Proton-Coupled Electron Transfer During the S-State Transitions of the Oxygen-Evolving Complex of Photosystem II

The oxygen-evolving complex (OEC) of photosystem II (PSII) is a unique Mn4O5Ca cluster that catalyzes water oxidation via four photoactivated electron transfer steps. As the protein influence on the redox and protonation chemistry of the OEC remains an open question, we present a classical valence model of the OEC that allows the redox state of each Mn and the protonation state of bridging μ-oxos and terminal waters to remain in equilibrium with the PSII protein throughout the redox cycle. We find that the last bridging oxygen loses its proton during the transition from S0 to S1. Two possible S2 states are found depending on the OEC geometry: S2 has Mn4(IV) with a proton lost from a terminal water (W1) trapped by the nearby D1-D61 if O5 is closer to Mn4, or Mn1(IV), with partial deprotonation of D1-H337 and D1-E329 if O5 is closer to Mn1. In S3, the OEC is Mn4(IV) with W2 deprotonated. The estimated OEC Em's range from +0.7 to +1.3 V, enabling oxidation by P680(+), the primary electron donor in PSII. In chloride-depleted PSII, the proton release increases during the S1 to S2 transition, leaving the OEC unable to properly advance through the water-splitting cycle.