Redox Potential of the Oxygen-Evolving Complex in the Electron Transfer Cascade of Photosystem II

In photosystem II (PSII), water oxidation occurs in the Mn4CaO5 cluster with the release of electrons via the redox-active tyrosine (TyrZ) to the reaction-center chlorophylls (P-D1/P-D2). Using a quantum mechanical/molecular mechanical approach, we report the redox potentials (E-m) of these cofactors in the PSII protein environment. The E-m values suggest that the Mn4CaO5 cluster, TyrZ, and P-D1/P-D2 form a downhill electron transfer pathway. E-m for the first oxidation step, E-m (S-1/S-2), is uniquely low (730 mV) and is similar to 100 mV lower than that for the second oxidation step, E-m (S-1/S-2) (830 mV) only when the O4 site of the Mn4CaO5 cluster is protonated in S-0. The O4-water chain, which directly forms a low-barrier H-bond with the Mn4CaO5 cluster and mediates protoncoupled electron transfer in the S-0 to S-1 transition, explains why the second lowest oxidation state, S-1 is the most stable and S-0 is converted to S-1 even in the dark.