Calcium EXAFS Establishes the Mn-Ca Cluster in the Oxygen-Evolving Complex of Photosystem II † (original) (raw)
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Biochemistry, 2004
The oxygen-evolving complex of photosystem II (PS II) in green plants and algae contains a cluster of four Mn atoms in the active site, which catalyzes the photoinduced oxidation of water to dioxygen. Along with Mn, calcium and chloride ions are necessary cofactors for proper functioning of the complex. The current study using polarized Sr EXAFS on oriented Sr-reactivated samples shows that Fourier peak II, which fits best to Mn at 3.5 Å rather than lighter atoms (C, N, O, or Cl), is dichroic, with a larger magnitude at 10°(angle between the PS II membrane normal and the X-ray electric field vector) and a smaller magnitude at 80°. Analysis of the dichroism of the Sr EXAFS yields a lower and upper limit of 0°and 23°for the average angle between the Sr-Mn vectors and the membrane normal and an isotropic coordination number (number of Mn neighbors to Sr) of 1 or 2 for these layered PS II samples. The results confirm the contention that Ca (Sr) is proximal to the Mn cluster and lead to refined working models of the heteronuclear Mn 4 Ca cluster of the oxygen-evolving complex in PS II.
The Journal of Physical Chemistry B, 1998
The oxygen-evolving complex of Photosystem II (PS II) in green plants and algae contains a cluster of four manganese atoms in the active site, which catalyzes the photoinduced oxidation of water to dioxygen. Along with Mn, calcium and chloride ions are necessary cofactors for proper functioning of the complex. A key unresolved question is whether Ca is close to the Mn cluster, within about 3.5 Å. To further test and verify this finding, we substituted strontium for Ca and probed from the Sr point-of-view for any nearby Mn. Sr has been shown to replace Ca and still maintain enzyme activity (about 40% of normal rate). The extended X-ray absorption fine structure (EXAFS) of Sr-PS II probes the local environment around the Sr cofactor to detect any nearby Mn. We focused on the functional Sr by removing nonessential, loosely bound Sr in the protein environment. For comparison, an inactive sample was prepared by treating the intact PS II with hydroxylamine to disrupt the Mn cluster and to produce nonfunctional enzyme. Sr EXAFS results indicate major differences in the phase and amplitude between the functional (intact) and nonfunctional (NH2OH-treated) samples. In intact samples, the Fourier transform of the Sr EXAFS shows a peak that is missing in inactive samples. This Fourier peak II is best simulated by two Mn neighbors at a distance of 3.5 Å. Thus, with X-ray absorption studies on Sr-reconstituted PS II, we confirm the proximity of Ca (Sr) cofactor to the Mn cluster and show that the active site is a Mn-Ca heteronuclear cluster.
Inorganic Chemistry, 2008
Light-driven oxidation of water to dioxygen in plants, algae and cyanobacteria is catalyzed within photosystem II (PS II) by a Mn 4 Ca cluster. Although the cluster has been studied by many different methods, the structure and the mechanism have remained elusive. Xray absorption and emission spectroscopy and EXAFS studies have been particularly useful in probing the electronic and geometric structure, and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn 4 Ca cluster geometry to a set of three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 Å-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-ray spectroscopy and crystallography are predominantly because of damage to the Mn 4 Ca cluster by X-rays under the conditions used for structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn 4 Ca catalytic center as it cycles through the five intermediate states known as the S i-states (i=0-4). The electronic structure of the Mn 4 Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formal oxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms that includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structure of the Mn 4 Ca cluster in the Sstates derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of the O-O bond formation during the photosynthetic water splitting process.
X-Ray spectroscopy of the photosynthetic oxygen-evolving complex
Coordination chemistry reviews, 2008
Water oxidation to dioxygen in photosynthesis is catalyzed by a Mn(4)Ca cluster with O bridging in Photosystem II (PS II) of plants, algae and cyanobacteria. A variety of spectroscopic methods have been applied to analyzing the participation of the complex. X-ray spectroscopy is particularly useful because it is element-specific, and because it can reveal important structural features of the complex with high accuracy and identify the participation of Mn in the redox chemistry. Following a brief history of the application of X-ray spectroscopy to PS II, an overview of newer results will be presented and a description of the present state of our knowledge based on this approach.
Journal of Biological Chemistry, 2006
X-ray absorption spectroscopy has provided important insights into the structure and function of the Mn 4 Ca cluster in the oxygenevolving complex of Photosystem II (PS II). The range of manganese extended x-ray absorption fine structure data collected from PS II until now has been, however, limited by the presence of iron in PS II. Using a crystal spectrometer with high energy resolution to detect solely the manganese K␣ fluorescence, we are able to extend the extended x-ray absorption fine structure range beyond the onset of the iron absorption edge. This results in improvement in resolution of the manganese-backscatterer distances in PS II from 0.14 to 0.09 Å. The high resolution data obtained from oriented spinach PS II membranes in the S 1 state show that there are three di-oxo-bridged manganese-manganese distances of ϳ2.7 and ϳ2.8 Å in a 2:1 ratio and that these three manganese-manganese vectors are aligned at an average orientation of ϳ60°relative to the membrane normal. Furthermore, we are able to observe the separation of the Fourier peaks corresponding to the ϳ3.2 Å manganese-manganese and the ϳ3.4 Å manganese-calcium interactions in oriented PS II samples and determine their orientation relative to the membrane normal. The average of the manganese-calcium vectors at ϳ3.4 Å is aligned along the membrane normal, while the ϳ3.2 Å manganese-manganese vector is oriented near the membrane plane. A comparison of this structural information with the proposed Mn 4 Ca cluster models based on spectroscopic and diffraction data provides input for refining and selecting among these models. Photosynthesis by green plants, algae, and cyanobacteria provides essentially all of the dioxygen in the biosphere as a byproduct of the electron transfer processes utilizing water as the ultimate electron source: 2H 2 O 3 O 2 ϩ 4H ϩ ϩ 4e Ϫ. Water oxidation is a light-driven reaction that is catalyzed by an oxygen-evolving complex (OEC) 4 of Photosystem II (PS II) (1-4). The active site of the OEC is known to be a proteinbound complex containing four manganese and one calcium atom. This complex cycles through a series of five intermediate redox states that are referred to as S states (S 0 to S 4) (5). The S state transitions are driven by successive light-induced oneelectron oxidations of the PS II reaction center. In each step the complex accumulates oxidizing equivalents until dioxygen is released during the spontaneous return from S 4 to S 0. Many of the proposed mechanisms of water oxidation depend critically on knowledge of the Mn 4 Ca cluster structure. To date, structural models of the OEC complex have been suggested based on EPR techniques (6-9), x-ray absorption spectroscopy (XAS) (10-14), x-ray diffraction (XRD) (15-17), and infrared spectroscopy (Fourier transform infrared) (18). The XRD studies (3.0-3.8 Å resolution) have located the Mn 4 Ca cluster in the density map (16, 17) and confirmed the presence of calcium in the OEC cluster, as had been shown previously by EPR (19-21) and by extended x-ray absorption fine structure (EXAFS) spectroscopy (22, 23). A recent XAS study showed that the OEC complex is very susceptible to reduction and disruption during x-ray exposure, under the conditions used in collecting the published XRD data (24). Consequently, the precise location of the manganese and calcium atoms has not been reliably established within active OEC centers by XRD, as acknowledged in the most recent study (17). Manganese XAS enables a detailed analysis of the Mn 4 Ca cluster in the OEC. X-ray absorption near-edge structure (XANES) contains information on the electronic structure and changes in oxidation states of the manganese that accompany S state transitions (25). EXAFS allows for a precise determination
Structural Changes of the Oxygen-evolving Complex in Photosystem II during the Catalytic Cycle
Journal of Biological Chemistry, 2013
Background: Mn 4 CaO 5 cluster catalyzes water oxidation in photosystem II. Results: Mn-Mn/Ca/ligand distances and changes in the structure of the Mn 4 CaO 5 cluster are determined for the intermediate states in the reaction using x-ray spectroscopy. Conclusion: Position of one bridging oxygen and related geometric changes may be critical during catalysis. Significance: Knowledge about structural changes during catalysis is crucial for understanding the O-O bond formation mechanism in PSII.
Journal of the American Chemical Society, 2005
Figure 1. Left: A schematic representation of the detection scheme. Mn and Fe Kα1 and Kα2 fluorescence peaks are ~5eV wide and split by ~11eV (not shown). The multi-crystal monochromator with ~1 eV resolution is tuned to the Kα1 peak (red). The fluorescence peaks broadened by the Ge-detector with 150-200eV resolution are shown below (blue). 17 Right: The PS II Mn K-edge EXAFS spectrum from the S 1 state sample obtained with a traditional energy-discriminating Ge-detector (blue) compared, with that collected using the highresolution crystal monochromator (red). Fe present in PS II does not pose a problem with the high-resolution detector (the Fe edge is marked by a green line). Inset: The inset shows the schematic for the crystal monochromator used in a backscattering configuration. 12