| 1. |
- Deák, Zsuzsanna, et al.
(författare)
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Methanol modification of the electron paramagnetic resonance signals from the S0 and S2 states of the water-oxidizing complex of Photosystem II
- 1999
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - 0005-2728. ; 1412:3, s. 240-249
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Tidskriftsartikel (refereegranskat)abstract
- The Mn-derived electron paramagnetic resonance (EPR) multiline signal from the S0 state of the water-oxidizing complex is observable only in the presence methanol. In the present study, we have characterized the effect of methanol on the EPR signals from the S0 and S2 states as well as on the EPR Signal IIslow originating from the TyrosineDox radical. The amplitudes of the S0 and S2 multiline signals increase with the methanol concentration in a similar way, whereas the S2 g=4.1 excited state signal amplitude shows a concomitant decrease. The methanol concentration at which half of the spectral change has occurred is ~0.2% and the effect is saturating around 5%. Methanol has an effect on the microwave power saturation of the S2 multiline signal, as well. The microwave power at half saturation (P1/2) is 85 mW in the presence of methanol, whereas the signal relaxes much slower (P1/2~27 mW) without. The relaxation of Signal IIslow in the presence of methanol has also been investigated. The P1/2 value of Signal IIslow oscillates with the S cycle in a similar way as without methanol, but the P1/2 values are consistently lower in the methanol-containing samples. From the results, we conclude that methanol modifies the magnetic properties of the S0 and S2 states in a similar way. The possible site and nature of methanol binding is discussed.
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| 2. |
- Geijer, Paulina, et al.
(författare)
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Comparative studies of the S0 and S2 multiline electron paramagnetic resonance signals from the manganese cluster in Photosystem II
- 2001
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - 0005-2728. ; 1503:1-2, s. 83-95
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Tidskriftsartikel (refereegranskat)abstract
- Electron paramagnetic resonance (EPR) spectroscopy is one of the major techniques used to analyse the structure and function of the water oxidising complex (WOC) in Photosystem II. The discovery of an EPR signal from the S0 state has opened the way for new experiments, aiming to characterise the S0 state and elucidate the differences between the different S states. We present a review of the biochemical and biophysical characterisation of the S0 state multiline signal that has evolved since its discovery, and compare these results to previous and recent data from the S2 multiline signal. We also present some new data from the S2 state reached on the second turnover of the enzyme.
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| 3. |
- Geijer, Paulina
(författare)
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Effects of Protons and Small Alcohols on the Oxygen-Evolving Complex of Photosystem II
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Higher plants and algae use oxygenic photosynthesis to convert solar energy to reducing equivalents and chemical energy. The ultimate electron donor to oxygenic photosynthesis is water. Oxidation of water to oxygen occurs in the oxygen-evolving complex (OEC) in Photosystem II (PSII). The OEC is composed of four Mn atoms, Ca2+ and Cl-. The OEC cycles through five states (S0 - S4) during oxidation of water to oxygen. The OEC can be studied with Electron Paramagnetic Resonance (EPR) spectroscopy. The shape of the EPR signals depends on the internal magnetic couplings between the Mn atoms and their ligand environment. To detect the EPR signal from the S0 state it is necessary to add a few percent of methanol. The saturation of the S0 multiline signal amplitude with increasing methanol concentration was compared to the saturation of the S2 multiline signal amplitude. The S0 and S2 multiline signals show the same saturation behaviour. It was concluded that methanol affects the S0 and S2 states in a similar way. The effect of methanol has given new spectroscopic probes of the Mn cluster of the OEC. These can be used to further understand how the OEC functions. The second modification of the OEC was a change of the ambient pH. The pH is important for efficient electron transfer through PSII and optimal oxygen evolution. Acidic and alkaline pH-values were shown to decrease the amplitudes of the S0 and S2 multiline signals. The decrease of the amplitudes is reversible. The alkaline pH-treatment also induced a signal from the OEC in the formal S3 state. The signal arises from the magnetic interaction between the Mn cluster in the S2 state and the YZ-radical. It was concluded that at acidic pH the Mn cluster or a ligand to the Mn cluster becomes protonated and this changes the magnetic couplings of the Mn cluster. In the S0 and S2 states, either the Mn cluster or YZ lowers the redox potential by the alkaline pH-treatment. In the S3 state, it is only YZ that lowers the redox potential. It is proposed that the difference between the S-states is due to the accessibility of protons. This shows that YZ has a very central role in the oxidation of water. The investigations presented in this thesis also shows that both the Mn cluster and YZ are solvent accessible.
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| 4. |
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| 5. |
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| 6. |
- Geijer, Paulina, et al.
(författare)
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Proton Equilibria in the Manganese Cluster of Photosystem II Control the Intensities of the S0 and S2 State g ~ 2 Electron Paramagnetic Resonance Signals
- 2000
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 39:23, s. 6763-6772
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the pH effect on the S0 and S2 multiline electron paramagnetic resonance (EPR) signals from the water-oxidizing complex of photosystem II. Around pH 6, the maximum signal intensities were detected. On both the acidic and alkaline sides of pH 6, the intensities of the EPR signals decreased. Two pKs were determined for the S0 multiline signal; pK1 = 4.2 ± 0.2 and pK2 = 8.0 ± 0.1, and for the S2 multiline signal the pKs were pK1 = 4.5 ± 0.1 and pK2 = 7.6 ± 0.1. The intensity of the S0-state EPR signal was partly restored when the pH was changed from acidic or alkaline pH back to pH ~ 6. In the S2 state we observed partial recovery of the multiline signal when going from alkaline pH back to pH 6, whereas no significant recovery of the S2 multiline signal was observed when the pH was changed from acidic pH back to pH 6. Several possible explanations for the intensity changes as a function of pH are discussed. Some are ruled out, such as disintegration of the Mn cluster or decay of the S states and formal Cl- and Ca2+ depletion. The altered EPR signal intensities probably reflect the protonation/deprotonation of ligands to the Mn cluster or the oxo bridges between the Mn ions. Also, the possibility of decreased multiline signal intensities at alkaline pH as an effect of changed redox potential of YZ is put forward.
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| 7. |
- Geijer, Paulina, et al.
(författare)
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Simultaneous detection of spin-coupled and decoupled QA- EPR-signals in Photosystem II complexes isolated with isoelectric focusing
- 1998
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Ingår i: Photosynthesis Research. - 0166-8595. ; 58:3, s. 231-243
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Tidskriftsartikel (refereegranskat)abstract
- The Photosystem II multisubunit protein complex can be extracted from thylakoid membranes with non-ionic detergents and subjected to various spectroscopical and biochemical investigations. This paper shows that after extraction with dodecyl-beta-D-maltoside, several Photosystem II complexes could be resolved by isoelectric focusing. Structurally, the various Photosystem II complexes differed from each other in polypeptide composition, especially with regard to the chlorophyll a/b-binding proteins, which gave rise to differing isoelectric points. Functionally, the various Photosystem II complexes differed from each other on the acceptor side, as judged by acceptor side-dependent electron transfer and electron paramagnetic resonance (EPR). The Q_{A}^- Fe2+-signal (g = 1.84), arising from Q_{A}^- spin-coupled to the acceptor-side iron, and a radical signal arising from decoupled Q_{A}^- (g = 2.0045) could be detected simultaneously in some of the Photosystem II complexes, and the amount of each of the two signals were inversely related. The results are discussed in relation to previously known heterogeneities in Photosystem II.
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| 8. |
- 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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| 9. |
- Morvaridi, Fatemeh, et al.
(författare)
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pH-dependent oxidation of Cytochrome b559 is different in the different S-states
- 2001
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Ingår i: PS2001 Proceedings. - 0643067116
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Konferensbidrag (refereegranskat)abstract
- We have studied the state of Cyt b559 and the Mn cluster by EPR and optical spectroscopy in the different S-states between pH 4.0-10.0. In these experiments, the pH was changed after the OEC turnover to the appropriate S-state (at pH 6.0) by 0-3 saturating flashes. Furthermore, the state of Cyt b559 after a subsequent flash given after the pH modification was also investigated. The results show that low (< pH 6.0) and high pH (> pH 9.0) resulted in an increase in the oxidized form of Cyt b559 in the S2 and S3 states. The effect was more pronounced at low than high pH. There was no further oxidation after the subsequent flash. Different to this, the pH induced oxidized Cyt b559 population in the S0 and S1 states was significantly smaller. In the S1 state, the pH dependent oxidation of Cyt b559 was very weak, with a slight increase from pH 10.0 to 4.0. A subsequent flash in S0 or S1 resulted in a significant Cyt b559 oxidation at both low and high pH’s. The ratio of the high and low potential forms of Cyt b559 at different pH’s was also different in different S-states. At low and normal pH’s both oxidized LP and HP forms were observed, while at high pH’s, only the LP form was oxidized. The possible involvement of Cyt b559 in electron donation to the Mn cluster during S-cycle at extreme pH is discussed.
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