20 research outputs found
Calcium EXAFS Establishes the Mn-Ca Cluster in the Oxygen-Evolving Complex of Photosystem II †
X-ray absorption spectroscopy on the calcium cofactor to the manganese cluster in photosynthetic oxygen evolution
Along with Mn, calcium and chloride ions are necessary cofactors for oxygen evolution in Photosystem II (PS II). To further test and verify whether Ca is close to the Mn cluster, we substituted strontium for Ca and probed from the Sr point of view for any nearby Mn. The extended X-ray absorption fine structure (EXAFS) of Sr-reactivated PS II indicates major differences between the intact and NH2OH-treated samples. In intact samples, the Fourier transform of the Sr EXAFS shows a Fourier peak that is missing in inactive samples. This peak II is best simulated by two Mn neighbors at a distance of 3.5 Angstrom, confirming the proximity of Ca (Sr) cofactor to the Mn cluster. In addition, polarized Sr EXAFS on oriented Sr-reactivated samples shows this peak II is dichroic: large magnitude at 10 degrees (angle between the PS II membrane normal and the x-ray electric field vector) and small at 80 degrees. Analysis of the dichroism yields the relative angle between the Sr-Mn vector and membrane normal (23 degrees plus or minus 4 degrees), and the isotropic coordination number for these layered samples. X-ray absorption spectroscopy has also been employed to assess the degree of similarity between the manganese cluster in PS II and a family of synthetic manganese complexes containing the distorted cubane [Mn4O3X] core (X = benzoate, acetate, methoxide, hydroxide, azide, fluoride, chloride or bromide). In addition, Mn4O3Cl complexes containing three or six terminal Cl ligands at three of the Mn were included in this study. The EXAFS method detects the small changes in the core structures as X is varied in this series, and serves to exclude these distorted cubanes of C3v symmetry as a topological model for the Mn catalytic cluster. The sulfur K-edge x-ray absorption near-edge structure (XANES) spectra for the amino acids cysteine, methionine, their corresponding oxidized forms cystine and methionine sulfoxide, and glutathione show distinct differences between the thiol and disulfide forms. Sulfur XANES is also used to detect changes (within 5 percent) of the thiol-to-disulfide ratio in whole human blood, plasma, and erythrocytes
Influence of the 33 kDa Manganese-Stabilizing Protein on the Structure and Substrate Accessibility of the Oxygen-Evolving Complex of Photosystem II†
Influence of the 33 kDa Manganese-Stabilizing Protein on the Structure and Substrate Accessibility of the Oxygen-Evolving Complex of Photosystem II †
Strontium EXAFS Reveals the Proximity of Calcium to the Manganese Cluster of Oxygen-Evolving Photosystem II
Orientation of Calcium in the Mn4Ca Cluster of the Oxygen-Evolving Complex Determined Using Polarized Strontium EXAFS of Photosystem II Membranes
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The Mn cluster in the S(0) state of the oxygen-evolving complex of photo system II studied by EXAFS spectroscopy: Are there three di-mu-oxo bridged Mn2 moieties in the tetranuclear Mn complex?
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Ca cofactor of the water-oxidation complex: Evidence for a Mn/Ca heteronuclear cluster
Calcium and chloride are necessary cofactors for the proper function of the oxygen-evolving complex (OEC) of Photosystem II (PS II). Located in the thylakoid membranes of green plants, cyanobacteria and algae, PS II and the OEC catalyze the light-driven oxidation of water into dioxygen (released into the biosphere), protons and electrons for carbon fixation. The actual chemistry of water oxidation is performed by a cluster of four manganese atoms, along with the requisite cofactors Ca{sup 2+} and Cl{sup -}. While the Mn complex has been extensively studied by X-ray absorption techniques, comparatively less is known about the Ca{sup 2+} cofactor. The fewer number of studies on the Ca{sup 2+} cofactor have sometimes relied on substituting the native cofactor with strontium or other metals, and have stirred some debate about the structure of the binding site. past efforts using Mn EXAFS on Sr-substituted PSII are suggestive of a close link between the Mn cluster and Sr, within 3.5 {angstrom}. The most recent published study using Sr EXAFS on similar samples confirms this finding of a 3.5 {angstrom} distance between Mn and Sr. This finding was base3d on a second Fourier peak (R {approx} 3 {angstrom}) in the Sr EXAFS from functional samples, but is absent from inactive, hydroxylamine-treated PS II. This Fourier peak II was found to fit best to two Mn at 3.5 {angstrom} rather than lighter atoms (carbon). Nevertheless, other experiments have given contrary results. They wanted to extend the technique by using polarized Sr EXAFS on layered Sr-substituted samples, to provide important angle information. Polarized EXAFS involves collecting spectra for different incident angles ({theta}) between the membrane normal of the layered sample and the X-ray electric field vector. Dichroism in the EXAFS can occur, depending on how the particular absorber-backscatterer (A-B) vector is aligned with the electric field. Through analysis of the dichroism, they extract the average number of scatterers per absorbing atom (N{sub iso}). Constraints on the structural model are then imposed by these parameters. In a complementary and definitive experiment, they use Ca K-edge EXAFS studies to probe the binding site of the native cofactor for any nearby Mn, within {approx} 4 {angstrom}. This is analogous to the Sr EXAFS studies already published, but it focuses on the native cofactor and avoids the treatments involving Ca depletion and Sr substitution. The samples examined were PS II membrane particles from spinach. This new technique promises to be a more sensitive and direct probe of the calcium binding site in PS II than Sr EXAFS. Clarifying whether the Ca cofactor is proximate to the Mn cluster, and finding its coordination environment at the various intermediate S-states of the OEC will reveal its important role in oxygen evolution
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The Mn cluster in the S(0) state of the oxygen-evolving complex of photo system II studied by EXAFS spectroscopy: Are there three di-mu-oxo bridged Mn2 moieties in the tetranuclear Mn complex?
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Calcium EXAFS establishes the Mn-Ca cluster in the oxygen-evolving complex of Photosystem II
The proximity of Ca to the Mn cluster of the photosynthetic water-oxidation complex is demonstrated by X-ray absorption spectroscopy. We have collected EXAFS data at the Ca K-edge using active PS II membrane samples that contain approximately 2 Ca per 4 Mn. These samples are much less perturbed than previously investigated Sr-substituted samples, which were prepared subsequent to Ca depletion. The new Ca EXAFS clearly shows backscattering from Mn at 3.4 angstroms, a distance that agrees with that surmised from previously recorded Mn EXAFS. This result is also consistent with earlier related experiments at the Sr K-edge, using samples that contained functional Sr, that show Mn is ~ 3.5 angstroms distant from Sr. The totality of the evidence clearly advances the notion that the catalytic center of oxygen evolution is a Mn-Ca heteronuclear cluster