PHOTOSYNTHETIC OXYGEN EVOLUTION - H D ISOTOPE EFFECTS AND THE COUPLING BETWEEN ELECTRON AND PROTON-TRANSFER DURING TRANSITIONS S-2-DOUBLE-RIGHT-ARROW-S-3 AND S-3-DOUBLE-RIGHT-ARROW-S-4-]S-0/

The oxygen evolving complex (OEC) of photosystem II (PS II) incorporat es a tetra Mn-cluster, tyrosine (Y-Z) and probably one histidine resid ue (X) as redox cofactors. Four quanta of light drive the OEC through the increasingly oxidized states S-0 double right arrow S-1 double rig ht arrow S-2 double right arrow S-3 double right arrow 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 oxidoreduc tion is electroneutral and linked to its deprotonation. To identify su ch steps we investigated the rates of electron transfer and proton rel ease as function of the D2O/H2O ratio, the pH, and the temperature in thylakoids and PS II core particles. Upon oxidation of X on S-2 double right arrow S-3, a rise of the pH from 5 to 8 increased the rate of t he electron transfer to Y-Z by a factor of 2,5 and substitution of D2O for H2O gave an isotopic ratio of 2.1. Contrastingly, during all othe r transitions, including the O-2-evolving step S-4-->S-0, the electron transfer rate was much less sensitive to these parameters (factors of less than or equal to 1.4). These results suggest a kinetical steerin g role of proton transfer only during S-2 double right arrow S-3. We p ropose that X(.) (His(.)?) serves as a hydrogen acceptor for bound wat er during S-4-->S-0.