pKa of the ligand water molecules in the oxygen-evolving Mn4CaO5 cluster in photosystem II
Keisuke Saito, Minesato Nakagawa, Hiroshi Ishikita
Abstract
Abstract 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 p K 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 p K 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. p K a (W1) ≈ p K a (W2) << p K a (W3) ≈ p K a (W4) in the Mn 4 CaO 5 cluster in water. However, p K a (W1) ≈ p K a (D1-Asp61) << p K a (W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.