The two water molecules serving as substrate for the oxygen evolution in Photosystem II are already bound in the S-2 state of the Kok-Joliot's cycle. Nevertheless, an additional water molecule is supposed to bind the duster during the transition between the S-2 and S-3 states, which has been recently revealed to have the Mn4CaO5 catalytic cluster arranged in an open cubane fashion. In this Letter, by means of ab initio calculations, we investigated the possible pathways for the binding of the upcoming water molecule. Upon the four different possibilities checked in our calculations, the binding of the crystallographic water molecule, originally located nearby the Cl- binding site, showed the lowest activation energy barrier. Our findings therefore support the view in which the W2 hydroxyl group and the O5 oxygen act as substrates for the oxygen evolution. Within this framework the role of the open and closed Mn4CaO5 conformers is clarified as well as the exact mechanistic events occurring along the S-2 to S-3 transition.
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