Photosynthetic oxygen evolution: H/D isotope effects and the coupling between electron and proton transfer during transitions S₂⟹₃ and S₃⟹S₄→S₀

The oxygen evolving complex (OEC) of photosystem II (PS II) incorporates a tetra Mn‐cluster, tyrosine (Y z ) and probably one histidine residue (X) as redox cofactors. Four quanta of light drive the OEC through the increasingly oxidized states S 0 ⟹S 1 ⟹S 2 ⟹S 3 ⟹S 4 to yield O 2 during S 4 →S 0 . It has been speculated that some oxidized cofactor abstracts hydrogen from bound water. This implies that its oxidoreduction is electroneutral and linked to its deprotonation. To identify such steps we investigated the rates of electron transfer and proton release as function of the D 2 O/H 2 O ratio, the pH, and the temperature in thylakoids and PS II core particles. Upon oxidation of X on S 2 ⟹S 3 , a rise of the pH from 5 to 8 increased the rate of the electron transfer to Y z by a factor of 2.5 and substitution of D 2 O for H 2 O gave an isotopic ratio of 2.1. Contrastingly, during all other transitions, including the O 2 ‐evolving step S 4 →S 0 , the electron transfer rate was much less sensitive to these parameters (factors of ≤ 1.4). These results suggest a kinetical steering role of proton transfer only during S 2 ⟹S 3 . We propose that X * (His * ?) serves as a hydrogen acceptor for bound water during S 4 ⟹S 0 .