O 2 evolution and recovery of the water-oxidizing enzyme.

In photosystem II, light-induced water oxidation occurs at the Mn4 CaO5 cluster. Here we demonstrate proton releases, dioxygen formation, and substrate water incorporation in response to Mn4 CaO5 oxidation in the protein environment, using a quantum mechanical/molecular mechanical approach and molecular dynamics simulations. In S2 , H2 O at the W1 site forms a low-barrier H-bond with D1-Asp61. In the S2 -to-S3 transition, oxidation of OW1 H- to OW1 •- , concerted proton transfer from OW1 H- to D1-Asp61, and binding of a water molecule Wn-W1 at OW1 •- are observed. In S4 , W n -W1 facilitates oxo-oxyl radical coupling between OW1 •- and corner μ-oxo O4. Deprotonation via D1-Asp61 leads to formation of OW1 =O4. As OW1 =O4 moves away from Mn, H2 O at W539 is incorporated into the vacant O4 site of the O2 -evolved Mn4 CaO4 cluster, forming a μ-oxo bridge (Mn3-OW539 -Mn4) in an exergonic process. Simultaneously, Wn-W1 is incorporated as W1, recovering the Mn4 CaO5 cluster.