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| 3. |
- Hillier, W, et al.
(författare)
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Kinetic determination of the fast exchanging substrate water molecule in the S3 state of photosystem II
- 1998
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Ingår i: Biochemistry. - Australian Natl Univ, Res Sch Biol Sci, Canberra, ACT 0200, Australia. Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley, CA 94720 USA. : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 37:48, s. 16908-16914
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Tidskriftsartikel (refereegranskat)abstract
- In a previous communication we showed from rapid isotopic exchange measurements that the exchangeability of the substrate water at the water oxidation catalytic site in the S-3 State undergoes biphasic kinetics although the fast phase could not be fully resolved at that time Flessinger, J., Badger, M., and Wydrzynski, T. (1995) Proc. Natl. Acad. Sci, U.S.A. 92, 3209-3213]. We have since improved the time resolution for these measurements by a further factor of 3 and report here the first detailed kinetics for the fast phase of exchange. First-order exchange kinetics were determined from mass spectrometric measurements of photogenerated O-2 as a function of time after injection of (H2O)-O-18 into spinach thylakoid samples preset in the S-3 State at 10 degrees C. For measurements made at m/e = 34 (i.e., for the mixed labeled O-16,18(2) product), the two kinetic components are observed: a slow component with k(1) = 2.2 +/- 0.1 s(-1) (t(1/2) similar to 315 ms) and a fast component with k(2) = 38 +/- 4 s(-1) (t(1/2) similar to 18 ms). When the isotopic exchange is measured at m/e = 36 (i.e,, for the double labeled O-18,18(2) product), only the slow component (k(1)) is observed, clearly indicating that the substrate water undergoing slow isotopic exchange provides the rate-limiting step in the formation of the double labeled O-18,18(2) product. When the isotopic exchange is measured as a function of temperature, the two kinetic components reveal different temperature dependencies in which k(1) increases by a factor of 10 over the range 0-20 degrees C while k(2) increases by only a factor of 3. Assuming simple Arrhenius behavior, the activation energies are estimated to be 78 +/- 10 kJ mol(-1) for the slow component and 39 +/- 5 kJ mol(-1) for the fast component. The different kinetic components in the O-18 isotopic exchange provide firm evidence that the two substrate water molecules undergo separate exchange processes at two different chemical sites in the S-3 state, prior to the O-2 release step (t(1/2) similar to 1 ms at 20 degrees C). The results are discussed in terms of how the substrate water may be bound at two separate metal sites.
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| 5. |
- KEBEKUS, U, et al.
(författare)
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STRUCTURAL-CHANGES IN THE WATER-OXIDIZING COMPLEX MONITORED VIA THE PH-DEPENDENCE OF THE REDUCTION RATE OF REDOX STATE S-1 BY HYDRAZINE AND HYDROXYLAMINE IN ISOLATED SPINACH THYLAKOIDS
- 1995
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Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 34:18, s. 6175-6182
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Tidskriftsartikel (refereegranskat)abstract
- A detailed kinetic analysis is presented for the pH dependence of the reduction of the water-oxidizing complex (WOC) in redox state S-1 by hydrophilic amines NH(2)R (R = NH2, OH) in suspensions of isolated thylakoids. Measurements of patterns of the oxygen yield induced by a train of single-turnover flashes and evaluation of the data within the framework of an extended Kok model [Messinger, J., Wacker, U., and Renger, G. (1991) Biochemistry 30, 7852-7862] led to the following results: (a) the rate constants k(s1)(NH(2)R) exhibit strikingly similar pH dependencies for NH2OH and NH2NH2 with ''titration waves'' at pH 5.3-5.6; 6.2-6.5, and above a critical pH value of about 7.4; (b) the differences in the reaction mechanism between NH2OH (1-electron reduction) and NH2NH2 (2-electron reduction) are almost pH-independent; (c) the ratio of the rate constants, k(s1)(NH2OH)/k(s1)(NH2NH2), decreases by a factor of about 9 within the range 5 < pH < 8.5. A detailed analysis reveals that these data cannot be consistently explained by the assumption that the unprotonated forms NH2OH and NH2NH2 are the active species while the protonated cations [NH3OH](+) and [N2H5](+) are nonreactive. A quantitative description is achieved by the additional postulate that pH-dependent structural changes take place in the WOC, thereby modulating the reactivity toward exogenous redox active amines of the type NH(2)R. On the basis of the results of this study and a recent report [Messinger, J., and Renger, G. (1994) Biochemistry 33, 10896-10905], it is inferred that the WOC undergoes three specific structural changes, with characteristic pH values of 5.3-5.5,6.2-6.5, and above 7.4.
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| 6. |
- Messinger, Johannes, 1963-, et al.
(författare)
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Detection of an EPR multiline signal for the S-0 state in photosystem II
- 1997
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Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 36:37, s. 11055-11060
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Tidskriftsartikel (refereegranskat)abstract
- The S-0* state was generated by incubation of dark-adapted (S-1 state) photosystem II membranes either with the exogenous two electron reductant hydrazine and subsequent 273 K illumination in the presence of DCMU or by dark incubation with low amounts of the one electron reductant hydroxylamine. In agreement with earlier reports, the S-1 and S-1 states were found to be electron paramagnetic resonance (EPR) silent. However, in the presence of 0.5-1.5% methanol, a weak EPR multiline signal centered around g = 2.0 was observed at 7 K for the S-0* states generated by both procedures. This signal has a similar average line splitting to the well-characterized S-2 state multiline EPR signal, but can be clearly distinguished from that and other modified S-2 multiline signals by differences in line position and intensities. In addition, at 4 K it can be seen that the S-0* multiline has a greater spectral breadth than the S-2 multilines and is composed of up to 26 peaks. The S-0* signal is not seen in the absence of methanol and is not affected by 1 mM EDTA in the buffer medium. We assign the S-0* multiline signal to the manganese cluster of the oxygen evolving complex in a mixed valence state of the form (MnMnMnMnIII)-Mn-II-Mn-III-Mn-III, (MnMnMnMnIV)-Mn-II-Mn-III-Mn-IV, or (MnMnMnMnIV)-Mn-III-Mn-III-Mn-III. Addition of methanol may be helpful in future to find an EPR signal originating from the natural S-0 state.
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| 7. |
- Messinger, Johannes, 1963-, et al.
(författare)
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DETECTION OF ONE SLOWLY EXCHANGING SUBSTRATE WATER MOLECULE IN THE S-3 STATE OF PHOTOSYSTEM-II
- 1995
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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. ; 92:8, s. 3209-3213
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Tidskriftsartikel (refereegranskat)abstract
- The exchangeability of the substrate water molecules at the catalytic site of water oxidation in photosystem II has been probed by isotope-exchange measurements using mass spectrometric detection of flash-induced oxygen evolution. A stirred sample chamber was constructed to reduce the lag time between injection of (H2O)-O-18 and the detecting flash by a factor of more than 1000 compared to the original experiments by R. Radmer and O. Ollinger [(1986) FEBS Lett. 195, 285-289]. Our data show that there is a slow (t(1/2) approximate to 500 ms, 10 degrees C) and a fast (t(1/2) < 25 ms, 10 degrees C) exchanging substrate water molecule in the S-3 State of photosystem II. The slow exchange is coupled with an activation energy of about 75 kJ/mol and is discussed in terms of a terminal manganese oxo ligand, while the faster exchanging substrate molecule may represent a water molecule not directly bound to the manganese center.
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| 8. |
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| 9. |
- Messinger, Johannes, 1963-, et al.
(författare)
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S-3 state of the water oxidase in photosystem II
- 1997
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Ingår i: Biochemistry. - : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 36:23, s. 6862-6873
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Tidskriftsartikel (refereegranskat)abstract
- The effect of the reductant hydrazine on the flash-induced oxygen oscillation patterns of spinach thylakoids was used to characterize a new super-reduced redox state of the water oxidase in photosystem II. The formation of a discrete S-3 state is evident from the shift of the first maximum of oxygen evolution from the 3rd flash through the 5th flash to the 7th flash during a 90 min incubation of dark-adapted thylakoids with 10 mM hydrazine sulfate at pH 6.8 on ice. A distinct period four oscillation with further maxima on the 11th and 15th flashes is still observed at this stage of the incubation. The data analysis within the framework of an extended Kok model reveals that a S-3 state population of almost 50% can be achieved by this treatment. A prolonged incubation of the S-3 sample with 10 mM hydrazine (and even 100 mM) does not lead to a further shift of the first maximum toward the 9th flash that could reflect the formation of the S-5 state. Instead, a slow oxidation of S-3 to S-2 takes place by an as yet unidentified electron acceptor. A consistent simulation of all the measured oxygen oscillation patterns of this study could, however, only be achieved by including the formal redox states S-4 and S-5 in the fits (S-4 + S-5 up to 35%). The implications of these findings for the oxidation states of the manganese in the tetranuclear cluster of the water oxidase are discussed.
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| 11. |
- Wydrzynski, T, et al.
(författare)
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On the functional significance of substrate accessibility in the photosynthetic water oxidation mechanism
- 1996
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Ingår i: Physiologia Plantarum. - : MUNKSGAARD INT PUBL LTD. - 0031-9317 .- 1399-3054. ; 96:2, s. 342-350
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Tidskriftsartikel (refereegranskat)abstract
- Recent evidence suggests that after selective perturbation of the protein structure in photosystem II, hydrogen peroxide is formed at the water oxidation catalytic site instead of molecular oxygen. In this communication, we review the interpretation of these observations and elaborate on the hypothesis that an essential factor in determining the end-product of photosynthetic water oxidation is one of substrate accessibility. It is argued that normally the access of water to the catalytic site is controlled by a hypdrophobic domain in the surrounding protein matrix and that the production of O-2 is optimized by an ordered binding of the two substrate water molecules. It is proposed that upon perturbation of the hydrophobic domain (for example, through the removal of various extrinsic proteins) the catalytic site becomes exposed to excess water from the external solvent phase. As a consequence, additional water binds at the catalytic site during intermediate oxidation steps and undergoes a partial oxidation reaction to form hydrogen peroxide. The importance of water accessibility to the structure/function relationships of photosystem II is discussed, particularly with respect to photoinhibitory damage through the formation of hydrogen peroxide.
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