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

The mechanism for photosynthetic O-2 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 O-18 exchange between the solvent water and the photogenerated O-2 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 S-0 state, while the second substrate water molecule binds in the S-3 state or possibly earlier. It may be that the second substrate water molecule only enters the catalytic sequence following the formation of the S-3 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 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) 2001 Elsevier Science B.V. All rights reserved.