Abstract
Release of the protons from the substrate water molecules is prerequisite for O2 evolution in photosystem II (PSII). Proton-releasing water molecules with low pKa 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 pKa values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca2+, of the Mn4CaO5 cluster. pKa(W1) ≈ pKa(W2) << pKa(W3) ≈ pKa(W4) in the Mn4CaO5 cluster in water. However, pKa(W1) ≈ pKa(D1-Asp61) << pKa(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.
Highlights
Release of the protons from the substrate water molecules is prerequisite for O2 evolution in photosystem II (PSII)
In S0 and S16, the ligand water molecules W1–W4 are H2O in quantum mechanical/molecular mechanical (QM/MM) models[3,7,8], whereas W2 is assumed to be OH− in simplified QM models[9,10,11]
The results show that pKa(W1) ≈ pKa(W2)
Summary
Release of the protons from the substrate water molecules is prerequisite for O2 evolution in photosystem II (PSII). We calculate the energy difference (ΔEwater) between the protonated and deprotonated states of hexa-aqua metal complexes in water (Fig. 2).
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