Where Water Is Oxidized to Dioxygen: Structure of the Photosynthetic Mn ₄ Ca Cluster

Abstract

The oxidation of water to dioxygen is catalyzed within photosystem II (PSII) by a Mn 4 Ca cluster, the structure of which remains elusive. Polarized extended x-ray absorption fine structure (EXAFS) measurements on PSII single crystals constrain the Mn 4 Ca cluster geometry to a set of three similar high-resolution structures. Combining polarized EXAFS and x-ray diffraction data, the cluster was placed within PSII, taking into account the overall trend of the electron density of the metal site and the putative ligands. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 angstrom-resolution x-ray structures or other previously proposed models.Oxygen, which makes up about 20% of Earth's atmosphere, comes mostly from photosynthesis that occurs in cyanobacteria, green algae, and higher plants (1). These organisms have within photosystem II (PSII) an oxygen-evolving complex (OEC), in which the energy of sunlight is used to oxidize water to molecular oxygen. The heart of the OEC is a cluster of four Mn atoms and one Ca atom (Mn 4 Ca) connected by mono-μ-oxo, di-μ-oxo, and/or hydroxo bridges. The specific protein environment and one chloride ion are also essential for the water-splitting activity (1). During the oxidation of water, the OEC cycles through five different oxidation states, which are known as S i states (where i ranges from 0 to 4), that couple the one-electron photochemistry of the PSII reaction center with the fourelectron chemistry of water oxidation (2).The structure of the Mn 4 Ca cluster and its role in the mechanism of water oxidation have been investigated with the use of spectroscopic methods (1), especially electron ‡ To whom correspondence should be addressed. messinger@mpi-muelheim. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript paramagnetic resonance and electron nuclear double-resonance spectroscopy (3-9), x-ray spectroscopy (10), and Fourier transform infrared (FTIR) spectroscopy (11). In addition, recent x-ray diffraction (XRD) studies of single crystals of PSII provide critical information about its structure at 3.8 to 3.0 Å resolution (12-16). However, even XRD data of the highest resolution presently available are insufficient to accurately determine the positions of Mn, Ca, and the bridging and terminal ligands. This is reflected by the differences in the placement of the metal ions and putative ligands in the 3.0 (16) and 3.5 Å (14) structures. Furthermore, at the x-ray dose and temperature used in the XRD studies, the geometry of the Mn 4 Ca cluster is disrupted, initiated by the rapid reduction of Mn(III) and Mn(IV) present in the dark-stable S 1 state to Mn(II), as shown by Mn x-ray absorption near-edge structure (XANES) studies and Mn x-ray absorption fine structure (EXAFS) studies of PSII single crystals (17).EXAFS experiments with PSII require a substantially lower x-ray dose than XRD measurements (17), and the onset of radiation damage can be precisely determined and controlled by monitoring the Mn K-edge positio...

Photoassembly of the water-oxidizing complex in photosystem II

“…The Imperial College group has postulated an assignment for all the elements in the WOC at 3.5 Å, while the Berlin group has left it as an open question by showing only the heavy atom scatterers [16,17]. A recent Mn EXAFS study of single crystals of these PSII core complexes has provided more evidence to distinguish between these models [18]. Also questions have arisen on the radiation damage to the cluster accrued by the X-rays used for the diffraction studies [19].…”

The PSII calcium site revisited

“…In light of this discovery, even the model shown here has to be viewed as provisional, although the authors have taken steps to minimize the radiation damage to their sample. A recent EXAFS study (which employed lower, non-damaging fluxes of X-rays) on oriented PSII crystals presents a modified version of the relationship between Ca 2+ and the Mn atoms (Yano et al 2006). In any case, it is clear that the Ca 2+ atom in PSII resides near the Mn cluster, at a distance of about 3.4-3.5 Å , and that until the problem of radiation damage is solved, the EXAFS structure is likely to be more reliable in making predictions about the position of Ca 2+ relative to that of the Mn atoms.…”

“…In contrast, Loll et al (2005b) proposes the structure of carboxylate oxo anion ligands to Ca 2+ from Glu189 and Ala 334, both of which provide oxo anion ligands to Mn atoms of the cluster (see Fig. 1), and Yano et al (2006) add oxo anion ligation from Asp 170. Additional ligands will no doubt be identified as the resolution of the structure improved.…”

“…Murray and Barber (2006) analyzed data in Ferreira et al (2004) and speculate that a Ca 2+ binding site is present in T. elongatus PsbO that is near the lumenal side of PSII. At the same time, all of the currently available models based on crystallographic data (Loll et al 2005b) or on EXAFS results (Yachandra 2005;Yano et al 2006) place the functional Ca 2+ site in the OEC in close proximity to the Mn cluster, and do not predict ligation by amino acid side chain residues of PsbO.…”

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…Manganese-oxo complexes have also been designed for PCET and HAT reactions, and have been studied extensively in the context of the special role of manganese oxo clusters in the oxygen evolving complex of photosystem II [60][61][62][63]. Borovik has disclosed a compound with the backbone structure 4, which features an unusual non-bridging terminal hydroxo ligand, and has measured its pK a and redox properties ( Figure 7) [64][65][66].…”