Conversion of the g=4.1 EPR signal to the multiline conformation during the S(2) to S(3) transition of the oxygen evolving complex of Photosystem II

The S1 to S2 and S2 to S3 state transitions in plant photosystem II: relevance to the functional and structural heterogeneity of the water oxidizing complex

In Photosystem II, light-induced water splitting occurs via the S state cycle of the CaMn4O5-cluster. To understand the role of various possible conformations of the CaMn4O5-cluster in this process, the temperature dependence of the S1 → S2 and S2 → S3 state transitions, induced by saturating laser flashes, was studied in spinach photosystem II membrane preparations under different conditions. The S1 → S2 transition temperature dependence was shown to be much dependent on the type of the cryoprotectant and presence of 3.5% methanol, resulting in the variation of transition half-inhibition temperature by 50 K. No similar effect was observed for the S2 → S3 state transition, for which we also show that both the low spin g = 2.0 multiline and high spin g = 4.1 EPR configurations of the S2 state advance with similar efficiency to the S3 state, both showing a transition half-inhibition temperature of 240 K. This was further confirmed by following the appearance of the Split S3 EPR signal. The results are discussed in relevance to the functional and structural heterogeneity of the water oxidizing complex intermediates in photosystem II.

Pivotal role of the redox-active tyrosine in driving the water splitting catalyzed by photosystem II

TyrZ oxidation state triggers hydrogen bond modification in the water oxidation catalysis.

A Comparison of Experimental and Broken Symmetry Density Functional Theory (BS-DFT) Calculated Electron Paramagnetic Resonance (EPR) Parameters for Intermediates Involved in the S2 to S3 State Transition of Nature's Oxygen Evolving Complex

A broken symmetry density functional theory (BS-DFT) magnetic analysis of the S₂, S₂Y_Z^•, and S₃ states of Nature's oxygen evolving complex is performed for both the native Ca and Sr substituted forms. Good agreement with experiment is observed between the tyrosyl calculated g-tensor and ¹H hyperfine couplings for the native Ca form. Changes in the hydrogen bonding environment of the tyrosyl radical in S₂Y_Z^• caused by Sr substitution lead to notable changes in the calculated g-tensor of the tyrosyl radical. Comparison of calculated and experimental ⁵⁵Mn hyperfine couplings for the S₃ state presently favors an open cubane form of the complex with an additional OH ligand coordinating to Mn_D. In Ca models, this additional ligation can arise by closed-cubane form deprotonation of the Ca ligand W3 in the S₂Y_Z^• state accompanied by spontaneous movement to the vacant Mn coordination site or by addition of an external OH group. For the Sr form, no spontaneous movement of W3 to the vacant Mn coordination site is observed in contrast to the native Ca form, a difference which may lead to the reduced catalytic activity of the Sr substituted form. BS-DFT studies on peroxo models of S₃ as indicated by a recent X-ray free electron laser (XFEL) crystallography study give rise to a structural model compatible with experimental data and an S = 3 ground state compatible with EPR studies.

Interaction of methanol with the oxygen-evolving complex: atomistic models, channel identification, species dependence, and mechanistic implications

Methanol has long being used as a substrate analogue to probe access pathways and investigate water delivery at the oxygen-evolving complex (OEC) of photosystem-II. In this contribution we study the interaction of methanol with the OEC by assembling available spectroscopic data into a quantum mechanical treatment that takes into account the local channel architecture of the active site. The effect on the magnetic energy levels of the Mn₄Ca cluster in the S₂ state of the catalytic cycle can be explained equally well by two models that involve either methanol binding to the calcium ion of the cluster, or a second-sphere interaction in the vicinity of the "dangler" Mn4 ion. However, consideration of the latest ¹³C hyperfine interaction data shows that only one model is fully consistent with experiment. In contrast to previous hypotheses, methanol is not a direct ligand to the OEC, but is situated at the end-point of a water channel associated with the O4 bridge. Its effect on magnetic properties of plant PS-II results from disruption of hydrogen bonding between O4 and proximal channel water molecules, thus enhancing superexchange (antiferromagnetic coupling) between the Mn3 and Mn4 ions. The same interaction mode applies to the dark-stable S₁ state and possibly to all other states of the complex. Comparison of protein sequences from cyanobacteria and plants reveals a channel-altering substitution (D1-Asn87 versus D1-Ala87) in the proximity of the methanol binding pocket, explaining the species-dependence of the methanol effect. The water channel established as the methanol access pathway is the same that delivers ammonia to the Mn4 ion, supporting the notion that this is the only directly solvent-accessible manganese site of the OEC. The results support the pivot mechanism for water binding at a component of the S₃ state and would be consistent with partial inhibition of water delivery by methanol. Mechanistic implications for enzymatic regulation and catalytic progression are discussed.

A five-coordinate Mn(iv) intermediate in biological water oxidation: spectroscopic signature and a pivot mechanism for water binding

Among the four photo-driven transitions of the water-oxidizing tetramanganese-calcium cofactor of biological photosynthesis, the second-last step of the catalytic cycle, that is the S₂ to S₃ state transition, is the crucial step that poises the catalyst for the final O-O bond formation. This transition, whose intermediates are not yet fully understood, is a multi-step process that involves the redox-active tyrosine residue and includes oxidation and deprotonation of the catalytic cluster, as well as the binding of a water molecule. Spectroscopic data has the potential to shed light on the sequence of events that comprise this catalytic step, which still lacks a structural interpretation. In this work the S₂-S₃ state transition is studied and a key intermediate species is characterized: it contains a Mn₃O₄Ca cubane subunit linked to a five-coordinate Mn(iv) ion that adopts an approximately trigonal bipyramidal ligand field. It is shown using high-level density functional and multireference wave function calculations that this species accounts for the near-infrared absorption and electron paramagnetic resonance observations on metastable S₂-S₃ intermediates. The results confirm that deprotonation and Mn oxidation of the cofactor must precede the coordination of a water molecule, and lead to identification of a novel low-energy water binding mode that has important implications for the identity of the substrates in the mechanism of biological water oxidation.

The first tyrosyl radical intermediate formed in the S2-S3 transition of photosystem II

The EPR "split signals" represent key intermediates of the S-state cycle where the redox active D1-Tyr161 (YZ) has been oxidized by the reaction center of the photosystem II enzyme to its tyrosyl radical form, but the successive oxidation of the Mn4CaO5 cluster has not yet occurred (SiYZ˙). Here we focus on the S2YZ˙ state, which is formed en route to the final metastable state of the catalyst, the S3 state, the state which immediately precedes O-O bond formation. Quantum chemical calculations demonstrate that both isomeric forms of the S2 state, the open and closed cubane isomers, can form states with an oxidized YZ˙ residue without prior deprotonation of the Mn4CaO5 cluster. The two forms are expected to lie close in energy and retain the electronic structure and magnetic topology of the corresponding S2 state of the inorganic core. As expected, tyrosine oxidation results in a proton shift towards His190. Analysis of the electronic rearrangements that occur upon formation of the tyrosyl radical suggests that a likely next step in the catalytic cycle is the deprotonation of a terminal water ligand (W1) of the Mn4CaO5 cluster. Diamagnetic metal ion substitution is used in our calculations to obtain the molecular g-tensor of YZ˙. It is known that the gx value is a sensitive probe not only of the extent of the proton shift between the tyrosine-histidine pair, but also of the polarization environment of the tyrosine, especially about the phenolic oxygen. It is shown for PSII that this environment is determined by the Ca(2+) ion, which locates two water molecules about the phenoxyl oxygen, indirectly modulating the oxidation potential of YZ.

Electron transfer pathways from the S2-states to the S3-states either after a Ca2+/Sr2+ or a Cl-/I- exchange in Photosystem II from Thermosynechococcus elongatus

The site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S0 to S4) before O2 is evolved. It consists of a Mn4CaO5-cluster close to a redox-active tyrosine residue (YZ). Cl- is also required for enzyme activity. By using EPR spectroscopy it has been shown that both Ca2+/Sr2+ exchange and Cl-/I- exchange perturb the proportions of centers showing high (S=5/2) and low spin (S=1/2) forms of the S2-state. The S3-state was also found to be heterogeneous with: i) a S=3 form that is detectable by EPR and not sensitive to near-infrared light; and ii) a form that is not EPR visible but in which Mn photochemistry occurs resulting in the formation of a (S2YZ)' split EPR signal upon near-infrared illumination. In Sr/Cl-PSII, the high spin (S=5/2) form of S2 shows a marked heterogeneity with a g=4.3 form generated at low temperature that converts to a relaxed form at g=4.9 at higher temperatures. The high spin g=4.9 form can then progress to the EPR detectable form of S3 at temperatures as low as 180K whereas the low spin (S=1/2) S2-state can only advance to the S3 state at temperatures≥235 K. Both of the two S2 configurations and the two S3 configurations are each shown to be in equilibrium at ≥235 K but not at 198 K. Since both S2 configurations are formed at 198 K, they likely arise from two specific populations of S1. The existence of heterogeneous populations in S1, S2 and S3 states may be related to the structural flexibility associated with the positioning of the oxygen O5 within the cluster highlighted in computational approaches and which has been linked to substrate exchange. These data are discussed in the context of recent in silico studies of the electron transfer pathways between the S2-state(s) and the S3-state(s).

Can we trap the S(3)Y(Z)· metalloradical intermediate during the S-state transitions of Photosystem II? An EPR investigation

We report the trapping of two metalloradical intermediates corresponding to the transitions S2 to S3 and S3 to S0 of the oxygen evolving complex (OEC) of Photosystem II (PSII), in preparations containing methanol, at temperatures near that of half inhibition of the respective S-state transitions. The first intermediate, with an EPR width of 160 G, is assigned to S2YZ, based on its similarity to the one previously characterized after trapping at 10 K. The second with a splitting of ∼80 G is tentatively assigned to S3YZ. The S3YZ EPR signal is weaker than the S2YZ one, and both are stable at cryogenic temperatures.

Proton transport facilitating water-oxidation: the role of second sphere ligands surrounding the catalytic metal cluster

The ability of PSII to extract electrons from water, with molecular oxygen as a by-product, is a remarkable biochemical and evolutionary innovation. From an evolutionary perspective, the invention of PSII approximately 2.7 Ga led to the accelerated accumulation of biomass in the biosphere and the accumulation of oxygen in the atmosphere, a combination that allowed for the evolution of a much more complex and extensive biosphere than would otherwise have been possible. From the biochemical and enzymatic perspective, PSII is remarkable because of the thermodynamic and kinetic obstacles that needed to have been overcome to oxidize water as the ultimate photosynthetic electron donor. This article focuses on how proton release is an integral part of how these kinetic and thermodynamic obstacles have been overcome: the sequential removal of protons from the active site of H2O-oxidation facilitates the multistep oxidation of the substrate water at the Mn4CaOx, the catalytic heart of the H2O-oxidation reaction. As noted previously, the facilitated deprotonation of the Mn4CaOx cluster exerts a redox-leveling function preventing the accumulation of excess positive charge on the cluster, which might otherwise hinder the already energetically difficult oxidation of water. Using recent results, including the characteristics of site-directed mutants, the role of the second sphere of amino acid ligands and the associated network of water molecules surrounding the Mn4CaOx is discussed in relation to proton transport in other systems. In addition to the redox-leveling function, a trapping function is assigned to the proton release step occurring immediately prior to the dioxygen chemistry. This trapping appears to involve a yet-to-be clarified gating mechanism that facilitates to coordinated release of a proton from the neighborhood of the active site thereby insuring that the backward charge-recombination reaction does not out-compete the forward reaction of dioxygen chemistry during this final step of H2O-oxidation.