Fig. 1: Light-driven reaction cycle of PSII water oxidation and active-site Mn4CaO5-6 cluster with water molecules and selected residues in its environment.
a Water oxidation cycle: Starting in the dark-stable S1 state, four consecutive light flashes drive PSII through the five semi-stable S-states of Kok’s classical S-state cycle (inner circle of Si-states6). By alternating oxidation (e─) and proton removal (H+) steps, the Mn4CaOx cluster accumulates up to 4 oxidation equivalents, while avoiding excessive charge accumulation (outer cycle of Sin/+ states11). The electrons are removed by transfer to the previously oxidized tyrosine residue (YZ); the protons move to the periphery of PSII; two water molecules are inserted3,87; O2 is formed and released. Approximate room temperature values of the respective time constants are indicated13. b Active-site structure: Water molecules and selected residues in the vicinity of the Mn4CaO5-6 cluster based on the 2.07 Å two-flash (S3-rich) crystallographic model (PDB ID: 6DHO, PSII from Thermosynechococcus vestitus BP-1; also used for Mn4CaO6 cluster structure in panel a)16. Residues from the D1, D2, and CP43 subunits are depicted in green, yellow, and pink, respectively; Mn in purple, Ca in yellow, Cl in green. Water molecules and six μ-oxo bridges (O1, O2,…, O6) are depicted as red or orange spheres. The pentagon of H-bonded water molecules, consisting of W26 to W30, has been termed “water wheel” and brought into play as an element of substrate water delivery16,17,19,46. Numbering of water molecules is based on ref. 16. Approximate boundaries of the Cl1, O4, and O1 channels near the Mn4CaO5-6 cluster are depicted in orange (Cl1), blue (O4), and yellow (O1). The proposed directionalities in proton-coupled electron transfer and water insertion in the oxygen-evolving S3→ S4→ S0 transition are indicated, as are the two key residues investigated here by genetic modification (D1-D61 and D1-N298).
