Extended Data Fig. 8. Complete picture of events in the oxygen-evolution transition.

Proton and electron transfers are highlighted by blue and red arrows, whereas relocations of heavy atoms are indicated by black arrows. Substrate oxygen atoms (O5 and O6) are depicted in red. The mechanism of the steps 1–5 was investigated in the present study, while the events occurring between the steps 5 and 7 (i.e. the release of molecular oxygen after the peroxide bond formation and the Mn₄CaO₅ cluster restoration) were described in ref. ⁸⁴. The events in the transition from 7 to 8 are plausible, but currently not backed up by calculations. The eight panels illustrate the following sequence of events: Oxidation of Tyr161 in the S₃ state, which is coupled to the proton transfer from Tyr161 to His190 (1), induces a conformational change involving the side chain of Lys317, resulting in the approach of Lys317 to Glu312 and deprotonation of Glus312 (2) within about 340 μs after the laser flash. Within the next 2.5 ms, the transfer of one electron from the O6 atom to Tyr161 coupled to a concerted Grotthus-type relocation of three protons (3), resulting in S₄ formation by radicalization of O6 coupled to protonation of Asp61 (4). Thereafter, the O6 radical forms a peroxide bond with the oxygen atom O5 (5) and the subsequent deprotonation of Asp61 and release of molecular oxygen. The vacancy site formed by the oxygen evolution step is rapidly refilled with a water molecule coordinated to the Ca²⁺ ion, with simultaneous proton transfer to the hydroxide ion bound to Mn4, and insertion of W5 into the coordination sphere of Ca²⁺ (6). The restoration of the Mn₄CaO₅ in the S₀ state is completed by the deprotonation of a water molecule coordinated to Mn4, relocation of Lys317 close to Asp61, and protonation of Glu312 (7 and 8).