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
(PD1/PD2). Using a quantum mechanical/molecular
mechanical approach, we report the redox potentials (Em) of these cofactors in the PSII protein environment.
The Em values suggest that the Mn4CaO5 cluster, TyrZ, and PD1/PD2 form a downhill electron transfer pathway. Em for the first oxidation step, Em(S0/S1), is uniquely low (730 mV) and is ∼100
mV lower than that for the second oxidation step, Em(S1/S2) (830 mV) only when the
O4 site of the Mn4CaO5 cluster is protonated
in S0. The O4-water chain, which directly forms a low-barrier
H-bond with the Mn4CaO5 cluster and mediates
proton-coupled electron transfer in the S0 to S1 transition, explains why the second lowest oxidation state, S1, is the most stable and S0 is converted to S1 even in the dark.