Fig. 1.

(A) Redox cofactors in PSII. In the OEC, manganese atoms are shown in purple, calcium is shown in green, and oxygens are shown in red [Protein Data Bank (PDB) ID code 4UB6]. (B) YZ and its hydrogen-bonding partners. Water is represented by blue spheres (PDB ID code 4UB6). Illustration of divalent ion substitution and calcium depletion, starting with Ca-PSII (Sr²⁺ is depicted in cyan sphere, and Ba²⁺ in yellow). (C) S-state cycle of OEC; the black arrow indicates the only S-state transition allowed at 190 K. A proton-release pattern is shown (58). (Inset) YZ is oxidized and reduced by a PCET reaction.