51
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Guo Y, Li H, He LL, Zhao DX, Gong LD, Yang ZZ. The open-cubane oxo–oxyl coupling mechanism dominates photosynthetic oxygen evolution: a comprehensive DFT investigation on O–O bond formation in the S4state. Phys Chem Chem Phys 2017; 19:13909-13923. [DOI: 10.1039/c7cp01617d] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
How is O2created in nature? Comprehensive DFT investigations determine the dominance of the open-cubane oxo–oxyl coupling mechanism over alternative possibilities.
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Affiliation(s)
- Yu Guo
- School of Chemistry and Chemical Engineering
- Liaoning Normal University
- Dalian 116029
- People's Republic of China
| | - Hui Li
- School of Chemistry and Chemical Engineering
- Liaoning Normal University
- Dalian 116029
- People's Republic of China
| | - Lan-Lan He
- School of Chemistry and Chemical Engineering
- Liaoning Normal University
- Dalian 116029
- People's Republic of China
| | - Dong-Xia Zhao
- School of Chemistry and Chemical Engineering
- Liaoning Normal University
- Dalian 116029
- People's Republic of China
| | - Li-Dong Gong
- School of Chemistry and Chemical Engineering
- Liaoning Normal University
- Dalian 116029
- People's Republic of China
| | - Zhong-Zhi Yang
- School of Chemistry and Chemical Engineering
- Liaoning Normal University
- Dalian 116029
- People's Republic of China
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52
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Sequential and Coupled Proton and Electron Transfer Events in the S2 → S3 Transition of Photosynthetic Water Oxidation Revealed by Time-Resolved X-ray Absorption Spectroscopy. Biochemistry 2016; 55:6996-7004. [DOI: 10.1021/acs.biochem.6b01078] [Citation(s) in RCA: 42] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
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53
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Bonke SA, Bond AM, Spiccia L, Simonov AN. Parameterization of Water Electrooxidation Catalyzed by Metal Oxides Using Fourier Transformed Alternating Current Voltammetry. J Am Chem Soc 2016; 138:16095-16104. [DOI: 10.1021/jacs.6b10304] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Shannon A. Bonke
- School of Chemistry and the
ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia
| | - Alan M. Bond
- School of Chemistry and the
ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia
| | - Leone Spiccia
- School of Chemistry and the
ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia
| | - Alexandr N. Simonov
- School of Chemistry and the
ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia
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54
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Vinyard DJ, Khan S, Brudvig GW. Photosynthetic water oxidation: binding and activation of substrate waters for O-O bond formation. Faraday Discuss 2016; 185:37-50. [PMID: 26447686 DOI: 10.1039/c5fd00087d] [Citation(s) in RCA: 51] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Photosynthetic water oxidation occurs at the oxygen-evolving complex (OEC) of Photosystem II (PSII). The OEC, which contains a Mn4CaO5 inorganic cluster ligated by oxides, waters and amino-acid residues, cycles through five redox intermediates known as S(i) states (i = 0-4). The electronic and structural properties of the transient S4 intermediate that forms the O-O bond are not well understood. In order to gain insight into how water is activated for O-O bond formation in the S4 intermediate, we have performed a detailed analysis of S-state dependent substrate water binding kinetics taking into consideration data from Mn coordination complexes. This analysis supports a model in which the substrate waters are both bound as terminal ligands and react via a water-nucleophile attack mechanism.
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Affiliation(s)
- David J Vinyard
- Department of Chemistry, Yale University, New Haven, CT, United States.
| | - Sahr Khan
- Department of Chemistry, Yale University, New Haven, CT, United States.
| | - Gary W Brudvig
- Department of Chemistry, Yale University, New Haven, CT, United States.
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55
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Retegan M, Pantazis DA. Interaction of methanol with the oxygen-evolving complex: atomistic models, channel identification, species dependence, and mechanistic implications. Chem Sci 2016; 7:6463-6476. [PMID: 28451104 PMCID: PMC5355959 DOI: 10.1039/c6sc02340a] [Citation(s) in RCA: 42] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2016] [Accepted: 06/28/2016] [Indexed: 12/29/2022] Open
Abstract
Methanol has long being used as a substrate analogue to probe access pathways and investigate water delivery at the oxygen-evolving complex (OEC) of photosystem-II. In this contribution we study the interaction of methanol with the OEC by assembling available spectroscopic data into a quantum mechanical treatment that takes into account the local channel architecture of the active site. The effect on the magnetic energy levels of the Mn4Ca cluster in the S2 state of the catalytic cycle can be explained equally well by two models that involve either methanol binding to the calcium ion of the cluster, or a second-sphere interaction in the vicinity of the "dangler" Mn4 ion. However, consideration of the latest 13C hyperfine interaction data shows that only one model is fully consistent with experiment. In contrast to previous hypotheses, methanol is not a direct ligand to the OEC, but is situated at the end-point of a water channel associated with the O4 bridge. Its effect on magnetic properties of plant PS-II results from disruption of hydrogen bonding between O4 and proximal channel water molecules, thus enhancing superexchange (antiferromagnetic coupling) between the Mn3 and Mn4 ions. The same interaction mode applies to the dark-stable S1 state and possibly to all other states of the complex. Comparison of protein sequences from cyanobacteria and plants reveals a channel-altering substitution (D1-Asn87 versus D1-Ala87) in the proximity of the methanol binding pocket, explaining the species-dependence of the methanol effect. The water channel established as the methanol access pathway is the same that delivers ammonia to the Mn4 ion, supporting the notion that this is the only directly solvent-accessible manganese site of the OEC. The results support the pivot mechanism for water binding at a component of the S3 state and would be consistent with partial inhibition of water delivery by methanol. Mechanistic implications for enzymatic regulation and catalytic progression are discussed.
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Affiliation(s)
- Marius Retegan
- Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , Germany .
| | - Dimitrios A Pantazis
- Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , Germany .
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56
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Ananyev G, Gates C, Dismukes GC. The Oxygen quantum yield in diverse algae and cyanobacteria is controlled by partitioning of flux between linear and cyclic electron flow within photosystem II. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2016; 1857:1380-1391. [PMID: 27117512 DOI: 10.1016/j.bbabio.2016.04.056] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/26/2015] [Revised: 04/20/2016] [Accepted: 04/21/2016] [Indexed: 01/07/2023]
Abstract
We have measured flash-induced oxygen quantum yields (O2-QYs) and primary charge separation (Chl variable fluorescence yield, Fv/Fm) in vivo among phylogenetically diverse microalgae and cyanobacteria. Higher O2-QYs can be attained in cells by releasing constraints on charge transfer at the Photosystem II (PSII) acceptor side by adding membrane-permeable benzoquinone (BQ) derivatives that oxidize plastosemiquinone QB(-) and QBH2. This method allows uncoupling PSII turnover from its natural regulation in living cells, without artifacts of isolating PSII complexes. This approach reveals different extents of regulation across species, controlled at the QB(-) acceptor site. Arthrospira maxima is confirmed as the most efficient PSII-WOC (water oxidizing complex) and exhibits the least regulation of flux. Thermosynechococcus elongatus exhibits an O2-QY of 30%, suggesting strong downregulation. WOC cycle simulations with the most accurate model (VZAD) show that a light-driven backward transition (net addition of an electron to the WOC, distinct from recombination) occurs in up to 25% of native PSIIs in the S2 and S3 states, while adding BQ prevents backward transitions and increases the lifetime of S2 and S3 by 10-fold. Backward transitions occur in PSIIs that have plastosemiquinone radicals in the QB site and are postulated to be physiologically regulated pathways for storing light energy as proton gradient through direct PSII-cyclic electron flow (PSII-CEF). PSII-CEF is independent of classical PSI/cyt-b6f-CEF and provides an alternative proton translocation pathway for energy conversion. PSII-CEF enables variable fluxes between linear and cyclic electron pathways, thus accommodating species-dependent needs for redox and ion-gradient energy sources powered by a single photosystem.
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Affiliation(s)
- Gennady Ananyev
- The Waksman Institute of Microbiology and the Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, United States
| | - Colin Gates
- The Waksman Institute of Microbiology and the Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, United States
| | - G Charles Dismukes
- The Waksman Institute of Microbiology and the Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, United States.
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57
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Recent developments in biological water oxidation. Curr Opin Chem Biol 2016; 31:113-9. [DOI: 10.1016/j.cbpa.2016.02.007] [Citation(s) in RCA: 87] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2016] [Accepted: 02/18/2016] [Indexed: 01/25/2023]
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58
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Pham LV, Messinger J. Probing S-state advancements and recombination pathways in photosystem II with a global fit program for flash-induced oxygen evolution pattern. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2016; 1857:848-59. [PMID: 27033305 DOI: 10.1016/j.bbabio.2016.03.013] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/26/2015] [Revised: 01/30/2016] [Accepted: 03/12/2016] [Indexed: 12/22/2022]
Abstract
The oxygen-evolving complex (OEC) in photosystem II catalyzes the oxidation of water to molecular oxygen. Four decades ago, measurements of flash-induced oxygen evolution have shown that the OEC steps through oxidation states S(0), S(1), S(2), S(3) and S(4) before O(2) is released and the S(0) state is reformed. The light-induced transitions between these states involve misses and double hits. While it is widely accepted that the miss parameter is S state dependent and may be further modulated by the oxidation state of the acceptor side, the traditional way of analyzing each flash-induced oxygen evolution pattern (FIOP) individually did not allow using enough free parameters to thoroughly test this proposal. Furthermore, this approach does not allow assessing whether the presently known recombination processes in photosystem II fully explain all measured oxygen yields during Si state lifetime measurements. Here we present a global fit program that simultaneously fits all flash-induced oxygen yields of a standard FIOP (2 Hz flash frequency) and of 11-18 FIOPs each obtained while probing the S(0), S(2) and S(3) state lifetimes in spinach thylakoids at neutral pH. This comprehensive data treatment demonstrates the presence of a very slow phase of S(2) decay, in addition to the commonly discussed fast and slow reduction of S(2) by YD and QB(-), respectively. Our data support previous suggestions that the S(0)→S(1) and S(1)→S(2) transitions involve low or no misses, while high misses occur in the S(2)→S(3) or S(3)→S(0) transitions.
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Affiliation(s)
- Long Vo Pham
- Department of Chemistry, Chemistry Biology Center (KBC), Umeå University, Linnaeus väg 6, SE-901 87 Umeå, Sweden
| | - Johannes Messinger
- Department of Chemistry, Chemistry Biology Center (KBC), Umeå University, Linnaeus väg 6, SE-901 87 Umeå, Sweden.
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59
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Nakamura S, Ota K, Shibuya Y, Noguchi T. Role of a Water Network around the Mn4CaO5 Cluster in Photosynthetic Water Oxidation: A Fourier Transform Infrared Spectroscopy and Quantum Mechanics/Molecular Mechanics Calculation Study. Biochemistry 2016; 55:597-607. [DOI: 10.1021/acs.biochem.5b01120] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Affiliation(s)
- Shin Nakamura
- Division
of Material Science,
Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
| | - Kai Ota
- Division
of Material Science,
Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
| | - Yuichi Shibuya
- Division
of Material Science,
Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
| | - Takumi Noguchi
- Division
of Material Science,
Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
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60
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Krewald V, Retegan M, Neese F, Lubitz W, Pantazis DA, Cox N. Spin State as a Marker for the Structural Evolution of Nature’s Water-Splitting Catalyst. Inorg Chem 2015; 55:488-501. [DOI: 10.1021/acs.inorgchem.5b02578] [Citation(s) in RCA: 71] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Affiliation(s)
- Vera Krewald
- Max Planck Institute for Chemical Energy Conversion, Stiftstr.
34–36, Mülheim an der Ruhr 45470, Germany
| | - Marius Retegan
- Max Planck Institute for Chemical Energy Conversion, Stiftstr.
34–36, Mülheim an der Ruhr 45470, Germany
| | - Frank Neese
- Max Planck Institute for Chemical Energy Conversion, Stiftstr.
34–36, Mülheim an der Ruhr 45470, Germany
| | - Wolfgang Lubitz
- Max Planck Institute for Chemical Energy Conversion, Stiftstr.
34–36, Mülheim an der Ruhr 45470, Germany
| | - Dimitrios A. Pantazis
- Max Planck Institute for Chemical Energy Conversion, Stiftstr.
34–36, Mülheim an der Ruhr 45470, Germany
| | - Nicholas Cox
- Max Planck Institute for Chemical Energy Conversion, Stiftstr.
34–36, Mülheim an der Ruhr 45470, Germany
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61
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Structural rearrangements preceding dioxygen formation by the water oxidation complex of photosystem II. Proc Natl Acad Sci U S A 2015; 112:E6139-47. [PMID: 26508637 DOI: 10.1073/pnas.1512008112] [Citation(s) in RCA: 49] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II. Recent studies implicate an oxo bridge atom, O5, of the Mn4CaO5 cluster, as the "slowly exchanging" substrate water molecule. The D1-V185N mutant is in close vicinity of O5 and known to extend the lag phase and retard the O2 release phase (slow phase) in this critical last [Formula: see text] transition of water oxidation. The pH dependence, hydrogen/deuterium (H/D) isotope effect, and temperature dependence on the O2 release kinetics for this mutant were studied using time-resolved O2 polarography, and comparisons were made with WT and two mutants of the putative proton gate D1-D61. Both kinetic phases in V185N are independent of pH and buffer concentration and have weaker H/D kinetic isotope effects. Each phase is characterized by a parallel or even lower activation enthalpy but a less favorable activation entropy than the WT. The results indicate new rate-determining steps for both phases. It is concluded that the lag does not represent inhibition of proton release but rather, slowing of a previously unrecognized kinetic phase involving a structural rearrangement or tautomerism of the S3 (+) ground state as it approaches a configuration conducive to dioxygen formation. The parallel impacts on both the lag and O2 formation phases suggest a common origin for the defects surmised to be perturbations of the H-bond network and the water cluster adjacent to O5.
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62
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Isobe H, Shoji M, Shen JR, Yamaguchi K. Strong Coupling between the Hydrogen Bonding Environment and Redox Chemistry during the S2 to S3 Transition in the Oxygen-Evolving Complex of Photosystem II. J Phys Chem B 2015; 119:13922-33. [DOI: 10.1021/acs.jpcb.5b05740] [Citation(s) in RCA: 48] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Affiliation(s)
- Hiroshi Isobe
- Photosynthesis
Research Center, Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
- The Institute
of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
| | - Mitsuo Shoji
- Graduate School
of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
| | - Jian-Ren Shen
- Photosynthesis
Research Center, Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
| | - Kizashi Yamaguchi
- The Institute
of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
- Institute for
NanoScience Design, Osaka University, Toyonaka, Osaka 560-0043, Japan
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63
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Nakamura S, Noguchi T. Infrared Detection of a Proton Released from Tyrosine YD to the Bulk upon Its Photo-oxidation in Photosystem II. Biochemistry 2015; 54:5045-53. [PMID: 26241205 DOI: 10.1021/acs.biochem.5b00568] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Photosystem II (PSII) has two symmetrically located, redox-active tyrosine residues, YZ and YD. Whereas YZ mediates the electron transfer from the water-oxidizing center to P680 in the main electron transfer pathway, YD functions as a peripheral electron donor to P680. To understand the mechanism of this functional difference between YZ and YD, it is essential to know where the proton is transferred upon its oxidation in the proton-coupled electron transfer process. In this study, we used Fourier transform infrared (FTIR) spectroscopy to examine whether the proton from YD is released from the protein into the bulk. The proton detection method previously used for water oxidation in PSII [Suzuki et al. (2009) J. Am. Chem. Soc. 131, 7849-7857] was applied to YD; a proton released into the bulk upon YD oxidation was trapped by a high-concentration Mes buffer, and the protonation reaction of Mes was monitored by FTIR difference spectroscopy. It was shown that 0.84 ± 0.10 protons are released into the bulk by oxidation of YD in one PSII center. This result indicates that the proton of YD is not transferred to the neighboring D2-His198 but is released from the protein; this is in sharp contrast to the YZ reaction, in which a proton is transferred to D1-His190 through a strong hydrogen bond. This functional difference is caused by differences in the hydrogen-bonded structures of YD and YZ, which are determined by the hydrogen bond partners at the Nπ sites of these His residues, i.e., D2-Arg294 and D1-Asn298, which function as a hydrogen bond donor and acceptor, respectively. This FTIR spectroscopy result supports the recent theoretical prediction [Saito et al. (2013) Proc. Natl. Acad. Sci. U.S.A. 110, 7690-7695] based on the X-ray crystallographic structure of PSII and explains the different rates of the redox reactions of YD and YZ.
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Affiliation(s)
- Shin Nakamura
- Division of Material Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan
| | - Takumi Noguchi
- Division of Material Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan
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64
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Boussac A, Rutherford AW, Sugiura M. Electron transfer pathways from the S2-states to the S3-states either after a Ca2+/Sr2+ or a Cl-/I- exchange in Photosystem II from Thermosynechococcus elongatus. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2015; 1847:576-86. [PMID: 25843552 DOI: 10.1016/j.bbabio.2015.03.006] [Citation(s) in RCA: 83] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/27/2014] [Revised: 03/25/2015] [Accepted: 03/29/2015] [Indexed: 01/12/2023]
Abstract
The site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S0 to S4) before O2 is evolved. It consists of a Mn4CaO5-cluster close to a redox-active tyrosine residue (YZ). Cl- is also required for enzyme activity. By using EPR spectroscopy it has been shown that both Ca2+/Sr2+ exchange and Cl-/I- exchange perturb the proportions of centers showing high (S=5/2) and low spin (S=1/2) forms of the S2-state. The S3-state was also found to be heterogeneous with: i) a S=3 form that is detectable by EPR and not sensitive to near-infrared light; and ii) a form that is not EPR visible but in which Mn photochemistry occurs resulting in the formation of a (S2YZ)' split EPR signal upon near-infrared illumination. In Sr/Cl-PSII, the high spin (S=5/2) form of S2 shows a marked heterogeneity with a g=4.3 form generated at low temperature that converts to a relaxed form at g=4.9 at higher temperatures. The high spin g=4.9 form can then progress to the EPR detectable form of S3 at temperatures as low as 180K whereas the low spin (S=1/2) S2-state can only advance to the S3 state at temperatures≥235 K. Both of the two S2 configurations and the two S3 configurations are each shown to be in equilibrium at ≥235 K but not at 198 K. Since both S2 configurations are formed at 198 K, they likely arise from two specific populations of S1. The existence of heterogeneous populations in S1, S2 and S3 states may be related to the structural flexibility associated with the positioning of the oxygen O5 within the cluster highlighted in computational approaches and which has been linked to substrate exchange. These data are discussed in the context of recent in silico studies of the electron transfer pathways between the S2-state(s) and the S3-state(s).
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Affiliation(s)
- Alain Boussac
- I(2)BC, CNRS UMR 9198, CEA Saclay, 91191 Gif-sur-Yvette, France.
| | | | - Miwa Sugiura
- Proteo-Science Research Center, Ehime University, Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan; Department of Chemistry, Graduate School of Science and Technology, Ehime University, Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan; PRESTO, Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan
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65
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Abstract
Technical progress in laser-sources and detectors has allowed the temporal and spatial resolution of chemical reactions down to femtoseconds and Å-units. In photon-excitable systems the key to chemical kinetics, trajectories across the vibrational saddle landscape, are experimentally accessible. Simple and thus well-defined chemical compounds are preferred objects for calibrating new methodologies and carving out paradigms of chemical dynamics, as shown in several contributions to this Faraday Discussion. Aerobic life on earth is powered by solar energy, which is captured by microorganisms and plants. Oxygenic photosynthesis relies on a three billion year old molecular machinery which is as well defined as simpler chemical constructs. It has been analysed to a very high precision. The transfer of excitation between pigments in antennae proteins, of electrons between redox-cofactors in reaction centres, and the oxidation of water by a Mn4Ca-cluster are solid state reactions. ATP, the general energy currency of the cell, is synthesized by a most agile, rotary molecular machine. While the efficiency of photosynthesis competes well with photovoltaics at the time scale of nanoseconds, it is lower by an order of magnitude for crops and again lower for bio-fuels. The enormous energy demand of mankind calls for engineered (bio-mimetic or bio-inspired) solar-electric and solar-fuel devices.
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Affiliation(s)
- Wolfgang Junge
- Dept. Biology & Chemistry, University of Osnabrück, R. 35/E42 Barbarastrasse 11, 49076 Osnabrück, Germany.
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66
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Abstract
Nature relies on a unique and intricate biochemical setup to achieve sunlight-driven water splitting. Combined experimental and computational efforts have produced significant insights into the structural and functional principles governing the operation of the water-oxidizing enzyme Photosystem II in general, and of the oxygen-evolving manganese-calcium cluster at its active site in particular. Here we review the most important aspects of biological water oxidation, emphasizing current knowledge on the organization of the enzyme, the geometric and electronic structure of the catalyst, and the role of calcium and chloride cofactors. The combination of recent experimental work on the identification of possible substrate sites with computational modeling have considerably limited the possible mechanistic pathways for the critical O-O bond formation step. Taken together, the key features and principles of natural photosynthesis may serve as inspiration for the design, development, and implementation of artificial systems.
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