| 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
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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
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 14:1
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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. |
- 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
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Ingår i: IUCrJ. - : International Union Of Crystallography. - 2052-2525. ; 10:6, s. 642-655
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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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| 4. |
- Bhowmick, Asmit, et al.
(författare)
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Structural evidence for intermediates during O2 formation in photosystem II
- 2023
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Ingår i: Nature. - : Springer Nature. - 0028-0836 .- 1476-4687. ; 617:7961, s. 629-636
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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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| 5. |
- Boniolo, Manuel, et al.
(författare)
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Water Oxidation by Pentapyridyl Base Metal Complexes? : A Case Study
- 2022
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:24, s. 9104-9118
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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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| 6. |
- D'Amario, Luca, et al.
(författare)
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Towards time resolved characterization of electrochemical reactions : electrochemically-induced Raman spectroscopy
- 2022
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Ingår i: Chemical Science. - : RSC Publishing. - 2041-6520 .- 2041-6539. ; 13:36, s. 10734-10742
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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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| 7. |
- de Lichtenberg, Casper, et al.
(författare)
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Assignment of the slowly exchanging substrate water of nature's water-splitting cofactor
- 2024
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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
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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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| 8. |
- 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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| 9. |
- de Lichtenberg, Casper, et al.
(författare)
-
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
-
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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| 10. |
- 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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| 11. |
- 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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| 12. |
- 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
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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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| 13. |
- Guo, Yu, et al.
(författare)
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Closing Kok’s cycle of nature’s water oxidation catalysis
- 2024
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 15:1
-
Tidskriftsartikel (refereegranskat)abstract
- The Mn4CaO5(6) cluster in photosystem II catalyzes water splitting through the Si state cycle (i = 0–4). Molecular O2 is formed and the natural catalyst is reset during the final S3 → (S4) → S0 transition. Only recently experimental breakthroughs have emerged for this transition but without explicit information on the S0-state reconstitution, thus the progression after O2 release remains elusive. In this report, our molecular dynamics simulations combined with density functional calculations suggest a likely missing link for closing the cycle, i.e., restoring the first catalytic state. Specifically, the formation of closed-cubane intermediates with all hexa-coordinate Mn is observed, which would undergo proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S0 state. Thereby, we theoretically identify the previously unknown structural isomerism in the S0 state that acts as the origin of the proposed structural flexibility prevailing in the cycle, which may be functionally important for nature’s water oxidation catalysis.
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| 14. |
- Guo, Yu, et al.
(författare)
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Closing Kok’s cycle of nature’s water oxidation catalysis
- 2024
-
Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 15:1
-
Tidskriftsartikel (refereegranskat)abstract
- The Mn4CaO5(6) cluster in photosystem II catalyzes water splitting through the Si state cycle (i = 0–4). Molecular O2 is formed and the natural catalyst is reset during the final S3 → (S4) → S0 transition. Only recently experimental breakthroughs have emerged for this transition but without explicit information on the S0-state reconstitution, thus the progression after O2 release remains elusive. In this report, our molecular dynamics simulations combined with density functional calculations suggest a likely missing link for closing the cycle, i.e., restoring the first catalytic state. Specifically, the formation of closed-cubane intermediates with all hexa-coordinate Mn is observed, which would undergo proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S0 state. Thereby, we theoretically identify the previously unknown structural isomerism in the S0 state that acts as the origin of the proposed structural flexibility prevailing in the cycle, which may be functionally important for nature’s water oxidation catalysis.
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| 15. |
- Guo, Y., et al.
(författare)
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Reversible Structural Isomerization of Nature's Water Oxidation Catalyst Prior to O-O Bond Formation
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:26, s. 11736-11747
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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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| 16. |
- Han, Guangye, et al.
(författare)
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Molecular basis for turnover inefficiencies (misses) during water oxidation in photosystem II
- 2022
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Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 13:29, s. 8667-8678
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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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| 17. |
- Hussein, Rana, et al.
(författare)
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Cryo-electron microscopy reveals hydrogen positions and water networks in photosystem II
- 2024
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 384:6702, s. 1349-1355
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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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| 18. |
- 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
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Ingår i: Photosynthesis Research. - : Springer Nature. - 0166-8595 .- 1573-5079. ; 158:2, s. 91-107
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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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| 19. |
- Ibrahim, Mohamed, et al.
(författare)
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Untangling the sequence of events during the S-2 -> S-3 transition in photosystem II and implications for the water oxidation mechanism
- 2020
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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. ; 117:23, s. 12624-12635
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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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| 20. |
- 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
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Ingår i: International journal of hydrogen energy. - : Elsevier. - 0360-3199 .- 1879-3487. ; 46:59, s. 30366-30372
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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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| 21. |
- Shevela, Dmitriy, 1979-, et al.
(författare)
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Measurements of oxygen evolution in photosynthesis
- 2024. - 2
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Ingår i: Photosynthesis. - New York : Humana Press. - 9781071637890 - 9781071637920 - 9781071637906 ; , s. 133-148
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Bokkapitel (refereegranskat)abstract
- This chapter compares two different techniques for monitoring photosynthetic O2 production; the wide-spread Clark-type O2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center.
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| 22. |
- Shevela, Dmitriy, 1979-, et al.
(författare)
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Solar energy conversion by photosystem II : principles and structures
- 2023
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Ingår i: Photosynthesis Research. - : Springer. - 0166-8595 .- 1573-5079. ; 156, s. 279-307
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Forskningsöversikt (refereegranskat)abstract
- Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.
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| 23. |
- Simon, Philipp S., et al.
(författare)
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Capturing the sequence of events during the water oxidation reaction in photosynthesis using XFELs
- 2023
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Ingår i: FEBS Letters. - : John Wiley & Sons. - 0014-5793 .- 1873-3468. ; 597:1, s. 30-37
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Tidskriftsartikel (refereegranskat)abstract
- Ever since the discovery that Mn was required for oxygen evolution in plants by Pirson in 1937 and the period-four oscillation in flash-induced oxygen evolution by Joliot and Kok in the 1970s, understanding of this process has advanced enormously using state-of-the-art methods. The most recent in this series of innovative techniques was the introduction of X-ray free-electron lasers (XFELs) a decade ago, which led to another quantum leap in the understanding in this field, by enabling operando X-ray structural and X-ray spectroscopy studies at room temperature. This review summarizes the current understanding of the structure of Photosystem II (PS II) and its catalytic centre, the Mn4CaO5 complex, in the intermediate Si (i = 0–4)-states of the Kok cycle, obtained using XFELs.
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| 24. |
- Tomás Graça, André, 1994-
(författare)
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Light’EM up! : structural characterization of light-driven membrane protein complexes by cryogenic electron microscopy
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photosynthesis is probably the most important process for allowing life to develop into the diverse forms we see today. In this process, solar radiation is used to convert CO2 into biomass. From this process, we obtain oxygen to breathe, sources of food (plant biomass), and the potential for clean and sustainable energy. Photosystem II (PSII) – a key enzyme in photosynthesis –, is a protein complex located in the thylakoid membrane of photosynthetic organisms. PSII and its light-harvesting antennae capture light energy, driving a charge separation process, which leads to the extraction of electrons from water molecules, forming and releasing molecular oxygen. A PSII dimer is composed of more than 20 unique proteins and hundreds of cofactors which fine-tune the mechanisms of light-harvesting and water oxidation, and stabilize the whole complex. While the arrangement of most (but not all!) of these proteins and cofactors is known, their dynamics and individual contributions are not yet fully understood.In my thesis work, I took on the challenge of resolving the structure of large protein complexes, such as PSII complexes from various photosynthetic organisms, using a technique called cryogenic electron microscopy (cryo-EM). This PhD dissertation focuses on structurally describing these macromolecular assemblies and how their components (protein, cofactors, and substrate) interact with each other or with their immediate cellular environment.Among the several outcomes of my research on PSII, I would like to highlight the following findings: 1) the usage of digitonin as a detergent to solubilize PSII destroys the catalytic activity and changes LHCII pigment content, among other consequences; 2) PSII does not seem to incorporate chlorophyll (Chl) a molecules with a farnesyl tail, and the Chl tails’ flexibility justifies not resolving the full-length of some of these molecules in PSII structures. We concluded that flexibility may be an advantage to PSII function; 3) cryo-EM is a technique with the potential to reveal information about electron/proton transfer processes within PSII, and provided us with data, for instance, to suggest a pathway for the protonation of QB, the final electron acceptor in PSII.In another project, also using cryo-EM, I studied the structure of the S-layer Deinoxanthin Binding Complex (SDBC), a membrane protein complex from Deinococcus radiodurans. This complex is an essential part of the cell envelope, the outermost barrier of this bacterium, and it is known to bind a carotenoid called deinoxanthin, which has significant spectroscopic and antioxidant properties. Additionally, we studied the function of this complex and showed that the SDBC is a quencher of UVC-UVB radiation and reactive oxygen species, with superoxide dismutase activity. This complex has an α-β coiled-coil stalk long enough to reach the inner membrane of the cell envelope.In summary, visualizing the structural organization and chemistry within these complexes allowed us to gain a new understanding of their function and diversity. Furthermore, this work demonstrates the potential of cryo-EM as a method to render complementary information at resolution superior to state-of-the-art X-ray diffraction methods.
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