Is There a Different Mechanism for Water Oxidation in Higher Plants?

The leading mechanism for the formation of O₂ in photosystem II (PSII) has, during the past decade, been established as the so-called oxyl-oxo mechanism. In that mechanism, O₂ is formed from a binding between an oxygen radical (oxyl) and a bridging oxo group. For the case of higher plants, that mechanism has recently been criticized. Instead, a nucleophilic attack of an oxo group on a five-coordinated Mn(V)═O group forming O₂ has been suggested in a so-called water-unbound (WU) mechanism. In the present study, the WU mechanism has been investigated. It is found that the WU mechanism is just a variant of a previously suggested mechanism but with a reactant and a transition state that have much higher energies. The addition of a water molecule on the empty site of the Mn(V)═O center is very exergonic and leads back to the previously suggested oxyl-oxo mechanism.