photoprotection mechanism in vascular plants indicates that conifers have developed an adaptative evolution trajectory for growing in harsh environments.
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| 3. |
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| 4. |
- Bergmann, U, et al.
(författare)
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High-resolution X-ray spectroscopy of rare events : a different look at local structure and chemistry
- 2001
-
Ingår i: Journal of Synchrotron Radiation. - Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley, CA 94720 USA. Univ Calif Davis, Dept Appl Sci, Davis, CA 95616 USA. CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA. : MUNKSGAARD INT PUBL LTD. - 0909-0495 .- 1600-5775. ; 8, s. 199-203
-
Tidskriftsartikel (refereegranskat)abstract
- The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal K beta fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the K beta spectrum resulting from valence-level and 'interatomic' ligand to metal transitions contain detailed information on the ligand-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorption fine structure (XAFS) spectra using hard ( 10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.
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| 5. |
- Bhowmick, Asmit, et al.
(författare)
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Going around the Kok cycle of the water oxidation reaction with femtosecond X-ray crystallography
- 2023
-
Ingår i: IUCrJ. - : International Union Of Crystallography. - 2052-2525. ; 10:6, s. 642-655
-
Forskningsöversikt (refereegranskat)abstract
- The water oxidation reaction in photosystem II (PS II) produces most of the molecular oxygen in the atmosphere, which sustains life on Earth, and in this process releases four electrons and four protons that drive the downstream process of CO2 fixation in the photosynthetic apparatus. The catalytic center of PS II is an oxygen-bridged Mn4Ca complex (Mn4CaO5) which is progressively oxidized upon the absorption of light by the chlorophyll of the PS II reaction center, and the accumulation of four oxidative equivalents in the catalytic center results in the oxidation of two waters to dioxygen in the last step. The recent emergence of X-ray free-electron lasers (XFELs) with intense femtosecond X-ray pulses has opened up opportunities to visualize this reaction in PS II as it proceeds through the catalytic cycle. In this review, we summarize our recent studies of the catalytic reaction in PS II by following the structural changes along the reaction pathway via room-temperature X-ray crystallography using XFELs. The evolution of the electron density changes at the Mn complex reveals notable structural changes, including the insertion of OX from a new water molecule, which disappears on completion of the reaction, implicating it in the O-O bond formation reaction. We were also able to follow the structural dynamics of the protein coordinating with the catalytic complex and of channels within the protein that are important for substrate and product transport, revealing well orchestrated conformational changes in response to the electronic changes at the Mn4Ca cluster.
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| 6. |
- Bhowmick, Asmit, et al.
(författare)
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Structural evidence for intermediates during O2 formation in photosystem II
- 2023
-
Ingår i: Nature. - : Springer Nature. - 0028-0836 .- 1476-4687. ; 617:7961, s. 629-636
-
Tidskriftsartikel (refereegranskat)abstract
- In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.
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| 7. |
- Boniolo, Manuel, et al.
(författare)
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Water Oxidation by Pentapyridyl Base Metal Complexes? : A Case Study
- 2022
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:24, s. 9104-9118
-
Tidskriftsartikel (refereegranskat)abstract
- The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.
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| 8. |
- Britt, R D, et al.
(författare)
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Recent pulsed EPR studies of the Photosystem II oxygen-evolving complex : implications as to water oxidation mechanisms
- 2004
-
Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - Univ Calif Davis, Dept Chem, Davis, CA 95616 USA. Univ Calif Berkeley, Lawrence Berkeley Lab, Melvin Calvin Lab, Phys Biosci Div, Berkeley, CA 94720 USA. Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA. : ELSEVIER. - 0005-2728 .- 1879-2650. ; 1655:1-3, s. 158-171
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Tidskriftsartikel (refereegranskat)abstract
- The pulsed electron paramagnetic resonance (EPR) methods of electron spin echo envelope modulation (ESEEM) and electron spill echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the structure of the Photosystem II oxygen-evolving complex (OEC), including the paramagnetic manganese cluster and its immediate surroundings. Recent unpublished results from the pulsed EPR laboratory at UC-Davis are discussed, along with aspects of recent publications, with a focus on substrate and cofactor interactions. New data on the proximity of exchangeable deuterons around the Mn cluster poised in the So-state are presented and interpreted. These pulsed EPR results are used in an evaluation of several recently proposed mechanisms for PSII water oxidation. We strongly favor mechanistic models where the substrate waters bind within the OEC early in the S-state cycle. Models in which the O-O bond is formed by a nucleophilic attack by a Ca2+-bound water on a strong S-4-state electrophile provide a good match to the pulsed EPR data. (C) 2004 Elsevier B.V. All rights reserved.
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| 9. |
- Chatterjee, Ruchira, et al.
(författare)
-
XANES and EXAFS of dilute solutions of transition metals at XFELs
- 2019
-
Ingår i: Journal of Synchrotron Radiation. - : INT UNION CRYSTALLOGRAPHY. - 0909-0495 .- 1600-5775. ; 26, s. 1716-1724
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Tidskriftsartikel (refereegranskat)abstract
- This work has demonstrated that X-ray absorption spectroscopy (XAS), both Mn XANES and EXAFS, of solutions with millimolar concentrations of metal is possible using the femtosecond X-ray pulses from XFELs. Mn XAS data were collected using two different sample delivery methods, a Rayleigh jet and a drop-on-demand setup, with varying concentrations of Mn. Here, a new method for normalization of XAS spectra based on solvent scattering that is compatible with data collection from a highly variable pulsed source is described. The measured XANES and EXAFS spectra of such dilute solution samples are in good agreement with data collected at synchrotron sources using traditional scanning protocols. The procedures described here will enable XFEL-based XAS on dilute biological samples, especially metalloproteins, with low sample consumption. Details of the experimental setup and data analysis methods used in this XANES and EXAFS study are presented. This method will also benefit XAS performed at high-repetition-rate XFELs such as the European XFEL, LCLS-II and LCLS-II-HE.
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| 10. |
- Cinco, R M, et al.
(författare)
-
Orientation of calcium in the Mn4Ca cluster of the oxygen-evolving complex determined using polarized strontium EXAFS of photosystem II membranes
- 2004
-
Ingår i: Biochemistry. - Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Melvin Calvin Lab, Berkeley, CA 94720 USA. Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA. : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 43:42, s. 13271-13282
-
Tidskriftsartikel (refereegranskat)abstract
- The oxygen-evolving complex of photosystem II (PS II) in green plants and algae contains a cluster of four Mn atoms in the active site, which catalyzes the photoinduced oxidation of water to dioxygen. Along with Mn, calcium and chloride ions are necessary cofactors for proper functioning of the complex. The current Study using polarized Sr EXAFS on oriented Sr-reactivated samples shows that Fourier peak II, which fits best to Mn at 3.5 Angstrom rather than lighter atoms (C, N, O, or Cl), is dichroic, with a larger magnitude at 10degrees (angle between the PS II membrane normal and the X-ray electric field vector) and a smaller magnitude at 80degrees. Analysis of the dichroism of the Sr EXAFS yields a lower and upper limit of 0degrees and 23degrees for the average angle between the Sr-Mn vectors and the membrane normal and an isotropic coordination number (number of Mn neighbors to Sr) of 1 or 2 for these layered PS II samples. The results confirm the contention that Ca (Sr) is proximal to the Mn cluster and lead to refined working models of the heteronuclear Mn4Ca cluster of the oxygen-evolving complex in PS II.
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| 11. |
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| 12. |
- Clausen, J, et al.
(författare)
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Evidence that bicarbonate is not the substrate in photosynthetic oxygen evolution
- 2005
-
Ingår i: Plant Physiology. - : AMER SOC PLANT BIOLOGISTS. - 0032-0889 .- 1532-2548. ; 139:3, s. 1444-1450
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Tidskriftsartikel (refereegranskat)abstract
- It is widely accepted that the oxygen produced by photosystem II of cyanobacteria, algae, and plants is derived from water. Earlier proposals that bicarbonate may serve as substrate or catalytic intermediate are almost forgotten, though not rigorously disproved. These latter proposals imply that CO2 is an intermediate product of oxygen production in addition to O-2. In this work, we investigated this possible role of exchangeable HCO3- in oxygen evolution in two independent ways. (1) We studied a possible product inhibition of the electron transfer into the catalytic Mn4Ca complex during the oxygen-evolving reaction by greatly increasing the pressure of CO2. This was monitored by absorption transients in the near UV. We found that a 3,000-fold increase of the CO2 pressure over ambient conditions did not affect the UV transient, whereas the S-3 -> S-4 -> S-0 transition was half-inhibited by raising the O-2 pressure only 10-fold over ambient, as previously established. (2) The flash-induced O-2 and CO2 production by photosystem II was followed simultaneously with membrane inlet mass spectrometry under approximately 15% H-2 O-18 enrichment. Light flashes that revealed the known oscillatory O-2 release failed to produce any oscillatory CO2 signal. Both types of results exclude that exchangeable bicarbonate is the substrate for (and CO2 an intermediate product of) oxygen evolution by photosynthesis. The possibility that a tightly bound carbonate or bicarbonate is a cofactor of photosynthetic water oxidation has remained.
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| 13. |
- Conlan, Brendon, et al.
(författare)
-
Thomas John Wydrzynski (8 July 1947–16 March 2018)
- 2019
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Ingår i: Photosynthesis Research. - : Springer. - 0166-8595 .- 1573-5079. ; 140:3, s. 253-261
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- With this Tribute, we remember and honor Thomas John (Tom) Wydrzynski. Tom was a highly innovative, independent and committed researcher, who had, early in his career, defined his life-long research goal. He was committed to understand how Photosystem II produces molecular oxygen from water, using the energy of sunlight, and to apply this knowledge towards making artificial systems. In this tribute, we summarize his research journey, which involved working on 'soft money' in several laboratories around the world for many years, as well as his research achievements. We also reflect upon his approach to life, science and student supervision, as we perceive it. Tom was not only a thoughtful scientist that inspired many to enter this field of research, but also a wonderful supervisor and friend, who is deeply missed (see footnote*).
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| 14. |
- Cox, Nicholas, et al.
(författare)
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Electronic Structure of a Weakly Antiferromagnetically Coupled MnIIMnIII Model Relevant to Manganese Proteins : A Combined EPR, 55Mn-ENDOR, and DFT Study
- 2011
-
Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 50:17, s. 8238-8251
-
Tidskriftsartikel (refereegranskat)abstract
- An analysis of the electronic structure of the [(MnMnIII)-Mn-II(mu-OH)-(mu-piv)(2)(Me(3)tacn)(2)] (ClO4)(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-mu-hydroxo bridge, bis-mu-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn-III. All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the Mn-55-ENDOR constrains the Mn-55 hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn-II and Mn-III ions. The large effective hyperfine tensor anisotropy of the Mn-II, a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn-III ion strongly perturbs the zero-field energy levels of the (MnMnIII)-Mn-II complex. An estimate of the fine structure parameter (d) for the Mn-III of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn-II ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn-III is consistent with the known coordination environment of the Mn-III ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.
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| 15. |
- D'Amario, Luca, et al.
(författare)
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Towards time resolved characterization of electrochemical reactions : electrochemically-induced Raman spectroscopy
- 2022
-
Ingår i: Chemical Science. - : RSC Publishing. - 2041-6520 .- 2041-6539. ; 13:36, s. 10734-10742
-
Tidskriftsartikel (refereegranskat)abstract
- Structural characterization of transient electrochemical species in the sub-millisecond time scale is the all-time wish of any electrochemist. Presently, common time resolution of structural spectro-electrochemical methods is about 0.1 seconds. Herein, a transient spectro-electrochemical Raman setup of easy implementation is described which allows sub-ms time resolution. The technique studies electrochemical processes by initiating the reaction with an electric potential (or current) pulse and analyses the product with a synchronized laser pulse of the modified Raman spectrometer. The approach was validated by studying a known redox driven isomerization of a Ru-based molecular switch grafted, as monolayer, on a SERS active Au microelectrode. Density-functional-theory calculations confirmed the spectral assignments to sub-ms transient species. This study paves the way to a new generation of time-resolved spectro-electrochemical techniques which will be of fundamental help in the development of next generation electrolizers, fuel cells and batteries.
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| 16. |
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| 17. |
- de Lichtenberg, Casper, et al.
(författare)
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Assignment of the slowly exchanging substrate water of nature's water-splitting cofactor
- 2024
-
Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences (PNAS). - 0027-8424 .- 1091-6490. ; 121:11
-
Tidskriftsartikel (refereegranskat)abstract
- Identifying the two substrate water sites of nature's water-splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O-O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time-Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time-Resolved 17O-Electron-electron Double resonance detected NMR (TR-17O-EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen-isotope label into the substrate sites after addition of H218O to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with H217O. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and WS are both enhanced by Ca2+→Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being WS. This strongly restricts options for the mechanism of water oxidation.
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| 18. |
- de Lichtenberg, Casper, et al.
(författare)
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Substrate water exchange in the S-2 state of photosystem II is dependent on the conformation of the Mn4Ca cluster
- 2020
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 22:23, s. 12894-12908
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Tidskriftsartikel (refereegranskat)abstract
- In photosynthesis, dioxygen formation from water is catalyzed by the oxygen evolving complex (OEC) in Photosystem II (PSII) that harbours the Mn4Ca cluster. During catalysis, the OEC cycles through five redox states, S-0 to S-4. In the S-2 state, the Mn4Ca cluster can exist in two conformations, which are signified by the low-spin (LS) g = 2 EPR multiline signal and the high-spin (HS) g = 4.1 EPR signal. Here, we employed time-resolved membrane inlet mass spectrometry to measure the kinetics of (H2O)-O-18/(H2O)-O-16 exchange between bulk water and the two substrate waters bound at the Mn4Ca cluster in the S-2(LS), S-2(HS), and the S-3 states in both Ca-PSII and Sr-PSII core complexes from T. elongatus. We found that the slowly exchanging substrate water exchanges 10 times faster in the S-2(HS) than in the S-2(LS) state, and that the S-2(LS) -> S-2(HS) conversion has at physiological temperature an activation barrier of 17 +/- 1 kcal mol(-1). Of the presently suggested S-2(HS) models, our findings are best in agreement with a water exchange pathway involving a S-2(HS) state that has an open cubane structure with a hydroxide bound between Ca and Mn1. We also show that water exchange in the S-3 state is governed by a different equilibrium than in S-2, and that the exchange of the fast substrate water in the S-2 state is unaffected by Ca/Sr substitution. These findings support that (i) O5 is the slowly exchanging substrate water, with W2 being the only other option, and (ii) either W2 or W3 is the fast exchanging substrate. The three remaining possibilities for O-O bond formation in PSII are discussed.
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| 19. |
- de Lichtenberg, Casper, et al.
(författare)
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The exchange of the fast substrate water in the S-2 state of photosystem II is limited by diffusion of bulk water through channels - implications for the water oxidation mechanism
- 2021
-
Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 12:38, s. 12763-12775
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Tidskriftsartikel (refereegranskat)abstract
- The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn4CaO5 cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the 'slow exchanging substrate water (W-s)', while, in the S-2 state, the terminal water ligands W2 and W3 are both discussed as the 'fast exchanging substrate water (W-f)'. A critical point for the assignment of W-f is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn4CaO5 cluster. In this study, we measured the rates of (H2O)-O-16/(H2O)-O-18 substrate water exchange in the S-2 and S-3 states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of W-f and W-s were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn4CaO5 cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that W-f exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S-2 state, namely the indifference of the rate of W-f exchange towards Ca/Sr substitution.
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| 20. |
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| 21. |
- Fransson, Thomas, et al.
(författare)
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Effects of x-ray free-electron laser pulse intensity on the Mn K beta(1,3) x-ray emission spectrum in photosystem II-A case study for metalloprotein crystals and solutions
- 2021
-
Ingår i: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 8:6
-
Tidskriftsartikel (refereegranskat)abstract
- In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and K beta x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the K beta XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn K beta(1,3) XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from & SIM;5 x 10(15) to 5 x 10(17) W/cm(2) at a pulse length of & SIM;35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.& nbsp;
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| 22. |
- Fransson, Thomas, et al.
(författare)
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Effects of x-ray free-electron laser pulse intensity on the Mn K β 1,3x-ray emission spectrum in photosystem II - A case study for metalloprotein crystals and solutions
- 2021
-
Ingår i: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 8:6
-
Tidskriftsartikel (refereegranskat)abstract
- In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and Kβ x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the Kβ XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn Kβ1,3 XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from ∼5 × 1015 to 5 × 1017 W/cm2 at a pulse length of ∼35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.
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| 23. |
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| 24. |
- Guo, Yu, et al.
(författare)
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Alternative Mechanism for O2 Formation in Natural Photosynthesis via Nucleophilic Oxo–Oxo Coupling
- 2023
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 145:7, s. 4129-4141
-
Tidskriftsartikel (refereegranskat)abstract
- O2 formation in photosystem II (PSII) is a vital event on Earth, but the exact mechanism remains unclear. The presently prevailing theoretical model is “radical coupling” (RC) involving a Mn(IV)-oxyl unit in an “open-cubane” Mn4CaO6 cluster, which is supported experimentally by the S3 state of cyanobacterial PSII featuring an additional Mn-bound oxygenic ligand. However, it was recently proposed that the major structural form of the S3 state of higher plants lacks this extra ligand, and that the resulting S4 state would feature instead a penta-coordinate dangler Mn(V)=oxo, covalently linked to a “closed-cubane” Mn3CaO4 cluster. For this proposal, we explore here a large number of possible pathways of O−O bond formation and demonstrate that the “nucleophilic oxo−oxo coupling” (NOOC) between Mn(V)=oxo and μ3-oxo is the only eligible mechanism in such a system. The reaction is facilitated by a specific conformation of the cluster and concomitant water binding, which is delayed compared to the RC mechanism. An energetically feasible process is described starting from the valid S4 state through the sequential formation of peroxide and superoxide, followed by O2 release and a second water insertion. The newly found mechanism is consistent with available experimental thermodynamic and kinetic data and thus a viable alternative pathway for O2 formation in natural photosynthesis, in particular for higher plants.
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| 25. |
- Guo, Y., et al.
(författare)
-
Reversible Structural Isomerization of Nature's Water Oxidation Catalyst Prior to O-O Bond Formation
- 2022
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:26, s. 11736-11747
-
Tidskriftsartikel (refereegranskat)abstract
- Photosynthetic water oxidation is catalyzed by a manganese-calcium oxide cluster, which experiences five "S-states" during a light-driven reaction cycle. The unique "distorted chair"-like geometry of the Mn4CaO5(6)cluster shows structural flexibility that has been frequently proposed to involve "open" and "closed"-cubane forms from the S1 to S3states. The isomers are interconvertible in the S1 and S2states, while in the S3state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S3+Yzstate to the S3nYz•→ S4+Yzstep, and report for the first time that the reversible isomerism, which is suppressed in the S3+Yzstate, is fully recovered in the ensuing S3nYz•state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese-ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O2formation, and exert a significant influence on the water oxidation mechanism in photosystem II.
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| 26. |
- Han, Guangye, et al.
(författare)
-
Hydration of the oxygen-evolving complex of photosystem II probed in the dark-stable S1 state using proton NMR dispersion profiles
- 2014
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; :16, s. 11924-11935
-
Tidskriftsartikel (refereegranskat)abstract
- The hydration of the oxygen-evolving complex (OEC) was characterized in the dark stable S1 state of photosystem II using water R1(ω) NMR dispersion (NMRD) profiles. The R1(ω) NMRD profiles were recorded over a frequency range from 0.01 MHz to 40 MHz for both intact and Mn-depleted photosystem II core complexes from Thermosynechococcus vulcanus (T. vulcanus). The intact-minus-(Mn)-depleted difference NMRD profiles show a characteristic dispersion from approximately 0.03 MHz to 1 MHz, which is interpreted on the basis of the Solomon-Bloembergen-Morgan (SBM) and the slow motion theories as being due to a paramagnetic enhanced relaxation (PRE) of water protons. Both theories are qualitatively consistent with the ST = 1, g = 4.9 paramagnetic state previously described for the S1 state of the OEC; however, an alternative explanation involving the loss of a separate class of long-lived internal waters due to the Mn-depletion procedure can presently not be ruled out. Using a point-dipole approximation the PRE-NMRD effect can be described as being caused by 1-2 water molecules that are located about 10 Å away from the spin center of the Mn4CaO5 cluster in the OEC. The application of the SBM theory to the dispersion observed for PSII in the S1 state is questionable, because the parameters extracted do not fulfil the presupposed perturbation criterion. In contrast, the slow motion theory gives a consistent picture indicating that the water molecules are in fast chemical exchange with the bulk (τw < 1 μs). The modulation of the zero-field splitting (ZFS) interaction suggests a (restricted) reorientation/structural equilibrium of the Mn4CaO5 cluster with a characteristic time constant of τZFS = 0.6-0.9 μs.
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| 27. |
- Han, Guangye, et al.
(författare)
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Molecular basis for turnover inefficiencies (misses) during water oxidation in photosystem II
- 2022
-
Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 13:29, s. 8667-8678
-
Tidskriftsartikel (refereegranskat)abstract
- Photosynthesis stores solar light as chemical energy and efficiency of this process isv highly important. The electrons required for CO2 reduction are extracted from water in a reaction driven by light-induced charge separations in the Photosystem II reaction center and catalyzed by the CaMn4O5-cluster. This cyclic process involves five redox intermediates known as the S-0-S-4 states. In this study, we quantify the flash-induced turnover efficiency of each S state by electron paramagnetic resonance spectroscopy. Measurements were performed in photosystem II membrane preparations from spinach in the presence of an exogenous electron acceptor at selected temperatures between -10 degrees C and +20 degrees C and at flash frequencies of 1.25, 5 and 10 Hz. The results show that at optimal conditions the turnover efficiencies are limited by reactions occurring in the water oxidizing complex, allowing the extraction of their S state dependence and correlating low efficiencies to structural changes and chemical events during the reaction cycle. At temperatures 10 degrees C and below, the highest efficiency (i.e. lowest miss parameter) was found for the S-1 -> S-2 transition, while the S-2 -> S-3 transition was least efficient (highest miss parameter) over the whole temperature range. These electron paramagnetic resonance results were confirmed by measurements of flash-induced oxygen release patterns in thylakoid membranes and are explained on the basis of S state dependent structural changes at the CaMn4O5-cluster that were determined recently by femtosecond X-ray crystallography. Thereby, possible "molecular errors" connected to the e(-) transfer, H+ transfer, H2O binding and O-2 release are identified.
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| 28. |
- Hillier, W, et al.
(författare)
-
Kinetic determination of the fast exchanging substrate water molecule in the S3 state of photosystem II
- 1998
-
Ingår i: Biochemistry. - Australian Natl Univ, Res Sch Biol Sci, Canberra, ACT 0200, Australia. Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley, CA 94720 USA. : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 37:48, s. 16908-16914
-
Tidskriftsartikel (refereegranskat)abstract
- In a previous communication we showed from rapid isotopic exchange measurements that the exchangeability of the substrate water at the water oxidation catalytic site in the S-3 State undergoes biphasic kinetics although the fast phase could not be fully resolved at that time Flessinger, J., Badger, M., and Wydrzynski, T. (1995) Proc. Natl. Acad. Sci, U.S.A. 92, 3209-3213]. We have since improved the time resolution for these measurements by a further factor of 3 and report here the first detailed kinetics for the fast phase of exchange. First-order exchange kinetics were determined from mass spectrometric measurements of photogenerated O-2 as a function of time after injection of (H2O)-O-18 into spinach thylakoid samples preset in the S-3 State at 10 degrees C. For measurements made at m/e = 34 (i.e., for the mixed labeled O-16,18(2) product), the two kinetic components are observed: a slow component with k(1) = 2.2 +/- 0.1 s(-1) (t(1/2) similar to 315 ms) and a fast component with k(2) = 38 +/- 4 s(-1) (t(1/2) similar to 18 ms). When the isotopic exchange is measured at m/e = 36 (i.e,, for the double labeled O-18,18(2) product), only the slow component (k(1)) is observed, clearly indicating that the substrate water undergoing slow isotopic exchange provides the rate-limiting step in the formation of the double labeled O-18,18(2) product. When the isotopic exchange is measured as a function of temperature, the two kinetic components reveal different temperature dependencies in which k(1) increases by a factor of 10 over the range 0-20 degrees C while k(2) increases by only a factor of 3. Assuming simple Arrhenius behavior, the activation energies are estimated to be 78 +/- 10 kJ mol(-1) for the slow component and 39 +/- 5 kJ mol(-1) for the fast component. The different kinetic components in the O-18 isotopic exchange provide firm evidence that the two substrate water molecules undergo separate exchange processes at two different chemical sites in the S-3 state, prior to the O-2 release step (t(1/2) similar to 1 ms at 20 degrees C). The results are discussed in terms of how the substrate water may be bound at two separate metal sites.
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| 29. |
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| 30. |
- Hussein, Rana, et al.
(författare)
-
Cryo-electron microscopy reveals hydrogen positions and water networks in photosystem II
- 2024
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 384:6702, s. 1349-1355
-
Tidskriftsartikel (refereegranskat)abstract
- Photosystem II starts the photosynthetic electron transport chain that converts solar energy into chemical energy and thus sustains life on Earth. It catalyzes two chemical reactions: water oxidation to molecular oxygen and plastoquinone reduction. Coupling of electron and proton transfer is crucial for efficiency; however, the molecular basis of these processes remains speculative owing to uncertain water binding sites and the lack of experimentally determined hydrogen positions. We thus collected high-resolution cryo-electron microscopy data of fully hydrated photosystem II from the thermophilic cyanobacterium Thermosynechococcus vestitus to a final resolution of 1.71 angstroms. The structure reveals several previously undetected partially occupied water binding sites and more than half of the hydrogen and proton positions. This clarifies the pathways of substrate water binding and plastoquinone B protonation.
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| 31. |
- Hussein, Rana, et al.
(författare)
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Evolutionary diversity of proton and water channels on the oxidizing side of photosystem II and their relevance to function
- 2023
-
Ingår i: Photosynthesis Research. - : Springer Nature. - 0166-8595 .- 1573-5079. ; 158:2, s. 91-107
-
Forskningsöversikt (refereegranskat)abstract
- One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the ‘water wheel’ to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proton release. The comparison does not eliminate the potential role of O4 channel as a water intake channel. However, the highly ordered hydrogen-bonded water wire connected to the Mn4CaO5 cluster via the O4 may strongly suggest that it functions in proton release, especially during the S0 → S1 transition (Saito et al., Nat Commun 6:8488, 2015; Kern et al., Nature 563:421–425, 2018; Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Sakashita et al., Phys Chem Chem Phys 22:15831–15841, 2020; Hussein et al., Nat Commun 12:6531, 2021).
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| 32. |
- Ibrahim, Mohamed, et al.
(författare)
-
Untangling the sequence of events during the S-2 -> S-3 transition in photosystem II and implications for the water oxidation mechanism
- 2020
-
Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : NATL ACAD SCIENCES. - 0027-8424 .- 1091-6490. ; 117:23, s. 12624-12635
-
Tidskriftsartikel (refereegranskat)abstract
- In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S-1, S-2, S-3, and S-0, showing that a water molecule is inserted during the S-2 -> S-3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O-2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S-2 -> S-3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, QA and QB, are observed. At the donor site, tyrosine YZ and His190 H-bonded to it move by 50 mu s after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of OX(H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a "water wheel"-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (t of similar to 350 mu s) during the S-2 -> S-3 transition mirrors the appearance of OX electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.
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| 33. |
- Isgandarova, S, et al.
(författare)
-
Functional differences of photosystem II from Synechococcus elongatus and spinach characterized by flash induced oxygen evolution patterns
- 2003
-
Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 42:30, s. 8929-8938
-
Tidskriftsartikel (refereegranskat)abstract
- Detailed comparative studies of flash induced oxygen evolution patterns in thylakoids from the thermophilic cyanobacterium Synechococcus elongatus (S. elongatus; also referred to as Thermosynechococcus elongatus) and from spinach led to the following results: (i) the miss parameter cc of S. elongatus thylakoids exhibits a pronounced temperature dependence with a minimum of 7% at 25 degreesC and values of 17 and 10% at 3 and 35 degreesC, respectively, while for spinach thylakoids alpha decreases continuously from 18% at 35 degreesC down to 8% at 3 degreesC; (ii) at all temperatures, the double hit probability exceeds in S. elongatus the corresponding values of spinach by an increment Deltabeta of about 3%; (iii) at 20 degreesC the slow relaxation of the oxidation states S-2 and S-3 is about 15 and 30 times, respectively, slower in S. elongatus than in spinach, while the reduction of these S states by tyrosine Y-D is 2-3 times faster; (iv) the reaction SOYDox --> S1YD is slower by a factor of 4 in S. elongatus as compared to spinach; and (v) the activation energies of S state dark relaxations in S. elongatus are all within a factor of 1.5 as compared to the previously reported values from spinach thylakoids [Vass, I., Deak, Z., and Hideg, E. (1990) Biochim. Biophys. Acta 1017, 63-69; Messinger, J., Schroder, W. P., and Renger, G. (1993) Biochemistry 32, 7658-7668], but the difference between the activation energies of the Slow S-2 and S-3 decays is significantly larger in S. elongatus than in spinach. These results are discussed in terms of differences between cyanobacteria and higher plants on the acceptor side of PSII and a shift of the redox potential of the couple Y-D/Y-D(ox). The obtained data are also suitable to address questions about effects of the redox state of Y-D on the miss probability and the possibility of an S state dependent miss parameter.
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| 34. |
- KEBEKUS, U, et al.
(författare)
-
STRUCTURAL-CHANGES IN THE WATER-OXIDIZING COMPLEX MONITORED VIA THE PH-DEPENDENCE OF THE REDUCTION RATE OF REDOX STATE S-1 BY HYDRAZINE AND HYDROXYLAMINE IN ISOLATED SPINACH THYLAKOIDS
- 1995
-
Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 34:18, s. 6175-6182
-
Tidskriftsartikel (refereegranskat)abstract
- A detailed kinetic analysis is presented for the pH dependence of the reduction of the water-oxidizing complex (WOC) in redox state S-1 by hydrophilic amines NH(2)R (R = NH2, OH) in suspensions of isolated thylakoids. Measurements of patterns of the oxygen yield induced by a train of single-turnover flashes and evaluation of the data within the framework of an extended Kok model [Messinger, J., Wacker, U., and Renger, G. (1991) Biochemistry 30, 7852-7862] led to the following results: (a) the rate constants k(s1)(NH(2)R) exhibit strikingly similar pH dependencies for NH2OH and NH2NH2 with ''titration waves'' at pH 5.3-5.6; 6.2-6.5, and above a critical pH value of about 7.4; (b) the differences in the reaction mechanism between NH2OH (1-electron reduction) and NH2NH2 (2-electron reduction) are almost pH-independent; (c) the ratio of the rate constants, k(s1)(NH2OH)/k(s1)(NH2NH2), decreases by a factor of about 9 within the range 5 < pH < 8.5. A detailed analysis reveals that these data cannot be consistently explained by the assumption that the unprotonated forms NH2OH and NH2NH2 are the active species while the protonated cations [NH3OH](+) and [N2H5](+) are nonreactive. A quantitative description is achieved by the additional postulate that pH-dependent structural changes take place in the WOC, thereby modulating the reactivity toward exogenous redox active amines of the type NH(2)R. On the basis of the results of this study and a recent report [Messinger, J., and Renger, G. (1994) Biochemistry 33, 10896-10905], it is inferred that the WOC undergoes three specific structural changes, with characteristic pH values of 5.3-5.5,6.2-6.5, and above 7.4.
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| 35. |
- Kubin, Markus, et al.
(författare)
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Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers
- 2017
-
Ingår i: Structural Dynamics. - : AMER INST PHYSICS. - 2329-7778. ; 4:5
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn similar to 6-15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions. (C) 2017 Author(s).
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| 36. |
- Kulik, L, et al.
(författare)
-
Pulse EPR, 55Mn-ENDOR and ELDOR-detected NMR of the S2-state of the oxygen evolving complex in Photosystem II
- 2005
-
Ingår i: Photosynthesis Research. - : SPRINGER. - 0166-8595 .- 1573-5079. ; 84:1-3, s. 347-353
-
Tidskriftsartikel (refereegranskat)abstract
- Pulse EPR, Mn-55-ENDOR and ELDOR-detected NMR experiments were performed on the S-2-state of the oxygen-evolving complex from spinach Photosystem II. The novel technique of random acquisition in ENDOR was used to suppress heating artefacts. Our data unambiguously shows that four Mn ions have significant hyperfine coupling constants. Numerical simulation of the Mn-55-ENDOR spectrum allowed the determination of the principal values of the hyperfine interaction tensors for all four Mn ions of the oxygen-evolving complex. The results of our Mn-55-ENDOR experiments are in good agreement with previously published data [Peloquin JM et al. (2000) J Am Chem Soc 122: 10926-10942]. For the first time ELDOR-detected NMR was applied to the S-2-state and revealed a broad peak that can be simulated numerically with the same parameters that were used for the simulation of the Mn-55-ENDOR spectrum. This provides strong independent support for the assigned hyperfine parameters.
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| 37. |
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| 38. |
- Kulik, L V, et al.
(författare)
-
Electron spin-lattice relaxation of the so state of the oxygen-evolving complex in photosystem II and of dinuclear manganese model complexes
- 2005
-
Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 44:26, s. 9368-9374
-
Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the electron spin-lattice relaxation time T, was measured for the So state of the oxygen-evolving complex (OEC) in photosystem II and for two dinuclear manganese model complexes by pulse EPR using the inversion-recovery method. For [Mn(III)Mn(IV)(mu-O)(2)bipy(4)]ClO4, the Raman relaxation process dominates at temperatures below 50 K. In contrast, Orbach type relaxation was found for [Mn(II)Mn(III)(mu-OH)(mu-piv)(2)(Me(3)tacn)(2)](ClO4)(2) between 4.3 and 9 K. For the latter complex, an energy separation of 24.7-28.0 cm(-1) between the ground and the first excited electronic state was determined. In the So state of photosystem II, the T-1 relaxation times were measured in the range of 4.3-6.5 K. A comparison with the relaxation data (rate and pre-exponential factor) of the two model complexes and of the S-2 state of photosystem II indicates that the Orbach relaxation process is dominant for the So state and that its first excited state lies 21.7 +/- 0.4 cm(-1) above its ground state. The results are discussed with respect to the structure of the OEC in photosystem II.
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| 39. |
- Kulik, Leonid V., et al.
(författare)
-
Electronic structure of the Mn4OxCa cluster in the S0 and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn-ENDOR and EPR Spectroscopy
- 2007
-
Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 129:44, s. 13421-13435
-
Tidskriftsartikel (refereegranskat)abstract
- The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S-0 to S-4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained Mn-55 hyperfine coupling constants of the S-0 and S-2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392-2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S-0 in methanol (3%) containing samples and thereby establishes that the oxidation states of the manganese ions in S-0 and S-2 are, at 4 K, Mn-4(III, III, III, IV) and Mn-4(III, IV, IV, IV), respectively. By applying a "structure filter" that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821-825] we (i) show that this new structural model is fully consistent with EPR and Mn-55-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 angstrom Mn-Mn distance in S-0 to 2.75 angstrom in S-1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459-7471] corresponds to a deprotonation of a mu-hydroxo bridge between Mn-A and Mn-B, i.e., between the outer Mn and its neighboring Mn of the mu(3)-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S-0 -> S-1 and S-1 -> S-2 transitions.
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| 40. |
- Kwong, Wai Ling, et al.
(författare)
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Electrochemical N2 reduction at ambient condition - Overcoming the selectivity issue via control of reactants' availabilities
- 2021
-
Ingår i: International journal of hydrogen energy. - : Elsevier. - 0360-3199 .- 1879-3487. ; 46:59, s. 30366-30372
-
Tidskriftsartikel (refereegranskat)abstract
- Ammonia production via the electrochemical N2 reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N2) on NRR using a FeOx-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton-and high N2 availabilities, which are achieved using 0.12 vol% water in LiClO4- ethyl acetate electrolyte and gaseous N2 supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 +/- 19 pmolNH3 s(-1) cm(-2) at a Faradaic efficiency of 70 +/- 24% at the applied potential of-0.1 V vs. NHE. Other conditions (high proton-, or low N2-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.
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| 41. |
- Lohmiller, Thomas, et al.
(författare)
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An EPR and ENDOR spectroscopic investigation of the Ca2+-depleted oxygen-evolving complex of photosystem II
- 2013
-
Ingår i: Photosynthesis research for food, fuel and future. - Berlin, Heidelberg : Springer Berlin/Heidelberg. - 9783642320330 - 9783642320347 ; , s. 239-243
-
Konferensbidrag (refereegranskat)abstract
- Multifrequency EPR and 55Mn ENDOR spectroscopy were used to characterize the S2′ state of the Ca2+- depleted oxygen-evolving complex. The 55Mn ENDOR spectrum of the S2′ state is broader than that of the native S2 state. Simulations of the data were performed using the spin Hamiltonian formalism. It was observed that the magnitudes of the four 55Mn hyperfine tensors (A1,iso≈ 300 MHz; A2, iso, A3, iso, A4, iso≈200MHz) are approximately the same as in the native S2 state. In addition, the geometries of the anisotropic hyperfine tensors are not changed. Thus, the same oxidation states are assigned to the Mn ions both in S2 and S2′ (MnA, MnB, MnC: IV, MnD: III). The isotropic hyperfine values of the individual Mn ions, especially A2, iso and A4 iso, do change upon Ca2+ depletion, indicating that, nonetheless, the electronic spin coupling scheme of the cluster is affected by Ca2+ removal.
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| 42. |
- Messinger, Johannes, 1963-, et al.
(författare)
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Absence of Mn-centered oxidation in the S2 → S3 Transition: : Implications for the mechanism of photosynthetic water oxidation
- 2001
-
Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 123:32, s. 7804-7820
-
Tidskriftsartikel (refereegranskat)abstract
- A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S-3 --> [S-4] --> So transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn K beta X-ray emission spectroscopy (K beta XES) to this problem for the first time. The K beta XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S-2 --> S3 transition, in contrast to the S-0 --> S-1 and S-1 --> S-2 transitions. does not involve a Mn-centered oxidation. On the basis of new structural data from the S-3-state, manganese mu -oxo bridge radical formation is proposed for the S-2 --> S-3 transition, and three possible mechanisms for the O-O bond formation are presented.
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| 43. |
- Messinger, Johannes, 1963-, et al.
(författare)
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ANALYSES OF PH-INDUCED MODIFICATIONS OF THE PERIOD-4 OSCILLATION OF FLASH-INDUCED OXYGEN EVOLUTION REVEAL DISTINCT STRUCTURAL-CHANGES OF THE PHOTOSYSTEM-II DONOR SIDE AT CHARACTERISTIC PH VALUES
- 1994
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Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 33:36, s. 10896-10905
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a thorough analysis of the reaction pattern of flash-induced oxygen evolution in spinach thylakoids as a function of pH (4.5 I pH I 9) and the redox state of tyrosine Y-D in polypeptide D2. Evaluation of the experimental data within the conventional Kok model [Kok, B., Forbush, B., and McGloin, M. (1970) Photochem. Photobiol. 11, 457-475] led to the following results: (1) the probability of the miss factor is strongly pH dependent (with a pronounced minimum near neutral pH) while the double hit factor is less affected; (2) a marked increase of the apparent S-0 population arises at alkaline pH in dark-adapted samples where most of the Y-D is reduced, but this effect is absent if the percentage of PS II containing the oxidized form Y-D(ox) is high; and (3) the lifetimes of S-2 and S-3 exhibit a characteristic pH dependence that is indicative of conformational changes of functional relevance within the water-oxidizing complex and its environment; (4) the kinetic interaction of redox states S-2 and S-3 With Y-D is characterized by a change of its behavior at a threshold pH of 6.5-7.0; and (5) at acidic pH values the extent of S-2 and S-3 reduction by Y-D decreases concomitant with the occurrence of a very fast decay kinetics. On the basis of a detailed discussion of these results and data from the literature, the water oxidase is inferred to undergo structural changes at pH values of 5-5.5 and 6.5-7.0. These transitions are almost independent of the redox state S-i and modify the reaction coordiates of the water oxidase toward endogenous reductants.
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| 46. |
- Messinger, Johannes, 1963-, et al.
(författare)
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Detection of an EPR multiline signal for the S-0 state in photosystem II
- 1997
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Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 36:37, s. 11055-11060
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Tidskriftsartikel (refereegranskat)abstract
- The S-0* state was generated by incubation of dark-adapted (S-1 state) photosystem II membranes either with the exogenous two electron reductant hydrazine and subsequent 273 K illumination in the presence of DCMU or by dark incubation with low amounts of the one electron reductant hydroxylamine. In agreement with earlier reports, the S-1 and S-1 states were found to be electron paramagnetic resonance (EPR) silent. However, in the presence of 0.5-1.5% methanol, a weak EPR multiline signal centered around g = 2.0 was observed at 7 K for the S-0* states generated by both procedures. This signal has a similar average line splitting to the well-characterized S-2 state multiline EPR signal, but can be clearly distinguished from that and other modified S-2 multiline signals by differences in line position and intensities. In addition, at 4 K it can be seen that the S-0* multiline has a greater spectral breadth than the S-2 multilines and is composed of up to 26 peaks. The S-0* signal is not seen in the absence of methanol and is not affected by 1 mM EDTA in the buffer medium. We assign the S-0* multiline signal to the manganese cluster of the oxygen evolving complex in a mixed valence state of the form (MnMnMnMnIII)-Mn-II-Mn-III-Mn-III, (MnMnMnMnIV)-Mn-II-Mn-III-Mn-IV, or (MnMnMnMnIV)-Mn-III-Mn-III-Mn-III. Addition of methanol may be helpful in future to find an EPR signal originating from the natural S-0 state.
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| 47. |
- Messinger, Johannes, 1963-, et al.
(författare)
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DETECTION OF ONE SLOWLY EXCHANGING SUBSTRATE WATER MOLECULE IN THE S-3 STATE OF PHOTOSYSTEM-II
- 1995
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : NATL ACAD SCIENCES. - 0027-8424 .- 1091-6490. ; 92:8, s. 3209-3213
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Tidskriftsartikel (refereegranskat)abstract
- The exchangeability of the substrate water molecules at the catalytic site of water oxidation in photosystem II has been probed by isotope-exchange measurements using mass spectrometric detection of flash-induced oxygen evolution. A stirred sample chamber was constructed to reduce the lag time between injection of (H2O)-O-18 and the detecting flash by a factor of more than 1000 compared to the original experiments by R. Radmer and O. Ollinger [(1986) FEBS Lett. 195, 285-289]. Our data show that there is a slow (t(1/2) approximate to 500 ms, 10 degrees C) and a fast (t(1/2) < 25 ms, 10 degrees C) exchanging substrate water molecule in the S-3 State of photosystem II. The slow exchange is coupled with an activation energy of about 75 kJ/mol and is discussed in terms of a terminal manganese oxo ligand, while the faster exchanging substrate molecule may represent a water molecule not directly bound to the manganese center.
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| 48. |
- Messinger, Johannes, 1963-
(författare)
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Evaluation of different mechanistic proposals for water oxidation in photosynthesis on the basis of Mn4OxCa structures for the catalytic site and spectroscopic data
- 2004
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - Max Planck Inst Bioinorgan Chem, D-45470 Mulheim, Germany. : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 6:20, s. 4764-4771
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Tidskriftsartikel (refereegranskat)abstract
- Recent progress in EPR and EXAFS spectroscopy, quantum mechanical calculations and X-ray crystallography led to a tremendous improvement in our picture of the catalytic site of water oxidation in photosystem II. It is now likely that the four Mn ions are grouped in a 3 + 1 fashion with three short Mn-Mn distances of about 2.7 Angstrom and one long Mn-Mn distance of 3.3 Angstrom. In addition, Ca has been firmly localized close to the Mn centers, with an average distance of 3.4 Angstrom and an average angle of the Mn-Ca vectors close to the membrane normal (less than or equal to23degrees). The recent crystal structure of Ferreira et al. (Science, 2004, 303, 1831-1838) suggests that the Mn-3 unit and Ca form a distorted cubane like structure. The fourth Mn is 'dangling' from this unit either via a mu(4)-oxo bridge or a mono mu-oxo bridge. However, the precise Mn-Mn distances and the bridging situation still need to be worked out. On this structural basis and the available spectroscopic data two possible mechanisms for photosynthetic water oxidation are discussed in order to highlight questions that still need to be solved for a full understanding of this fascinating reaction.
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| 49. |
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| 50. |
- Messinger, Johannes, 1963-, et al.
(författare)
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GENERATION, OXIDATION BY THE OXIDIZED FORM OF THE TYROSINE OF POLYPEPTIDE D2, AND POSSIBLE ELECTRONIC CONFIGURATION OF THE REDOX STATE-S(0), STATE-S-1, AND STATE-S-2 OF THE WATER OXIDASE IN ISOLATED SPINACH THYLAKOIDS
- 1993
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Ingår i: Biochemistry. - TECH UNIV BERLIN,MAX VOLMER INST BIOPHYS & PHYS CHEM,STR 17 JUNI 135,D-10623 BERLIN,GERMANY. : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 32:36, s. 9379-9386
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Tidskriftsartikel (refereegranskat)abstract
- Suitable treatment of thylakoids with hydrazine permits a high population of the redox states S0, S-1, and S-2 in the water oxidase. Experiments performed with dark-adapted samples enriched either in the oxidized or reduced form of the redox-active tyrosine, Y(D), of PolYpeptide D2 reveal that Y(D)ox is a unique endogenous oxidant within the PS II complex which causes a one-electron abstraction from the water oxidase in states S0, S-1, and S-2, respectively. A kinetic analysis of the period four oscillation of oxygen yield induced by a train of short flashes in dark-adapted samples permits the determination of the rate constants of electron abstraction from the reduced water oxidase by Y(D)ox. A value of 9 x 10(-4) s-1 was found for the oxidation of S0 and S-2, while S-1 becomes oxidized with a rate constant of 4 x 10(-4) s-1 at 20-degrees-C and pH 7.2. The redox state S0 generated either from S1 via the three-flash-induced oxidative pathway through S4 or from a one-flash oxidation of the S-1 state obtained by S1 reduction with NH2NH2 exhibits the same kinetics as S0 oxidation by Y(D)ox. On the basis of these findings and data taken from the literature, the electronic configuration of the Manganese atoms in the tetranuclear cluster is discussed. It is assumed that the dimer model of two binuclear manganese groups within the tetranuclear cluster comprises a functional heterogeneity: (i) one binuclear center, referred to as the catalytic group, is proposed to be involved in the oxidative pathway leading to the eventual oxidation of water to dioxygen, and (ii) the other binuclear center, symbolized as component C, is redox-inert during the oxidative pathway but can be reduced by exogenous (and endogenous?) components, thereby forming the states S-1 and S-2 Of the water oxidase. An asymmetric protein matrix around the tetranuclear manganese cluster is assumed to be responsible for the functional heterogeneity of the manganese centers.
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