| 1. |
- Shevela, Dmitriy, et al.
(författare)
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Membrane-inlet mass spectrometry reveals a high driving force for oxygen production by photosystem II
- 2011
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : NATL ACAD SCIENCES. - 0027-8424 .- 1091-6490. ; 108:9, s. 3602-3607
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Tidskriftsartikel (refereegranskat)abstract
- Oxygenic photosynthesis is the basis for aerobic life on earth. The catalytic Mn4OxCaYZ center of photosystem II (PSII), after fourfold oxidation, extracts four electrons from two water molecules to yield dioxygen. This reaction cascade has appeared as a single four-electron transfer that occurs in typically 1 ms. Inevitable redox intermediates have so far escaped detection, probably because of very short lifetime. Previous attempts to stabilize intermediates by high O-2-back pressure have revealed controversial results. Here we monitored by membrane-inlet mass spectrometry (MIMS) the production of O-18(2) from O-18-labeled water against a high background of O-16(2) in a suspension of PSII-core complexes. We found neither an inhibition nor an altered pattern of O-2 production by up to 50-fold increased concentration of dissolved O-2. Lack of inhibition is in line with results from previous X-ray absorption and visible-fluorescence experiments, but contradictory to the interpretation of previous UV-absorption data. Because we used essentially identical experimental conditions in MIMS as had been used in the UV work, the contradiction was serious, and we found it was not to be resolved by assuming a significant slowdown of the O-2 release kinetics or a subsequent slow conformational relaxation. This calls for reevaluation of the less direct UV experiments. The direct detection of O-2 release by MIMS shows unequivocally that O-2 release in PSII is highly exothermic. Under the likely assumption that one H+ is released in the S-4 -> S-0 transition, the driving force at pH 6.5 and atmospheric O-2 pressure is at least 220 meV, otherwise 160 meV.
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| 2. |
- Shutova, Tatiana, 1964-, et al.
(författare)
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The photosystem II-associated Cah3 in Chlamydomonas enhances the O-2 evolution rate by proton removal
- 2008
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Ingår i: EMBO Journal. - : Wiley. - 0261-4189 .- 1460-2075. ; 27:5, s. 782-791
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Tidskriftsartikel (refereegranskat)abstract
- Water oxidation in photosystem II ( PSII) is still insufficiently understood and is assumed to involve HCO3-. A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O-2 evolution in the absence of HCO3-. The O-2 evolution for saturating, continuous illumination (R-O2) was slower than in the wild type, but was elevated by HCO3- and increased further by Cah3. The R-O2 limitation in the absence of Cah3/HCO3- was amplified by H2O/D2O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO3- in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO3- effect at the donor side of PSII. Time-resolved delayed fluorescence and O-2-release measurements suggest specific effects on proton-release steps but not on electron transfer. We propose that Cah3 promotes proton removal from the Mn complex by locally providing HCO3-, which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O-2 formation and thus, limit water oxidation under high light. Our results underlie the general importance of proton release at the donor side of PSII during water oxidation.
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