An evaluation of structural models for the photosynthetic water-oxidizing complex derived from spectroscopic and X-ray diffraction signatures

Abstract

Four of the five intermediate oxidation states (S-states) in the catalytic cycle of water oxidation used by O2-evolving photoautotrophs have been previously characterized by EPR and/or ENDOR spectroscopy, with the first reports for the S0, S1, and S3 states available in just the last three years. The first electron density map of the Mn cluster derived from X-ray diffraction measurements of single crystals of photosystem II at 3.8-4.2 A resolution has also appeared this year. This wealth of new information has provided significant insight into the structure of the inorganic core (Mn4OxCa1Cl1-2), the Mn oxidation states, and the location and function of the essential Ca2+ cofactor within the water-oxidizing complex (WOC). We summarize these advances and provide a unified interpretation of debated structural proposals and Mn oxidation states, based on an integrated analysis of the published data, particularly from Mn X-ray absorption spectroscopy (XAS) and EPR/ENDOR data. Only three magnetic spin-exchange models for the inter-manganese interactions are possible from consideration of the EPR data for the S0, S1, S2 and S(-N) (NO-reduced) states. These models fall into one of three types denoted butterfly, funnel, or tetrahedron. A revised set of eight allowed chemical structures for the Mn4Ox core can be deduced that are shown to be consistent with both EPR and XAS. The popular "dimer-of-dimers" structural model is not compatible with the possible structural candidates. EPR data have identified two inter-manganese couplings that are sensitive to the S-state, suggesting two possible bridging sites for substrate water molecules. Spin densities derived from 55Mn hyperfine data together with Mn K-edge energies from Ca-depleted samples provide an internally consistent assignment for the Mn oxidation states of Mn4(3III,IV) for the S2 state. EPR and XAS data also provide a consistent picture, locating Ca2+ as an integral part of the inorganic core, probably via shared bridging ligands with Mn (aqua/hydroxo/carboxylato/chloro). XAS data reveal that the Ca2+ cofactor increases the Mn(1s-->4p) transition energy by 0.6-1 eV with minimal structural perturbation versus the Ca-depleted WOC. Thus, calcium binding appears to increase the Mn-ligand covalency by increasing electron transfer from shared ligands to Mn, suggesting a direct role for Ca2+ in substrate water oxidation. Consideration of both the XAS and the EPR data, together with reactivity studies on two model complexes that evolve O2, suggest two favored structure types as feasible models for the reactive S4 state that is precursor to the O2 evolution step. These are a calcium-capped "cuboidal" core and a calcium-capped "funnel" core.

Photoassembly of the water-oxidizing complex in photosystem II

“…Photoactivation kinetics and EPR spectroscopy reveal that when Ca 2+ binds in the dark to its specific site it induces the formation of ions, forming the active water oxidase. The photoactivation and spectroscopic data noted above provide a self-consistent picture locating the Ca effector site as an integral part of the Mn 4 core, in direct association with Mn probably via shared bridging ligands to Mn [28], [61].…”

“…Proposals ranging from homolytic O-O bond formation from two facial bridging electrondeficient oxo's [25] to heterolytic O-O bond formation via nucleophilic-electrophilic oxocoupling have been presented [26,27]. And although various spectroscopic and structural markers (such as EPR and EXAFS) [28,29,30], mutagenesis [31] and water exchange rates of the individual S-states have been considered [32], no commonly agreed on mechanism of water oxidation has been convincingly proved. It is for these reasons that the photoassembly reaction of the Mn 4 Ca cluster provides special insights as it can dissect the roles of the individual cofactors and enables examination by substitution, a topic we refer to as "inorganic mutants".…”

“…On this basis, it has been proposed that the nondirectional bonding properties of these alkaline Earth ions and the ionic charge density of Ca 2+ are critical for allowing formation of a peroxide intermediate, Ca 2+ (O 2 2− ), between oxo bridges in the step leading to O 2 evolution from the S 4 state of the Mn 4 O 4 Ca cluster [25,28]. A role for Ca 2+ as Lewis acid catalyst for proton ionization of substrate water and generation of a nucleophilic hydroxide has also been considered [24,25], although this ignores the direct effect that Ca 2+ has on the electronic energy of the Mn ions [28]. 4 Ca cluster from its free metal ions it is necessary to deprotonate four or more aqua ligands attached to the initially bound Mn 2+ .…”

Functional implications on the mechanism of the function of photosystem II including water oxidation based on the structure of photosystem II

“…Two of them are shown in gure 5. An arrangement, where three of the four Mn atoms are arranged in triangular formation was also favoured by Carrell et al (2002), because it ful ls the requirement of 'spin frustration' (spins are not able achieve an anti-parallel orientation), which is necessary to get a con dent interpretation of the EPR spectra in the different S states. Dismukes and co-workers exclude all linear or cubic arrangements, because the spins can be orientated anti-parallel in these geometries.…”

Spectroscopic Evidence for Ca2+Involvement in the Assembly of the Mn4Ca Cluster in the Photosynthetic Water-Oxidizing Complex

“…However, the relative position of Ca 2+ within the Mn 4 O x cluster differs according to which type of data is emphasized and the assumptions of the models used to interpret the raw data. Spectroscopic and XRD data have constrained the possible core topologies to only a few types, with the structures given in Chart 1 proposed on the basis of one or more lines of evidence (9)(10)(11)13). These structural proposals have led to a number of mechanistic proposals for the role of calcium in O 2 evolution reviewed elsewhere (11,(14)(15)(16)(17).…”