| 1. |
- Cox, Nicholas, et al.
(författare)
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Effect of Ca(2+)/Sr(2+) substitution on the electronic structure of the oxygen-evolving complex of photosystem II : a combined multifrequency EPR, (55)Mn-ENDOR, and DFT study of the S(2) State
- 2011
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 133:10, s. 3635-3648
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structures of the native Mn(4)O(x)Ca cluster and the biosynthetically substituted Mn(4)O(x)Sr cluster of the oxygen evolving complex (OEC) of photosystem II (PSII) core complexes isolated from Thermosynechococcus elongatus, poised in the S(2) state, were studied by X- and Q-band CW-EPR and by pulsed Q-band (55)Mn-ENDOR spectroscopy. Both wild type and tyrosine D less mutants grown photoautotrophically in either CaCl(2) or SrCl(2) containing media were measured. The obtained CW-EPR spectra of the S(2) state displayed the characteristic, clearly noticeable differences in the hyperfine pattern of the multiline EPR signal [Boussac et al. J. Biol. Chem.2004, 279, 22809-22819]. In sharp contrast, the manganese ((55)Mn) ENDOR spectra of the Ca and Sr forms of the OEC were remarkably similar. Multifrequency simulations of the X- and Q-band CW-EPR and (55)Mn-pulsed ENDOR spectra using the Spin Hamiltonian formalism were performed to investigate this surprising result. It is shown that (i) all four manganese ions contribute to the (55)Mn-ENDOR spectra; (ii) only small changes are seen in the fitted isotropic hyperfine values for the Ca(2+) and Sr(2+) containing OEC, suggesting that there is no change in the overall spin distribution (electronic coupling scheme) upon Ca(2+)/Sr(2+) substitution; (iii) the changes in the CW-EPR hyperfine pattern can be explained by a small decrease in the anisotropy of at least two hyperfine tensors. It is proposed that modifications at the Ca(2+) site may modulate the fine structure tensor of the Mn(III) ion. DFT calculations support the above conclusions. Our data analysis also provides strong support for the notion that in the S(2) state the coordination of the Mn(III) ion is square-pyramidal (5-coordinate) or octahedral (6-coordinate) with tetragonal elongation. In addition, it is shown that only one of the currently published OEC models, the Siegbahn structure [Siegbahn, P. E. M. Acc. Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al. Phys. Chem. Chem. Phys.2009, 11, 6788-6798], is consistent with all data presented here. These results provide important information for the structure of the OEC and the water-splitting mechanism. In particular, the 5-coordinate Mn(III) is a potential site for substrate 'water' (H(2)O, OH(-)) binding. Its location within the cuboidal structural unit, as opposed to the external 'dangler' position, may have important consequences for the mechanism of O-O bond formation.
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| 2. |
- Su, Ji-Hu, et al.
(författare)
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The electronic structures of the S(2) states of the oxygen evolving complexes of photosystem II in plants and cyanobacteria in the presence and absence of methanol
- 2011
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Ingår i: Biochimica et Biophysica Acta. - Amsterdam : Elsevier. - 0006-3002 .- 1878-2434 .- 0005-2728 .- 1879-2650. ; 1807:7, s. 829-840
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties of the Mn(4)O(x)Ca cluster in the S(2) state of the oxygen evolving complex (OEC) were studied using X- and Q-band EPR and Q-band (55)Mn-ENDOR using photosystem II preparations isolated from the thermophilic cyanobacterium T. elongatus and higher plants (spinach). The data presented here show that there is very little difference between the two species. Specifically it is shown that: (i) only small changes are seen in the fitted isotropic hyperfine values, suggesting that there is no significant difference in the overall spin distribution (electronic coupling scheme) between the two species; (ii) the inferred fine-structure tensor of the only Mn(III) ion in the cluster is of the same magnitude and geometry for both species types, suggesting that the Mn(III) ion has the same coordination sphere in both sample preparations; and (iii) the data from both species are consistent with only one structural model available in the literature, namely the Siegbahn structure [Siegbahn, P. E. M. Accounts Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al., Phys. Chem. Chem. Phys.2009, 11, 6788-6798]. These measurements were made in the presence of methanol because it confers favorable magnetic relaxation properties to the cluster that facilitate pulse-EPR techniques. In the absence of methanol the separation of the ground state and the first excited state of the spin system is smaller. For cyanobacteria this effect is minor but in plant PS II it leads to a break-down of the S(T)=½ spin model of the S(2) state. This suggests that the methanol-OEC interaction is species dependent. It is proposed that the effect of small organic solvents on the electronic structure of the cluster is to change the coupling between the outer Mn (Mn(A)) and the other three Mn ions that form the trimeric part of the cluster (Mn(B), Mn(C), Mn(D)), by perturbing the linking bis-μ-oxo bridge. The flexibility of this bridging unit is discussed with regard to the mechanism of O-O bond formation.
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| 3. |
- Ames, William, et al.
(författare)
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Theoretical Evaluation of Structural Models of the S(2) State in the Oxygen Evolving Complex of Photosystem II : Protonation States and Magnetic Interactions
- 2011
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 133:49, s. 19743-19757
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Tidskriftsartikel (refereegranskat)abstract
- Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, steering the mechanistic pathway, which involves at least five redox state intermediates S(n) (n = 0-4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S(2) state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn(4)O(5)Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (Mn(A)) of the cluster is deprotonated in the S(2) state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal-metal distances. Deprotonation of Ca(2+)-bound water molecules is strongly disfavored in the S(2) state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to Mn(III). The present results impose limits for the total charge and the proton configuration of the OEC in the S(2) state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.
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| 4. |
- Navarro, Montserrat Perez, et al.
(författare)
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Ammonia binding to the oxygen-evolving complex of photosystem II identifies the solvent-exchangeable oxygen bridge (µ-oxo) of the manganese tetramer
- 2013
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 110:39, s. 15561-15566
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Tidskriftsartikel (refereegranskat)abstract
- The assignment of the two substrate water sites of the tetramanganese penta-oxygen calcium (Mn4O5Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (mu-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn4O5Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522-3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable mu-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that (i) the exchangable mu-oxo bridge is O5 {using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55-60]}; (ii) ammonia displaces a water ligand to the outer manganese (Mn-A4-W1); and (iii) as W1 is trans to O5, ammonia binding elongates the Mn-A4-O5 bond, leading to the perturbation of the mu-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.
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| 5. |
- Shafaat, Hannah S., et al.
(författare)
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Electronic Structural Flexibility of Heterobimetallic Mn/Fe Cofactors : R2lox and R2c Proteins
- 2014
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 136:38, s. 13399-13409
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of the Mn/Fe cofactor identified in a new class of oxidases (R2lox) described by Andersson and Hogbom [Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 5633] is reported. The R2lox protein is homologous to the small subunit of class Ic ribonucleotide reductase (R2c) but has a completely different in vivo function. Using multifrequency EPR and related pulse techniques, it is shown that the cofactor of R2lox represents an antiferromagnetically coupled Mn-III/Fe-III dimer linked by a mu-hydroxo/bis-mu-carboxylato bridging network. The Mn-III ion is coordinated by a single water ligand. The R2lox cofactor is photoactive, converting into a second form (R2lox(photo)) upon visible illumination at cryogenic temperatures (77 K) that completely decays upon warming. This second, unstable form of the cofactor more closely resembles the Mn-III/Fe-III cofactor seen in R2c. It is shown that the two forms of the R2lox cofactor differ primarily in terms of the local site geometry and electronic state of the Mn-III ion, as best evidenced by a reorientation of its unique Mn-55 hyperfine axis. Analysis of the metal hyperfine tensors in combination with density functional theory (DFT) calculations suggests that this change is triggered by deprotonation of the mu-hydroxo bridge. These results have important consequences for the mixed-metal R2c cofactor and the divergent chemistry R2lox and R2c perform.
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