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1.
Photosystem II (PSII) catalyzes the oxidation of water to O2 at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC;
four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash
illumination, the OEC cycles among five S
n
states. Chloride depletion inhibits O2 evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has
not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles
PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived
structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising
from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands
were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra
assigned to the S3 to S0 (third flash) and S0 to S1 transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the
first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes,
which alter protein dynamics during the S state cycle.
Supported by NSF MCB 03-55421. 相似文献
2.
Masami Kusunoki 《BBA》2007,1767(6):484-492
The molecular mechanism of the water oxidation reaction in photosystem II (PSII) of green plants remains a great mystery in life science. This reaction is known to take place in the oxygen evolving complex (OEC) incorporating four manganese, one calcium and one chloride cofactors, that is light-driven to cycle four intermediates, designated S0 through S4, to produce four protons, five electrons and lastly one molecular oxygen, for indispensable resources in biosphere. Recent advancements of X-ray crystallography models established the existence of a catalytic Mn4Ca cluster ligated by seven protein amino acids, but its functional structure is not yet resolved. The 18O exchange rates of two substrate water molecules were recently measured for four Si-state samples (i = 0-3) leading to 34O2 and 36O2 formations, revealing asymmetric substrate binding sites significantly depending on the Si-state. In this paper, we present a chemically complete model for the Mn4Ca cluster and its surrounding enzyme field, which we found out from some possible models by using the hybrid density functional theoretic geometry optimization method to confirm good agreements with the 3.0 Å resolution PSII model [B. Loll, J. Kern, W. Saenger, A. Zouni , J. Biesiadka, Nature 438 (2005) 1040-1044] and the S-state dependence of 18O exchange rates [W. Hillier and T. Wydrzynski, Phys. Chem. Chem. Phys. 6 (2004) 4882-4889]. Furthermore, we have verified that two substrate water molecules are bound to asymmetric cis-positions on the terminal Mn ion being triply bridged (μ-oxo, μ-carboxylato, and a hydrogen bond) to the Mn3CaO3(OH) core, by developing a generalized theory of 18O exchange kinetics in OEC to obtain an experimental evidence for the cross exchange pathway from the slow to the fast exchange process. Some important experimental data will be discussed in terms of this model and its possible tautomers, to suggest that a cofactor, Cl− ion, may be bound to CP43-Arg357 nearby Ca2+ ion and that D1-His337 may be used to trap a released proton only in the S2-state. 相似文献
3.
Carina Gl?ckner Jan Kern Matthias Broser Athina Zouni Vittal Yachandra Junko Yano 《The Journal of biological chemistry》2013,288(31):22607-22620
The oxygen-evolving complex (OEC) in the membrane-bound protein complex photosystem II (PSII) catalyzes the water oxidation reaction that takes place in oxygenic photosynthetic organisms. We investigated the structural changes of the Mn4CaO5 cluster in the OEC during the S state transitions using x-ray absorption spectroscopy (XAS). Overall structural changes of the Mn4CaO5 cluster, based on the manganese ligand and Mn-Mn distances obtained from this study, were incorporated into the geometry of the Mn4CaO5 cluster in the OEC obtained from a polarized XAS model and the 1.9-Å high resolution crystal structure. Additionally, we compared the S1 state XAS of the dimeric and monomeric form of PSII from Thermosynechococcus elongatus and spinach PSII. Although the basic structures of the OEC are the same for T. elongatus PSII and spinach PSII, minor electronic structural differences that affect the manganese K-edge XAS between T. elongatus PSII and spinach PSII are found and may originate from differences in the second sphere ligand atom geometry. 相似文献
4.
Photosystem II (PSII), the light-absorbing complex of photosynthesis that evolves oxygen, requires chloride for activation
of the oxygen evolving complex (OEC). In this study, fluoride was characterized as an inhibitor of Cl−-activated oxygen evolution in higher plant PSII. It was confirmed to be primarily a competitive inhibitor in intact PSII,
with Cl−-competitive inhibition constant Ki = 2 mM and uncompetitive inhibition constant
\textK\texti¢ {\text{K}}_{\text{i}}^{\prime } = 79 mM. A pH dependence study showed that fluoride inhibition was more pronounced at lower pH values. In order to determine
the location of the fluoride effect, PSII preparations lacking various amounts of the PsbQ subunit were prepared. The competitive
F− inhibition constant and the Michaelis constant for Cl− activation increased with loss of the PsbQ subunit, while the uncompetitive F− inhibition constant was relatively insensitive to loss of PsbQ. The S2 state EPR signals from PSII lacking PsbQ responded to Ca2+ and Cl− removal and to F− treatment similar to intact PSII, with enhancement of the g = 4.1 signal and suppression of the multiline signal, but the effects were more pronounced in PSII lacking PsbQ. Together,
these results support the interpretation that the PsbQ subunit has a role in retaining anions within the OEC. 相似文献
5.
《BBA》2020,1861(12):148301
In photosystem II (PSII), photosynthetic water oxidation occurs at the O2-evolving complex (OEC), a tetramanganese-calcium cluster that cycles through light-induced redox intermediates (S0–S4) to produce oxygen from two substrate water molecules. The OEC is surrounded by a hydrogen-bonded network of amino-acid residues that plays a crucial role in proton transfer and substrate water delivery. Previously, we found that D1-S169 was crucial for water oxidation and its mutation to alanine perturbed the hydrogen-bonding network. In this study, we demonstrate that the activation energy for the S2 to S1 transition of D1-S169A PSII is higher than wild-type PSII with a ~1.7–2.7× slower rate of charge recombination with QA− relative to wild-type PSII. Arrhenius analysis of the decay kinetics shows an Ea of 5.87 ± 1.15 kcal mol−1 for decay back to the S1 state, compared to 0.80 ± 0.13 kcal mol−1 for the wild-type S2 state. In addition, we find that ammonia does not affect the S2-state EPR signal, indicating that ammonia does not bind to the Mn cluster in D1-S169A PSII. Finally, a QM/MM analysis indicates that an additional water molecule binds to the Mn4 ion in place of an oxo ligand O5 in the S2 state of D1-S169A PSII. The altered S2 state of D1-S169A PSII provides insight into the S2➔S3 state transition. 相似文献
6.
Chemeris Yu. K. Korolkov N. S. Seifullina N. Kh. Rubin A. B. 《Russian Journal of Plant Physiology》2004,51(5):584-589
Incubation of green alga Chlorella pyrenoidosa Chick in darkness at 37–38°C for 10–30 h resulted in inactivation of the oxygen-evolving complex (OEC): the maximum yield of oxygen evolution during a series of short light flashes shifted from the third to the fifth flash; the transition of S2- and S3-states of OEC to a stable S1-state was markedly accelerated. This inactivation of OEC was accompanied by the accumulation of inactive complexes of photosystem II (PSII), in which the reduction of primary quinone acceptor and the conversion into the closed state occurred with a low efficiency, even in the presence of 5 M DCMU. The treatment of light-grown algal cells with hydroxylamine impaired OEC functioning, in similarity to the effect of dark incubation, but caused no accumulation of inactive PSII complexes. We conclude that the inactivation of OEC is not the cause of the inactivation of PSII complex. The decline in the efficiency of electron-transport reactions, both on the donor and acceptor sides of PSII may be related to modification of major proteins in the PS II reaction center. 相似文献
7.
The NMR paramagnetic relaxation enhancement (NMR-PRE) produced in the solvent proton resonance by manganese in the S0 and S2 states of the oxygen evolving center (OEC) has been recorded for three Photosystem II (PS II)-enriched preparations: (1) PS II-enriched thylakoid membrane fragments (TMF-2 particles); (2) salt-washed (2M NaCl) TMF-2 particles; and (3) the octylglucopyranoside (OGP)-solubilized PS II complex. The second and third preparations, but not the first, are depleted of the peripheral 17 and 23 kD polypeptides associated with the OEC. It has been proposed that depletion of these polypeptides increases the exposure of OEC manganese to the aqueous phase. The NMR-PRE response measures the quantity (T1m+m)-1, where T1m is the spin relaxation time and m is the mean residence time with respect to chemical exchange reactions of solvent protons in the manganese coordination sphere, and, thus, the NMR-PRE provides a direct measure of the solvent proton chemical exchange rate constant m
-1. This study tested whether the 17 and 23 kD polypeptides shield the OEC from the solvent phase and whether their depletion enhances the S2 and S0 NMR-PRE signals by removing a kinetic barrier to the solvent proton chemical exchange reaction. The amplitude of the S2 NMR-PRE signal, measured in its chemical exchange-limited regime (m>T1m), is slightly decreased, rather than increased, in preparations (2) and (3) relative to (1), indicating that removal of the 17 and 23 kD polypeptides slightly slows, rather than accelerates, the rate-limiting steps of the solvent proton chemical exchange reactions. In addition, the lifetime of the S2 state was shortened several-fold in the solubilized PS II complex and in salt-washed TMF-2 membranes relative to untreated TMF-2 control samples. The S0 NMR-PRE signal, which is present in TMF-2 suspensions, was not detected in suspensions of the solubilized PS II complex, even though these samples contained high concentrations of active manganese centers (approximately double those of the TMF-2 control) and exhibited an S2 NMR-PRE signal of comparable amplitude to that of the TMF-2 preparation. These results suggest that the 17 and 23 kD extrinsic polypeptides do not shield the NMR-visible water binding site in the OEC from the aqueous phase, although their removal substantially alters the proton relaxation efficiency by shortening T1m.Abbreviations ADRY
acceleration of the deactivation reactions of the water splitting enzyme Y
- BBY
Photosystem II-enriched membrane fragments prepared by the method of Berthold et al. (1981)
- CCCP
carbonyl cyanide m-chlorophenyl hydrazone
- Chl
chlorophyll
- DCBQ
2,5-dichlorobenzoquinone
- MES
morpholinoethanesulfonate
- NMR
nuclear magnetic resonance
- OEC
oxygen evolving complex
- OGP
octylglucopyranoside
- PRE
paramagnetic relaxation enhancement
- PS II
Photosystem II
- SDS-PAGE
sodium dodecyl sulfate polyacrylamide gel electrophoresis
- TMF-2
Photosystem II-enriched thylakoid membrane fragments prepared by the method of Radmer et al. (1986)
- T1, T2
longitudinal and transverse nuclear spin relaxation times 相似文献
8.
Damage to the oxygen-evolving complex by superoxide anion, hydrogen peroxide, and hydroxyl radical in photoinhibition of photosystem II 总被引:1,自引:0,他引:1
Under strong illumination of a photosystem II (PSII) membrane, endogenous superoxide anion, hydrogen peroxide, and hydroxyl
radical were successively produced. These compounds then cooperatively resulted in a release of manganese from the oxygen-evolving
complex (OEC) and an inhibition of oxygen evolution activity. The OEC inactivation was initiated by an acceptor-side generated
superoxide anion, and hydrogen peroxide was most probably responsible for the transportation of reactive oxygen species (ROS)
across the PSII membrane from the acceptor-side to the donor-side. Besides ROS being generated in the acceptor-side induced
manganese loss; there may also be a ROS-independent manganese loss in the OEC of PSII. Both superoxide anion and hydroxyl
radical located inside the PSII membrane were directly identified by a spin trapping-electron spin resonance (ESR) method
in combination with a lipophilic spin trap, 5-(diethoxyphosphoryl)-5-phenethyl-1-pyrroline N-oxide (DEPPEPO). The endogenous hydrogen peroxide production was examined by oxidation of thiobenzamide. 相似文献
9.
This mini review is an attempt to briefly summarize our current knowledge on light driven oxidative water splitting in photosynthesis. The reaction leading to molecular oxygen and four protons via photosynthesis comprises thermodynamic and kinetic constraints that require a balanced fine tuning of the reaction coordinates. The mode of coupling between electron (ET) and proton transfer (PT) reactions is shown to be of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The WOC is characterized by peculiar energetics of its oxidation steps in the WOC. In all oxygen evolving photosynthetic organisms the redox state S1 is thermodynamically most stable and therefore this general feature is assumed to be of physiological relevance. Available information on the Gibbs energy differences between the individual redox states Si+1 and Si and on the activation energies of their oxidative transitions are used to construct a general reaction coordinate of oxidative water splitting in photosystem II (PS II). Finally, an attempt is presented to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and the active role of the protein environment in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place within a multistate equilibrium at the redox level of S3, comprising both redox isomerism and proton tautomerism. It is proposed that one state, S3(P), attains an electronic configuration and nuclear geometry that corresponds with a hydrogen bonded peroxide which acts as the entatic state for the generation of complexed molecular oxygen through S3(P) oxidation by YZox. 相似文献
10.
Chloride is required for the maximum activity of the oxygen evolving complex (OEC) while formate inhibits the function of OEC. On the basis of the measurements of oxygen evolution rates and the S2 state multiline EPR signal, an interaction between the action of chloride and formate at the donor side of PS II has been suggested. Moreover, the Fe2+Q–A EPR signals were measured to investigate a common binding site of both these anions at the PS II acceptor side. Other monovalent anions like bromide, nitrate etc. could influence the effects of formate to a small extent at the donor side of PS II, but not significantly at the acceptor side of PS II. The results presented in this paper clearly suggest a competitive binding of formate and chloride at the PS II acceptor side. 相似文献
11.
Ruchira Chatterjee Louise Lassalle Sheraz Gul Franklin D. Fuller Iris D. Young Mohamed Ibrahim Casper de Lichtenberg Mun Hon Cheah Athina Zouni Johannes Messinger Vittal K. Yachandra Jan Kern Junko Yano 《Physiologia plantarum》2019,166(1):60-72
In nature, an oxo‐bridged Mn4CaO5 cluster embedded in photosystem II (PSII), a membrane‐bound multi‐subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light‐induced charge separations in the reaction center of PSII. The Mn4CaO5 cluster accumulates four oxidizing equivalents to enable the four‐electron four‐proton catalysis of two water molecules to one dioxygen molecule and cycles through five intermediate S‐states, S0 – S4 in the Kok cycle. One important question related to the catalytic mechanism of the oxygen‐evolving complex (OEC) that remains is, whether structural isomers are present in some of the intermediate S‐states and if such equilibria are essential for the mechanism of the O‐O bond formation. Here we compare results from electron paramagnetic resonance (EPR) and X‐ray absorption spectroscopy (XAS) obtained at cryogenic temperatures for the S2 state of PSII with structural data collected of the S1, S2 and S3 states by serial crystallography at neutral pH (~6.5) using an X‐ray free electron laser at room temperature. While the cryogenic data show the presence of at least two structural forms of the S2 state, the room temperature crystallography data can be well‐described by just one S2 structure. We discuss the deviating results and outline experimental strategies for clarifying this mechanistically important question. 相似文献
12.
Photosynthesis Research - Light-driven water oxidation in photosynthesis occurs at the oxygen-evolving center (OEC) of photosystem II (PSII). Chloride ions (Cl?) are essential for oxygen... 相似文献
13.
《BBA》2014,1837(2):264-269
Photosystem II (PSII) is the pigment–protein complex which converts sunlight energy into chemical energy by catalysing the process of light-driven oxidation of water into reducing equivalents in the form of protons and electrons. Three-dimensional structures from x-ray crystallography have been used extensively to model these processes. However, the crystal structures are not necessarily identical to those of the solubilised complexes. Here we compared picosecond fluorescence of solubilised and crystallised PSII core particles isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The fluorescence of the crystals is sensitive to the presence of artificial electron acceptors (K3Fe(CN)3) and electron transport inhibitors (DCMU). In PSII with reaction centres in the open state, the picosecond fluorescence of PSII crystals and solubilised PSII is indistinguishable. Additionally we compared picosecond fluorescence of native PSII with PSII in which Ca2 in the oxygen evolving complex (OEC) is biosynthetically replaced by Sr2 +. With the Sr2 + replaced OEC the average fluorescence decay slows down slightly (81 ps to 85 ps), and reaction centres are less readily closed, indicating that both energy transfer/trapping and electron transfer are affected by the replacement. 相似文献
14.
D. Koulougliotis 《Photosynthetica》2009,47(4):567-574
Progressive microwave power saturation (P1/2) measurements have been performed on the tyrosine D radical (YD
•) of photosystem II (PSII) in order to examine its relaxation enhancement by the oxygen-evolving complex (OEC) poised to the
reduced S−1 and S−2 oxidation states by NO treatment. Analysis of the power saturation curves showed that the S−1 oxidation state of the OEC does not enhance the relaxation of YD
•: it therefore possesses a diamagnetic ground state. In contrast, the Mn(II)-Mn(III) multiline electron paramagnetic resonance
(EPR) signal characteristic of the S−2 oxidation state of the OEC was shown to provide a relaxation enhancement pathway for YD
•, however less efficient relative to the one provided by the S2-state multiline EPR signal. We also examined the YD
• relaxation enhancement characteristics of the EPR-silent oxidation state produced after brief (1–5 min) dark incubation at
0°C of a PSII sample poised to the EPRactive S−2 state. This EPR-silent oxidation state denoted as “0°C incubation” state was shown to possess remarkably similar P1/2 values with the EPR-active S−2 state in the overall examined temperature range (6–20 K). In addition, these values remained unchanged after successive cycles
of the OEC between the EPR-active S−2 state and the “0°C incubation” state. The data presented in this work point to the conclusion that the “0°C incubation” state
is indeed an S−2 oxidation state with half-integer spin. 相似文献
15.
Renger G 《Photosynthesis research》2007,92(3):407-425
This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on
the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457–476) as a framework, the energetics and kinetics
of two different types of reactions comprising the overall process are discussed: (i) P680+• reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yzox induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen.
The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox
turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to
be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available
information about the Gibbs energy differences between the individual redox states S
i+1 and S
i
and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made
to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation
of the essential O–O bond and on the active role of the protein in tuning the local proton activity that depends on time and
redox state S
i
. The O–O linkage is assumed to take place at the level of a complexed peroxide. 相似文献
16.
The photosystem II (PSII) in green plants exhibits marvelous oxygen production in neutral environments. However, artificially developed oxygen evolution catalysts (OECs) show much less activity, and the oxygen evolution reaction (OER) is now becoming a bottleneck in many energy‐related issues. Here, the PSII is mimicked to design an efficient OER system in neutral environments by introducing an oleylamine (OAm) organic layer to cap the Co3O4 OEC, and employing buffers as proton shuttles in the system. Consequently, the activity is largely enhanced. The current density can reach 10 mA cm?2 at an overpotential (η) of 390 mV in the best case in neutral environment. The turnover frequency is 0.0117 at η of 400 mV, almost the same as that in 1 m KOH solutions. The surface chemistry of the Co3O4 OEC indicates that the OAm can promote the activity. The reason that buffers as proton shuttles can greatly facilitate the reaction is ascribed to the proton‐coupled electron transfer process in the OER mechanism. These results may stimulate new perspectives on mimicking natural systems as well as new insights in electrocatalysis. 相似文献
17.
Mayer JM Rhile IJ Larsen FB Mader EA Markle TF DiPasquale AG 《Photosynthesis research》2006,87(1):21-24
The coupling of proton and electron transfers is a key part of the chemistry of photosynthesis. The oxidative side of photosystem
II (PS II) in particular seems to involve a number of proton-coupled electron transfer (PCET) steps in the S-state transitions.
This mini-review presents an overview of recent studies of PCET model systems in the authors’ laboratory. PCET is defined
as a chemical reaction involving concerted transfer of one electron and one proton. These are thus distinguished from stepwise
pathways involving initial electron transfer (ET) or initial proton transfer (PT). Hydrogen atom transfer (HAT) reactions
are one class of PCET, in which H+ and e
− are transferred from one reagent to another: AH+B→A+BH, roughly along the same path. Rate constants for many HAT reactions
are found to be well predicted by the thermochemistry of hydrogen transfer and by Marcus Theory. This includes organic HAT
reactions and reactions of iron-tris(α-diimine) and manganese-(μ-oxo) complexes. In PS II, HAT has been proposed as the mechanism
by which the tyrosine Z radical (YZ) oxidizes the manganese cluster (the oxygen evolving complex, OEC). Another class of PCET reactions involves transfer of
H+ and e
− in different directions, for instance when the proton and electron acceptors are different reagents, as in AH–B+C+→A–HB++C. The oxidation of YZ by the chlorophyll P680 + has been suggested to occur by this mechanism. Models for this process – the oxidation of phenols
with a pendent base – are described. The oxidation of the OEC by YZ could also occur by this second class of PCET reactions, involving an Mn–O–H fragment of the OEC. Initial attempts to model
such a process using ruthenium-aquo complexes are described.
An erratum to this article can be found at 相似文献
18.
Yano J Walker LM Strickler MA Service RJ Yachandra VK Debus RJ 《The Journal of biological chemistry》2011,286(11):9257-9267
The effect of replacing a histidine ligand on the properties of the oxygen-evolving complex (OEC) and the structure of the Mn4Ca cluster in Photosystem II (PSII) is studied by x-ray absorption spectroscopy using PSII core complexes from the Synechocystis sp. PCC 6803 D1 polypeptide mutant H332E. In the x-ray crystallographic structures of PSII, D1-His332 has been assigned as a direct ligand of a manganese ion, and the mutation of this histidine ligand to glutamate has been reported to prevent the advancement of the OEC beyond the S2Yz• intermediate state. The manganese K-edge (1s core electron to 4p) absorption spectrum of D1-H332E shifts to a lower energy compared with that of the native WT samples, suggesting that the electronic structure of the manganese cluster is affected by the presence of the additional negative charge on the OEC of the mutant. The extended x-ray absorption spectrum shows that the geometric structure of the cluster is altered substantially from that of the native WT state, resulting in an elongation of manganese-ligand and manganese-manganese interactions in the mutant. The strontium-H332E mutant, in which calcium is substituted by strontium, confirms that strontium (calcium) is a part of the altered cluster. The structural perturbations caused by the D1-H332E mutation are much larger than those produced by any biochemical treatment or mutation examined previously with x-ray absorption spectroscopy. The substantial structural changes provide an explanation not only for the altered properties of the D1-H332E mutant but also the importance of the histidine ligand for proper assembly of the Mn4Ca cluster. 相似文献
19.
James P McEvoy Jose A Gascon Victor S Batista Gary W Brudvig 《Photochemical & photobiological sciences》2005,4(12):940-949
Oxygenic photosynthesis, which provides the biosphere with most of its chemical energy, uses water as its source of electrons. Water is photochemically oxidized by the protein complex photosystem II (PSII), which is found, along with other proteins of the photosynthetic light reactions, in the thylakoid membranes of cyanobacteria and of green plant chloroplasts. Water splitting is catalyzed by the oxygen-evolving complex (OEC) of PSII, producing dioxygen gas, protons and electrons. O(2) is released into the atmosphere, sustaining all aerobic life on earth; product protons are released into the thylakoid lumen, augmenting a proton concentration gradient across the membrane; and photo-energized electrons pass to the rest of the electron-transfer pathway. The OEC contains four manganese ions, one calcium ion and (almost certainly) a chloride ion, but its precise structure and catalytic mechanism remain unclear. In this paper, we develop a chemically complete structure of the OEC and its environment by using molecular mechanics calculations to extend and slightly adjust the recently-obtained X-ray crystallographic model with reference to this structure and to some important recent experimental results. 相似文献
20.
《BBA》2023,1864(4):148998
Chloride (Cl−) is essential for O2 evolution during photosynthetic water oxidation. Two chlorides near the water-oxidizing complex (WOC) in Photosystem II (PSII) structures from Thermosynechococcus elongatus (and T. vulcanus) have been postulated to transfer protons generated from water oxidation. We monitored four criteria: primary charge separation flash yield (P* → P+QA−), rates of water oxidation steps (S-states), rate of proton evolution, and flash O2 yield oscillations by measuring chlorophyll variable fluorescence (P* quenching), pH-sensitive dye changes, and oximetry. Br-substitution slows and destabilizes cellular growth, resulting from lower light-saturated O2 evolution rate (−20 %) and proton release (−36 % ΔpH gradient). The latter implies less ATP production. In Br- cultures, protonogenic S-state transitions (S2 → S3 → S0’) slow with increasing light intensity and during O2/water exchange (S0’ → S0 → S1), while the non-protonogenic S1 → S2 transition is kinetically unaffected. As flash rate increases in Cl− cultures, both rate and extent of acidification of the lumen increase, while charge recombination is suppressed relative to Br−. The Cl− advantage in rapid proton escape from the WOC to lumen is attributed to correlated ion-pair movement of H3O+Cl− in dry water channels vs. separated Br− and H+ ion movement through different regions (>200-fold difference in Bronsted acidities). By contrast, at low flash rates a previously unreported reversal occurs that favors Br− cultures for both proton evolution and less PSII charge recombination. In Br− cultures, slower proton transfer rate is attributed to stronger ion-pairing of Br− with AA residues lining the water channels. Both anions charge-neutralize protons and shepherd them to the lumen using dry aqueous channels. 相似文献