Photosynthesis research, September 2019, Vol.141(3), pp.331-341
The oxidation of water to O is catalyzed by the Oxygen Evolving Complex (OEC), a MnCaO complex in Photosystem II (PSII). The OEC is sequentially oxidized from state S to S. The S state, (Mn)(Mn), coexists in two redox isomers: S, where Mn4 is Mn and S, where Mn1 is Mn. Mn4 has two terminal water ligands, whose proton affinity is affected by the Mn oxidation state. The relative energy of the two S redox isomers and the protonation state of the terminal water ligands are analyzed using classical multi-conformer continuum electrostatics (MCCE). The Monte Carlo simulations are done on QM/MM optimized S and S structures docked back into the complete PSII, keeping the protonation state of the protein at equilibrium with the OEC redox and protonation states. Wild-type PSII, chloride-depleted PSII, PSII in the presence of oxidized Y/protonated D1-H190, and the PSII mutants D2-K317A, D1-D61A, and D1-S169A are studied at pH 6. The wild-type PSII at pH 8 is also described. In qualitative agreement with experiment, in wild-type PSII, the S redox isomer is the lower energy state; while chloride depletion or pH 8 stabilizes the S state and the mutants D2-K317A, D1-D61A, and D1-S169A favor the S state. The protonation states of D1-E329, D1-E65, D1-H337, D1-D61, and the terminal waters on Mn4 (W1 and W2) are affected by the OEC oxidation state. The terminal W2 on Mn4 is a mixture of water and hydroxyl in the S state, indicating the two water protonation states have similar energy, while it remains neutral in the S and S states. In wild-type PSII, advancement to S leads to negligible proton loss and so there is an accumulation of positive charge. In the analyzed mutations and Cl depleted PSII, additional deprotonation is found upon formation of S state.
Grand Canonical Monte Carlo Simulations ; Linear Response Approximation (Lra) ; Oxygen Evolving Complex (Oec) ; Photosystem II ; Proton Transfer ; Pka
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