Oxygen ligand exchange at metal sites - Implications for the O₂ evolving mechanism of photosystem II

The mechanism for photosynthetic O₂ evolution by photosystem II is currently 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 bond form. Recent measurements of the ¹⁸O exchange between the solvent water and the photogenerated O₂ as a function of the S-state cycle have provided some surprising insights to these questions (W. Hillier, T. Wydrzynski, Biochemistry 39 (2000) 4399-4405). The results show that one substrate water molecule is bound at the beginning of the catalytic sequence, in the S₀ state, while the second substrate water molecule binds in the S₃ state or possibly earlier. It may be that the second substrate water molecule only enters the catalytic sequence following the formation of the S₃ state. Most importantly, comparison of the observed exchange rates with oxygen ligand exchange in various metal complexes reveal that the two substrate water molecules are most likely bound to separate Mn(III) ions, which do not undergo metal-centered oxidations through to the S₃ state. The implication of this analysis is that in the S₁ state, all four Mn ions are in the +3 oxidation state. This minireview summarizes the arguments for this proposal. Copyright (C) 2001 Elsevier Science B.V.