view article
Figure 1
Overview of the water oxidation reaction in PS II. (a) Overall structure of PS II, embedded in the thylakoid membrane of cyano­bacteria, algae and higher plants. The PS II enzyme is a dimer with pseudo-C2 symmetry about the membrane axis as shown. Light absorption at the reaction center triggers charge separation that drives the water oxidation reaction at the oxygen evolving complex (OEC). The OEC (Mn4CaO5 cluster), where water oxidation takes place, is located on the lumenal side of the membrane. Three distinct water channels [depicted in red (O1 channel), green (Cl1 channel) and blue (O4 channel)] connect the cluster to the lumen and are thought to be involved in water and proton transport. (b) Detailed view of the OEC in the dark-stable S1 state with the coordinating waters and the water channels that extend towards the bulk from the catalytic center. Mn atoms are shown in purple, Ca atoms in green and O atoms in red. (c) The Kok cycle for the water oxidation reaction. Starting from the most reduced S0 state, each transition in the Kok cycle is initiated by the absorption of a photon of light by the reaction center in PS II, leading to the extraction of one electron from the OEC and advancement to the next S-state. Water oxidation and oxygen release take place after the formation of the transient S4 state and the cluster resets to S0.

IUCrJ
Volume 10| Part 6| November 2023| Pages 642-655
ISSN: 2052-2525
https://doi.org/10.1107/S2052252523008928