Stoichiometry of Proton Release from the Catalytic Center in Photosynthetic Water Oxidation

The catalytic center (CC) of water oxidation in photosystem II passes through four stepwise increased oxidized states (S0–S4) before O2 evolution takes place from 2H2O in the S4 → S0 transition. The pattern of the release of the four protons from the CC cannot be followed directly in the medium, because proton release from unknown amino acid residues also takes place. However, pH-independent net charge oscillations of 0:0:1:1 in S0:S1:S2:S3 have been considered as an intrinsic indicator for the H+ release from the CC. The net charges have been proposed to be created as the charge difference between electron abstraction and H+release from the CC. Then the H+ release from the CC is 1:0:1:2 for the S0 → S1 → S2 → S3 → S0 transition. Strong support for this conclusion is given in this work with the analysis of the pH-dependent pattern of H+ release in the medium measured directly by a glass electrode between pH 5.5 and 7.2. Improved and crystallizable photosystem II core complexes from the cyanobacterium Synechococcus elongatus were used as material. The pattern can be explained by protons released from the CC with a stoichiometry of 1:0:1:2 and protons from an amino acid group (pK ≈ 5.7) that is deprotonated and reprotonated through electrostatic interaction with the oscillating net charges 0:0:1:1 in S0:S1:S2:S3. Possible water derivatives that circulate through the S states have been named.