Fig. 5.

Protein environment of the apo-OEC binding site from Zouni and coworkers (PDB ID code 5MX2) (31). Protons have been added to titratable groups based on the predominant species observed in MCCE calculations (SI Appendix, Fig. S6). The HAS represents the location where Mn²⁺ binds during photoassembly. To form intermediate C (Fig. 1), oxidized Mn³⁺ must migrate deeper into the apo site (purple arrow). Water must be deprotonated for μ-hydroxo bridge formation, and the proton(s) must be shuttled out of the site (cyan arrow). Residues D1-E333 and D1-61 facilitate these 2 processes. When chloride is removed, the pK_a values of CP43-E354, D1-E333, and D1-D61 (tan sticks) decrease and the pK_a value of D1-D342 (purple sticks) increases.