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- Gadjieva, Rena, et al.
(författare)
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Interconversion of Low- and High-Potential Forms of Cytochrome b559 in Tris-Washed Photosystem II Membranes under Aerobic and Anaerobic Conditions
- 1999
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 38:32, s. 10578-10584
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the reversible conversion between the high- (HP) and low-potential (LP) forms of Cytb559 has been analyzed in Tris-washed photosystem II (PSII) enriched membranes. These samples are deprived of the Mn cluster of the water-oxidizing complex (WOC) and the extrinsic regulatory proteins. The results obtained by application of optical and EPR spectroscopy reveal that (i) under aerobic conditions, the vast majority of Cytb559 exhibits a low midpoint potential, (ii) after removal of O2 in the dark, a fraction of Cytb559 is converted to the high-potential form which reaches level of about 25% of the total Cytb559, (iii) a similar dark transformation of LP HP Cytb559 occurs under reducing conditions (8 mM hydroquinone), (iv) under anaerobic conditions and in the presence of 8 mM hydroquinone, about 60% of the Cytb559 attains the HP form, (v) the interconversion is reversible with the re-establishment of aerobic conditions, and (vi) aerobic and oxidizing conditions (2 mM ferricyanide or 0.5 mM potassium iridate) induce a decrease of the amount of the HP form, also showing that the conversion is reversible. This reversible interconversion between LP and HP Cytb559 is not observed in PSII membrane fragments with an intact WOC. On the basis of these findings, the possibility is discussed that the O2-dependent conversion of Cytb559 in PSII complexes lacking a functionally competent WOC is related to a protective role of Cytb559 in photoinhibition and/or that it is involved in the regulation of the assembly of a competent water-oxidizing complex in PSII.
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- Noering, Birgit, et al.
(författare)
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Effects of methanol on the Si-state transitions in photosynthetic water-splitting
- 2008
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Ingår i: Photosynthesis Research. - : SPRINGER. - 0166-8595 .- 1573-5079. ; 98:1-3, s. 251-260
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Tidskriftsartikel (refereegranskat)abstract
- From a chemical point of view methanol is one of the closest analogues of water. Consistent with this idea EPR spectroscopy studies have shown that methanol binds at-or at least very close to-the Mn4OxCa cluster of photosystem II (PSII). In contrast, Clark-type oxygen rate measurements demonstrate that the O-2 evolving activity of PSII is surprisingly unaffected by methanol concentrations of up to 10%. Here we study for the first time in detail the effect of methanol on photosynthetic water-splitting by employing a Joliot-type bare platinum electrode. We demonstrate a linear dependence of the miss parameter for Si state advancement on the methanol concentrations in the range of 0-10% (v/v). This finding is consistent with the idea that methanol binds in PSII with similar affinity as water to one or both substrate binding sites at the Mn(4)OxCa cluster. The possibility is discussed that the two substrate water molecules bind at different stages of the cycle, one during the S-4 -> S-0 and the other during the S-2 -> S-3 transition.
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- Shevela, Dmitriy, et al.
(författare)
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Characterization of the water oxidizing complex of photosystem II of the Chl d-containing cyanobacterium Acaryochloris marina via its reactivity towards endogenous electron donors and acceptors
- 2006
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 8:29, s. 3460-3466
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Tidskriftsartikel (refereegranskat)abstract
- Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the Si state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different Si states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of Y-Z are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S-2 and S-3 states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S-2 and S-3 decay between A. marina cells and spinach thylakoids reflect a shift of the E-m of Y-D/Y-D(OX) to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S-0 oxidation to S-1 by Y-D(OX). A slightly increased double hit probability in A. marina cells is indicative of a faster Q(A)(-) to Q(B) electron transfer in these cells compared to spinach thylakoids.
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