Structural Change of the Mn Cluster during the S₂→S₃ State Transition of the Oxygen-Evolving Complex of Photosystem II. Does It Reflect the Onset of Water/Substrate Oxidation? Determination by Mn X-ray Absorption Spectroscopy

Abstract

The oxygen-evolving complex of Photosystem II in plants and cyanobacteria catalyzes the oxidation of two water molecules to one molecule of dioxygen. A tetranuclear Mn complex is believed to cycle through five intermediate states (S 0 -S 4 ) to couple the four-electron oxidation of water with the one-electron photochemistry occurring at the Photosystem II reaction center. We have used X-ray absorption spectroscopy to study the local structure of the Mn complex and have proposed a model for it, based on studies of the Mn K-edges and the extended X-ray absorption fine structure of the S 1 and S 2 states. The proposed model consists of two di-μ-oxo-bridged binuclear Mn units with Mn-Mn distances of ~2.7 Å that are linked to each other by a mono-μ-oxo bridge with a Mn-Mn separation of ~3.3 Å. The Mn-Mn distances are invariant in the native S 1 and S 2 states. This report describes the application of X-ray absorption spectroscopy to S 3 samples created under physiological conditions with saturating flash illumination. Significant changes are observed in the Mn-Mn distances in the S 3 state compared to the S 1 and the S 2 states. The two 2.7 Å Mn-Mn distances that characterize the di-μ-oxo centers in the S 1 and S 2 states are lengthened to ~2.8 and 3.0 Å in the S 3 state, respectively. The 3.3 Å Mn-Mn and Mn-Ca distances also increase by 0.04-0.2 Å. These changes in Mn-Mn distances are interpreted as consequences of the onset of substrate/water oxidation in the S 3 state. Mn-centered oxidation is evident during the S 0 →S 1 and S 1 →S 2 transitions. We propose that the changes in Mn-Mn distances during the S 2 →S 3 transition are the result of ligand or water oxidation, leading to the Supporting Information Available: E-space S 3 state EXAFS spectrum; the data in k-space and the background that was removed to reduce the low-frequency contributions that show up as peaks at <1 Å in the Fourier transform; and the Fourier isolate of the k-space S 3 spectrum, shown overplotted on the S 3 EXAFS spectrum (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. NIH Public Access

An Intrinsically Disordered Photosystem II Subunit, PsbO, Provides a Structural Template and a Sensor of the Hydrogen-bonding Network in Photosynthetic Water Oxidation

“…Comparison of the integrated areas showed that urea or CaCl 2 treatments removed 82 and 92% of native PsbO, respectively. Reconstitution with 12 C-or 13 C-PsbO to urea-PSII gave quantitative (120%) rebinding. This analysis is consistent with previous results for a different PSII preparation (24,79).…”

“…Flash excitation can be coupled with spectroscopic techniques to identify structural changes associated with each S state transition (for examples, see Refs. [12][13][14][15].…”

“…Samples (PSII, NaCl-PSII, urea-PSII, CaCl 2 -PSII, native-PSII, 12 C-PSII, or 13 C-PSII) for FT-IR experiments were suspended in 400 mM sucrose, 50 mM MES-NaOH (pH 6.0), 60 mM NaCl, and 20 mM CaCl 2 by three rounds of centrifugation (50,000 ϫ g, 4°C, 15 min) and resuspension and frozen (Ϫ70°C) until use. Samples were thawed and potassium ferricyanide (7 mM) was added, and the sample was centrifuged (50,000 ϫ g, 4°C, 15 min) to produce a pellet.…”

“…Difference spectra were constructed to measure the isotope-induced downshift. In A, data were derived from 12 C/ 13 C-PsbO bound to PSII ( 12 C-minus 13 C-PSII; black), and in B, data were derived from PsbO in D 2 O buffer ( 12 C-minus 13 C-PsbO; red). In A, samples were prepared in 400 mM sucrose, 50 mM MES (pH 6.0), 60 mM NaCl, and 20 mM CaCl 2 , as described under "Experimental Procedures" for reaction-induced FT-IR spectroscopy.…”

Focusing the view on nature's water-splitting catalyst

“…Interestingly, the structure of the cluster changes dramatically upon S 3 -state formation (Liang et al 2000;Haumann et al 2005) and is also different in the S 0 state Haumann et al 2005). This structural flexibility may be key to the catalytic activity of the WOC.…”