Reprint of PSII manganese cluster: protonation of W2, O5, O4 and His337 in the S1 state explored by combined quantum chemical and electrostatic energy computations

pKa of the ligand water molecules in the oxygen-evolving Mn4CaO5 cluster in photosystem II

Release of the protons from the substrate water molecules is prerequisite for O₂ evolution in photosystem II (PSII). Proton-releasing water molecules with low pK_a values at the catalytic moiety can be the substrate water molecules. In some studies, one of the ligand water molecules, W2, is regarded as OH^-. However, the PSII crystal structure shows neither proton acceptor nor proton-transfer pathway for W2, which is not consistent with the assumption of W2 = OH^-. Here we report the pK_a values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca²⁺, of the Mn₄CaO₅ cluster. pK_a(W1) ≈ pK_a(W2) << pK_a(W3) ≈ pK_a(W4) in the Mn₄CaO₅ cluster in water. However, pK_a(W1) ≈ pK_a(D1-Asp61) << pK_a(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.

Room-Temperature Energy-Sampling Kβ X-ray Emission Spectroscopy of the Mn4Ca Complex of Photosynthesis Reveals Three Manganese-Centered Oxidation Steps and Suggests a Coordination Change Prior to O2 Formation

In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn4Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state cycle are of great interest. In room-temperature (in situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn Kβ1,3 line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The Kβ1,3 line energies suggest a high-valence configuration of the Mn4Ca complex with Mn(III)3Mn(IV) in S0, Mn(III)2Mn(IV)2 in S1, Mn(III)Mn(IV)3 in S2, and Mn(IV)4 in S3 and, thus, manganese oxidation in each of the three accessible oxidizing transitions of the water-oxidizing complex. There are no indications of formation of a ligand radical, thus rendering partial water oxidation before reaching the S4 state unlikely. The difference spectra of both manganese Kβ1,3 emission and K-edge X-ray absorption display different shapes for Mn(III) oxidation in the S2 → S3 transition when compared to Mn(III) oxidation in the S1 → S2 transition. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environments suggests a change in the manganese ligand environment in the S2 → S3 transition, which could be oxidation of five-coordinated Mn(III) to six-coordinated Mn(IV). Conceivable options for the rearrangement of (substrate) water species and metal-ligand bonding patterns at the Mn4Ca complex in the S2 → S3 transition are discussed.

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).