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

Abstract

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 18O 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.