Fig. 5. Crystal structures of the native OEC and the synthetic Mn₄Ca complex I.⁷⁵ (a) Mn₄CaO₅ core of the native OEC (8). (b) Mn₄CaO₄ core of I. (c) Structure of the native OEC, including ligating protein sidechains and water molecules. (d) Structure of I, including all ligand groups. Structures of complexes with exchanged terminal ligands. Adapted from ref. 75. Copyright© 2015 The American Association for the Advancement of Science. The complexes differ from I by ligand exchange either on (e) Ca(ii) or (f) Mn₄(iii). (g) Design of a [4Fe–4S] binding site in CcP to mimic the heme[4Fe–4S] center in native SiR.⁷⁷ Adapted from ref. 77. Copyright© 2018 The American Association for the Advancement of Science.