| 1. |
|
|
| 2. |
- Deák, Zsuzsanna, et al.
(författare)
-
Methanol modification of the electron paramagnetic resonance signals from the S0 and S2 states of the water-oxidizing complex of Photosystem II
- 1999
-
Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - 0005-2728. ; 1412:3, s. 240-249
-
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.
|
|
| 3. |
- Geijer, Paulina, et al.
(författare)
-
Comparative studies of the S0 and S2 multiline electron paramagnetic resonance signals from the manganese cluster in Photosystem II
- 2001
-
Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - 0005-2728. ; 1503:1-2, s. 83-95
-
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.
|
|
| 4. |
- Peterson Årsköld, Sindra, et al.
(författare)
-
EPR studies of the oxygen-evolving complex reveal a light-adaption process in Photosystem II
- 2001
-
Ingår i: PS2001 Proceedings. - 0643067116
-
Konferensbidrag (refereegranskat)abstract
- Photosystem II utilizes solar energy to drive electrons from the Mn cluster at the lumenal side to the quinone at the stromal side of the thylakoid membrane. The source of electrons is H2O, which is split to oxygen by the OEC. The water-splitting process involves cycling of the Mn-cluster through four semi-stable oxidation states, termed the S0, S1, S2, and S3 states. The OEC can be trapped in the different S-states, and studied by EPR techniques. The S2 state is often obtained by exposing a dark-adapted (S1 state) sample to continuous illumination at low temperature, or to a single flash of light at room temperature followed by freezing. Both procedures capture the S2 state, as reached by a single oxidation of the OEC following a period of dark-adaptation. With a laser flash procedure, the S2 state can be obtained a second time after 5 flashes, when the S-cycle has been completed once. We have discovered differences between the S2 state formed after one flash and the S2 state formed after five flashes, in terms of the relaxation behavior of the S2 multiline EPR signal. These data indicate a change within the Mn cluster that builds up during the first turnovers after dark-adaptation. Pulsed field-swept spectra of samples given 0-5 flashes of light show a similar trend: the S1 state obtained by zero and four flashes differ significantly from one another. Based on the data presented and similar reports in the literature, we propose that the OEC undergoes a light-adaptation process during the first two turnovers after dark-adaptation, adjusting the system for efficient continuous water-splitting. We tentatively suggest that light-adaptation involves rearrangements of the proton network in the OEC, possibly as a means of setting up proton channels.
|
|
| 5. |
- Peterson Årsköld, Sindra, et al.
(författare)
-
Flash-induced relaxation changes of the EPR signals from the manganese cluster and YD reveal a light-adaptation process of Photosystem II
- 2003
-
Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 42:9, s. 2748-2758
-
Tidskriftsartikel (refereegranskat)abstract
- By exposing photosystem II (PSII) samples to an incrementing number of excitation flashes at room temperature, followed by freezing, we could compare the Mn-derived multiline EPR signal from the S-2 oxidation state as prepared by 1, 5, 10, and 25 flashes of light. While the S-2 multiline signals exhibited by these samples differed very little in spectral shape, a significant increase of the relaxation rate of the signal was detected in the multiflash samples as compared to the S-2-state produced by a single oxidation. A similar relaxation rate increase was observed for the EPR signal from Y-D(.). The temperature dependence of the multiline spin-lattice relaxation rate is similar after 1 and 5 flashes. These data are discussed together with previously reported phenomena in terms of a light-adaptation process of PSII, which commences on the third flash after dark-adaptation and is completed after 10 flashes. At room temperature, the fast-relaxing, light-adapted state falls back to the slow-relaxing, dark-adapted state with t(1/2) = 80 s. We speculate that light-adaptation involves changes necessary for efficient continuous water splitting. This would parallel activation processes found in many other large redox enzymes, such as Cytochrome c oxidase and Ni-Fe hydrogenase. Several mechanisms of light-adaptation are discussed, and we find that the data may be accounted for by a change of the PSII protein matrix or by the light-induced appearance of a paramagnetic center on the PSII donor side. At this time, no EPR signal has been detected that correlates with the increase of the relaxation rates, and the nature of such a new paramagnet remains unclear. However, the relaxation enhancement data could be used, in conjunction with the known Mn-Y-D distance, to estimate the position of such an unknown relaxer. If positioned between Y-D and the Mn cluster, it would be located 7-8 Angstrom from the spin center of the S-2 multiline signal.
|
|
| 6. |
- Peterson Årsköld, Sindra, et al.
(författare)
-
The EPR Signals from the S0 and S2 States of the Mn Cluster in Photosystem II Relax Differently
- 1999
-
Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 38:46, s. 15223-15230
-
Tidskriftsartikel (refereegranskat)abstract
- The oxygen evolving complex (OEC) of photosystem II (PSII) gives rise to manganese-derived electron paramagnetic resonance (EPR) signals in the S0 and S2 oxidation states. These signals exhibit different microwave power saturation behavior between 4 and 10 K. Below 8 K, the S0 state EPR signal is a faster relaxer than the S2 multiline signal, but above 8 K, the S0 signal is the slower relaxer of the two. The different temperature dependencies of the relaxation of the S0 and S2 ground-state Mn signals are due to differences in the spin-lattice relaxation process. The dominating spin-lattice relaxation mechanism is concluded to be a Raman mechanism in the S0 state, with a T4.1 temperature dependence of the relaxation rate. It is proposed that the relaxation of the S2 state arises from a Raman mechanism as well, with a T6.8 temperature dependence of the relaxation rate, although the data also fit an Orbach process. If both signals relax through a Raman mechanism, the different exponents are proposed to reflect structural differences in the proteins surrounding the Mn cluster between the S0 and S2 states. The saturation of SIIslow from the YDox radical on the D2 protein was also studied, and found to vary between the S0 and the S2 states of the enzyme in a manner similar to the EPR signals from the OEC. Furthermore, we found that the S2 multiline signal in the second turnover of the enzyme is significantly more difficult to saturate than in the first turnover. This suggests differences in the OEC between the first and second cycles of the enzyme. The increased relaxation rate may be caused by the appearance of a relaxation enhancer, or it may be due to subtle structural changes as the OEC is brought into an active state.
|
|
| 7. |
- Åhrling, Karin A., et al.
(författare)
-
An oscillating manganese electron paramagnetic resonance signal from the S0 state of the oxygen evolving complex in Photosystem II
- 1997
-
Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 36:43, s. 13148-13152
-
Tidskriftsartikel (refereegranskat)abstract
- Photosynthesis produces the oxygen necessary for all aerobic life. During this process, the manganese-containing oxygen evolving complex (OEC) in photosystem II (PSII), cycles through five oxidation states, S0-S4. One of these, S2, is known to be paramagnetic and gives rise to electron paramagnetic resonance (EPR) signals used to probe the catalytic structure and function of the OEC. The S0 state has long been thought to be paramagnetic. We report here a Mn EPR signal from the previously EPR invisible S0 state. The new signal oscillates with a period of four, indicating that it originates from fully active PSII centers. Although similar to the S2 state multiline signal, the new signal is wider (2200 gauss compared with 1850 gauss in samples produced by flashing), with different peak intensity and separation (82 gauss compared with 89 gauss). These characteristics are consistent with the S0 state EPR signal arising from a coupled MnII-MnIII intermediate. The new signal is more stable than the S2 state signal and its decay in tens of minutes is indicative of it originating from the S0 state. The S0 state signal will provide invaluable information toward the understanding of oxygen evolution in plants.
|
|
| 8. |
- Åhrling, Karin A, et al.
(författare)
-
Light adaptation of Photosystem II is mediated by the plastoquinone pool.
- 2003
-
Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 42:25, s. 7655-7662
-
Tidskriftsartikel (refereegranskat)abstract
- During the first few enzymatic turnovers after dark adaptation of photosystem II (PSII), the relaxation rate of the EPR signals from the Mn cluster and YD(dot) are significantly enhanced. This light-adaptation process has been suggested to involve the appearance of a new paramagnet on the PSII donor side [Peterson, Åhrling, Hogblom, and Styring, Biochemistry 2003, asap]. In the present study, a correlation is established between the observed relaxation enhancement and the redox state of the quinone pool. It is shown that the addition of quinol to dark-adapted PSII membrane fragments induces relaxation enhancement already after a single oxidation of the Mn, comparable to that observed after five oxidations in samples with quinones (PPBQ or DQ) added. The saturation behavior of YD(dot) revealed that with quinol added in the dark, a single flash was necessary for the relaxation enhancement to occur. The quinol-induced relaxation enhancement of PSII was also activated by illumination at 200 K. Whole thylakoids, with no artificial electron acceptor present but with an intact plastoquinone pool, displayed the same relaxation enhancement on the fifth flash as membrane fragments with exogenous quinones present. We conclude that (i) reduction of the quinone pool induces the relaxation enhancement of the PSII donor-side paramagnets, (ii) light is required for the quinol to effect the relaxation enhancement, and (iii) light-adaptation occurs in the intact thylakoid system, when the endogenous plastoquinone pool is gradually reduced by PSII turnover. It seems clear that a species on the PSII donor side is reduced by the quinol, to become a potent paramagnetic relaxer. On the basis of XANES reports, we suggest that this species may be the Mn ions not involved in the cyclic redox changes of the oxygen-evolving complex.
|
|
| 9. |
|
|
| 10. |
|
|
