pK a of the ligand water molecules in the oxygen-evolving Mn 4 CaO 5 cluster in photosystem II.

Release of the protons from the substrate water molecules is prerequisite for O 2 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 Ca 2+ , of the Mn 4 CaO 5 cluster. pK a (W1) ≈ pK a (W2) << pK a (W3) ≈ pK a (W4) in the Mn 4 CaO 5 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.