Oxygen ligand exchange at metal sites - implications for the O-2 evolving mechanism of photosystem II

The mechanism for photosynthetic O-2 evolution by photosystem II is current ly a topic of intense debate. Important questions remain as to what is the nature of the binding sites for the substrate water and how does the O-O bo nd form. Recent measurements of the O-18 exchange between the solvent water and the photogenerated O-2 as a function of the S-state cycle have provide d some surprising insights to these questions (W. Hillier, T. Wydrzynski, B iochemistry 39 (2000) 4399-4405). The results show that one substrate water molecule is bound at the beginning of the catalytic sequence, in the S-0 s tate, while the second substrate water molecule binds in the S-3 state or p ossibly earlier. It may be that the second substrate water molecule only en ters the catalytic sequence following the formation of the S-3 state. Most importantly, comparison of the observed exchange rates with oxygen ligand e xchange in various metal complexes reveal that the two substrate water mole cules are most likely bound to separate Mn-III ions, which do not undergo m etal-centered oxidations through to the S-3 state. The implication of this analysis is that in the S-1 state, all four Mn ions are in the +3 oxidation state. This minireview summarizes the arguments for this proposal. (C) 200 1 Elsevier Science B.V. All rights reserved.