| 1. |
- Bernat, Gabor, et al.
(författare)
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pH dependence of the four individual transitions in the catalytic S-cycle during photosynthetic oxygen evolution.
- 2002
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 41:18, s. 5830-5843
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the pH dependence for each individual redox transition in the S-cycle of the oxygen evolving complex (OEC) of photosystem II by electron paramagnetic resonance (EPR) spectroscopy. In the experiments, OEC is advanced to the appropriate S-state at normal pH. Then, the pH is rapidly changed, and a new flash is given. The ability to advance to the next S-state in the cycle at different pHs is determined by measurements of the decrease or increase of characteristic EPR signals from the OEC in different S-states. In some cases the measured EPR signals are very small (this holds especially for the S0 ML signal at pH >7.5 and pH <4.8). Therefore, we refrain from providing error limits for the determined pK's. Our results indicate that the S1 --> S2 transition is independent of pH between 4.1 and 8.4. All other S-transitions are blocked at low pH. In the acidic region, the pK's for the inhibition of the S2 --> S3, the S3 --> [S4] --> S0, and the S0 --> S1 transitions are about 4.0, 4.5, and 4.7, respectively. The similarity of these pK values indicates that the inhibition of the steady-state oxygen evolution in the acidic range, which occurs with pK approximately 4.8, is a consequence of similar pH blocks in three of the redox steps involved in the oxygen evolution. In the alkaline region, we report a clear pH block in the S3 --> [S4] --> S0 transition with a pK of about 8.0. Our study also indicates the existence of a pH block at very high pH (pK approximately 9.4) in the S2 --> S3 transition. The S0 --> S1 transition is not affected, at least up to pH 9.0. This suggests that the inhibition of the steady-state oxygen evolution, which occurs with a pK of 8.0, is dominated by the inhibition of the S3 --> [S4] --> S0 transition. Our results are obtained in the presence of 5% methanol (v/v). However, it is unlikely that the determined pK's are affected by the presence of methanol since our results also show that the pH dependence of the steady-state oxygen evolution is not affected by methanol. The results in the alkaline region are in good agreement with a model, which suggests that the redox potential of Y(Z*)/Y(Z) is directly affected by high pH. At high pH the Y(Z*)/Y(Z) potential becomes lower than that of S2/S1 and S3/S2. The acidic block, with a pK of 4-5 in three S-transitions, implies that the inhibition mechanism is similar, and we suggest that it reflects protonation of a carboxylic side chain in the proton relay that expels protons from the OEC.
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| 2. |
- Geijer, Paulina, et al.
(författare)
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The S3 State of the Oxygen-Evolving Complex in Photosystem II Is Converted to the S2YZ State at Alkaline pH
- 2001
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 40:36, s. 10881-10891
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Tidskriftsartikel (refereegranskat)abstract
- Here we report an EPR signal that is induced by a pH jump to alkaline pH in the S3 state of the oxygen-evolving complex in photosystem II. The S3 state is first formed with two flashes at pH 6. Thereafter, the pH is changed in the dark prior to freezing of the sample. The EPR signal is 90-100 G wide and centered around g = 2. The signal is reversibly induced with a pK = 8.5 ± 0.3 and is very stable with a decay half-time of 5-6 min. If the pH is changed in the dark from pH 8.6 to 6.0, the signal disappears although the S3 state remains. We propose that the signal arises from the interaction between the Mn cluster and YZ, resulting in the spin-coupled S2YZ signal. Our data suggest that the potential of the YZ/YZ redox couple is sensitive to the ambient pH in the S3 state. The alkaline pH decreases the potential of the YZ/YZ couple so that YZ can give back an electron to the S3 state, thereby obtaining the S2YZ EPR signal. The tyrosine oxidation also involves proton release from YZ, and the results support a mechanism where this proton is released to the bulk medium presumably via a close-lying base. Thus, the equilibrium is changed from S3YZ to S2YZ by the alkaline pH. At normal pH (pH 5.5-7), this equilibrium is set strongly to the S3YZ state. The results are discussed in relation to the present models of water oxidation. Consequences for the relative redox potentials of YZ/YZ and S3/S2 at different pH values are discussed. We also compare the pH-induced S2YZ signal with the S2YZ signal from Ca2+-depleted photosystem II.
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| 3. |
- Han, Guangye, et al.
(författare)
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Direct quantification of the four individual S states in Photosystem II using EPR spectroscopy
- 2008
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1777:6, s. 496-503
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Tidskriftsartikel (refereegranskat)abstract
- Car, carotenoid; Chl, chlorophyll; ChlZ, secondary chlorophyll electron donor to P680+; Cytb559, cytochrome b559; EPR, electron paramagnetic resonance; DMSO, dimethylsulfoxide; MES, 2-(N-morpholino) ethanesulfonic acid; NIR, near-infrared; OEC, oxygen evolving complex; P680, primary electron donor chlorophylls in PSII; PpBQ, phenyl-p-benzoquinone; PSII, Photosystem II; QA and QB, primary and secondary plastoquinone acceptors of Photosystem II; YD, tyrosine 161 of the PSII D2 polypeptide; YZ, tyrosine 161 of the PSII D1 polypeptide
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| 4. |
- Ho, Felix M., et al.
(författare)
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Enhancement of Y-D(center dot) spin relaxation by the CaMn4 cluster in photosystem II detected at room temperature : A new probe for the S-cycle
- 2007
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1767:1, s. 5-14
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Tidskriftsartikel (refereegranskat)abstract
- The long-lived, light-induced radical Y-D(.) of the Tyr161 residue in the D2 protein of Photosystem 11 (PSII) is known to magnetically interact with the CaMn4 cluster, situated similar to 30 angstrom away. In this study we report a transient step-change increase in YD EPR intensity upon the application of a single laser flash to S, state-synchronised PSII-enriched membranes from spinach. This transient effect was observed at room temperature and high applied microwave power (100 mW) in samples containing PpBQ, as well as those containing DCMU. The subsequent decay lifetimes were found to differ depending on the additive used. We propose that this flash-induced signal increase was caused by enhanced spin relaxation of YD by the OEC in the S-2 state, as a consequence of the single laser flash turnover. The post-flash decay reflected S-2 -> S-1 back-turnover, as confirmed by their correlations with independent measurements of S-2 multiline EPR signal and flash-induced variable fluorescence decay kinetics under corresponding experimental conditions. This flash-induced effect opens up the possibility to study the kinetic behaviour of S-state transitions at room temperature using YD as a probe.
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| 5. |
- Morvaridi, Susan F
(författare)
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The Oxygen Evolving cycle in Photosystem II studied with EPR Spectroscopy at different pH and Temperature
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- PSII catalyses the light-induced water splitting process which leads to electron transfer through the thylakoid membrane and formation of energy rich molecules. Several redox components including a Mn4Ca, an amino acid residueYZ, two special Chl molecules P680, two Pheo and two quinone molecules participate in this process. Another component of PSII which interacts magnetically with Mn4Ca is D2-Tyr161 and is positioned symmetrically with YZ to P680+. The active site of this enzyme is the Mn4Ca cluster where two substrate water molecules bind/ oxidize, and an O2 is released. The Mn4Ca cluster oscillates through 5 semi-stable intermediate oxidation states (Si, i = 0-4), where the S1 state is the most dark-stable state and the S4 state is a transient state, where O2 is released. The EPR, laser flash and low temperature freezing techniques have been used to study the properties of semi-stable states of the Mn4Ca cluster at different pH and temperatures under experimental conditions. The flash-induced Si state turnover at different pHs revealed transition inhibition at extreme pH. The inhibition pKs in the acidic region are 4.0, 4.5 and 4.7 for S2/S3, S3/S0 and S0/S1 respectively. The inhibition pKs at alkaline pH are 9.4 and 8.0 for S2/S3 and S3/S0 respectively. The S0/S1 and S1/S2 transition are open at alkaline and both acidic and alkaline pH respectively. We suggest the inhibition pKs at acidic pH are due to: (1) protonation of carboxylic residues ligand to the Mn4Ca cluster and function at the proton transfer pathway from active site to the lumen; (2) increasing of YZ/YZox redox potential close to P680 by lowering pH where P680+ is no longer able to oxidize Yz. The inhibition of Si state turnover at alkaline pH is suggested to be due to reduction of the YZ/Yzox couple redox potential close to S3/S2 thereby unable to oxidize S2 state. This shift the equilibrium S3YZ ? S2YZ? to the right, radical split signal at high pH induced. The signal intensity increased with increasing pH to a maximum value around pH 8.5-8.9. A single pK was determined to be around 8.5. We suggest this proton originates from the His-YZ moiety and the YZ radical is trapped at high pH due to proton deficiency and lowering of Yz/YZox reduction potential. The oscillation of Si state of the Mn4Ca cluster at different temperatures at pH 6 revealed two types of miss: (1) real misses which have mechanistic origin are where no charge separation occurs upon the applied flash of light; (2) apparent misses, where a flash-induced charge-separated state of the reaction centre is lost due to recombination reactions at higher temperature. At optimal temperature, where the apparent misses are close to zero the misses are very Si state-dependent and they only originate from higher Si states. This is consistent with the higher activation energy of the Si state turnover at the higher oxidation state of the Mn4Ca. The YD residue is known to interact magnetically with the Mn4Ca at cryogenic temperature. We utilized the relaxation property of the YD radical at the S1 and S2 state to study S2 state decay kinetics after flash-induced S2 state formation at room temperature. The decay kinetics were found to be biphasic in the presence of both PpBQ and DCMU. The half- life time with PpBQ was about 0.7 s (10-15% of total amplitude) and 4 minutes (85-90%) and in the presence of DCMU was about 0.23 s (about 30%) and 1.8 s (70%).
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