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| 2. |
- Babcock, Gerald T, et al.
(författare)
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Tyrosyl radicals in enzyme catalysis: Some properties and a focus on photosynthetic water oxidation
- 1997
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Ingår i: Acta Chemica Scandinavica. - 0904-213X. ; 51:5, s. 533-540
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Tidskriftsartikel (refereegranskat)abstract
- Enzymes that require a redox-active amino acid for catalysis or function have emerged as a distinct class of proteins. For the tyrosine-based radical enzymes, we show that the spin-density distribution in the radical follows an odd alternate pattern that is invariant to within 10% across the class. General properties of the radical enzymes are summarized from which we conclude that their essential role in catalysis is to initiate substrate metabolism by hydrogen-atom abstraction. These ideas are extended to the Y-Z and Y-D tyrosines in Photosystem II and a radical-based hydrogen-atom abstraction model for water oxidation is discussed. Differences in rates of oxidation of Y-Z and Y-D by the reaction-center chlorophyll, P680(+), under various conditions, are considered and rationalized on the basis of changes in reorganization energy induced by the local protein structure and by the presence or absence of the (Mn)(4) cluster that binds substrate water.
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| 3. |
- Blomberg, Margareta R A, et al.
(författare)
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A Quantum Chemical Study of Hydrogen Abstraction from Manganese-Coordinated Water by a Tyrosyl Radical: A Model for Water Oxidation in Photosystem II
- 1997
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 119:35, s. 8285-8292
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Tidskriftsartikel (refereegranskat)abstract
- Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, TyrZ. This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstraction energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction model, the present calculations support this model. Possible functions of a coordinated chloride and a nearby calcium complex are suggested. Five- or six-coordination and ferro- or antiferromagnetic spin couplings of the manganese centers are discussed.
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| 4. |
- Styring, Stenbjörn, et al.
(författare)
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pH Dependence of the Donor Side Reactions in Ca2+-Depleted Photosystem II
- 2003
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 42:20, s. 6185-6192
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Tidskriftsartikel (refereegranskat)abstract
- We have studied how low pH affects the water-oxidizing complex in Photosystem II when depleted of the essential Ca2+ ion cofactor. For these samples, it was found that the EPR signal from the YZ radical decays faster at low pH than at high pH. At 20 C, YZ decays with biphasic kinetics. At pH 6.5, the fast phase encompasses about 65% of the amplitude and has a lifetime of ~0.8 s, while the slow phase has a lifetime of ~22 s. At pH 3.9, the kinetics become totally dominated by the fast phase, with more than 90% of the signal intensity operating with a lifetime of ~0.3 s. The kinetic changes occurred with an approximate pKa of 4.5. Low pH also affected the induction of the so-called split radical EPR signal from the S2YZ state that is induced in Ca2+-depleted PSII membranes because of an inability of YZ to oxidize the S2 state. At pH 4.5, about 50% of the split signal was induced, as compared to the amplitude of the signal that was induced at pH 6.5-7, using similar illumination conditions. Thus, the split-signal induction decreased with an apparent pKa of 4.5. In the same samples, the stable multiline signal from the S2 state, which is modified by the removal of Ca2+, was decreased by the illumination to the same extent at all pHs. It is proposed that decreased induction of the S2YZ state at lower pH was not due to inability to oxidize the modified S2 state induced by the Ca2+ depletion. Instead, we propose that the low pH makes YZ able to oxidize the S2 state, making the S2 S3 transition available in Ca2+-depleted PSII. Implications of these results for the catalytic role of Ca2+ and the role of proton transfer between the Mn cluster and YZ during oxygen evolution is discussed.
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| 5. |
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| 6. |
- Styring, Stenbjörn, et al.
(författare)
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The pH-dependence of the donor side reactions in Ca2+ -depleted Photosystem II: A role for Ca2+ in the proton expulsion pathway.
- 2001
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Ingår i: PS2001 Proceedings. - 0643067116
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Konferensbidrag (refereegranskat)abstract
- Removal of Ca2+ from PSII lead to impaired electron transfer from YZ to P680+ and to inhibition of S state transitions which results in a modified stable S2 state (probed by a stable multiline EPR signal) and the induction of the S2YZox state ("split" signal) by illumination at 0° C. We have performed an EPR study on the pH dependence of these reactions in Ca2+-depleted PSII (prepared by citrate treatment at pH 3.0). The stable multiline, the "split" signal and the induction and decay kinetics of YZox over a pH range from 3.5 to 8.0 were investigated. The stable S2 multiline signal showed no significant pH dependence. In contrast, the induction of the "split" signal (two forms of 110 G and 160 G wide were studied) was decreased by 50-60% with an apparent pK of 4.5 when the pH was decreased. The decay of the induced YZox at optimal pH values (pH 6-7) shows two kinetic phases with half times of about 0.5 and 20 s. This decay phases became of about 0.1 and 10 s at pH 4.0. Our results indicate that YZox is easier to reduce at low pH values in Ca2+- depleted PSII. Possible donors and the relevance to the proton translocation reactions during S cycle are discussed.
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| 7. |
- Tommos, Cecilia, et al.
(författare)
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Stepwise Disintegration of the Photosynthetic Oxygen-Evolving Complex
- 1998
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 120:40, s. 10441-10452
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Tidskriftsartikel (refereegranskat)abstract
- Photosynthetic water oxidation catalyzed by Photosystem II takes place at a site that comprises a redox-active tyrosine, YZ, a tetramanganese cluster, and, in addition to its redox components, two inorganic cofactors, calcium and chloride. Recent work suggests that YZ and the metal site are intimately linked in the oxidation and deprotonation reactions of substrate water. The metal cluster stores oxidizing equivalents and provides binding sites for the substrate from which YZ is proposed to abstract hydrogen atoms during the catalytic cycle of photosystem II. Intrinsic to this hydrogen-abstraction mechanism for water oxidation is an intimate structural and functional relationship between the metal site and YZ, which predicts that the local YZ environment will be sensitive to the composition and integrity of the metal cluster. To test this postulate, we have examined the YZ site and its status with respect to solvent exposure under varying degrees of disassembly of the oxygen-evolving complex. 1H2H-isotope exchange was carried out for various times in samples devoid of Mn, Ca2+, and Cl-, and in samples depleted exclusively of Ca2+. The YZ and S2YZ species were cryotrapped to high yield in these two preparations, respectively, and the radical site was characterized by using electron spin-echo envelope modulation spectroscopy. The isotope exchange at the YZ site was completed with an upper limit on the minutes time scale in both the (Mn)4-depleted and the Ca-depleted samples. The number of isotope-exchangeable protons in the site and their distances to YZ were found to be different in the two systems, indicating that YZ is shielded from the solvent in the Ca-depleted system and, upon removal of the (Mn)4 cluster, becomes accessible to bulk water. The results from an electron spin-echo analysis of S2YZ, in the weak-coupling limit, suggest that YZ in samples that retain the (Mn)4 cluster, but lack Ca2+, is involved in a bifurcated hydrogen bond. The data for both classes of samples are consistent with a hydrogen-abstraction function for YZ in water oxidation and provide insight into the light-driven assembly of the (Mn)4 cluster.
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