title: Computational insights on crystal structures of the oxygen-evolving complex of photosystem II with either Ca(2+) or Ca(2+) substituted by Sr(2+).
abstract: The oxygen-evolving complex of photosystem II can function with either Ca(2+) or Sr(2+) as the heterocation, but the reason for different turnover rates remains unresolved despite reported X-ray crystal structures for both forms. Using quantum mechanics/molecular mechanics (QM/MM) calculations, we optimize structures with each cation in both the resting state (S1) and in a series of reduced states (S0, S-1, and S-2). Through comparison with experimental data, we determine that the X-ray crystal structures with either Ca(2+) or Sr(2+) are most consistent with the S-2 state (i.e., Mn4[III,III,III,II] with O4 and O5 protonated). As expected, the QM/MM models show that Ca(2+)/Sr(2+) substitution results in the elongation of the heterocation bonds and the displacement of terminal waters W3 and W4. The optimized structures also show that hydrogen-bonded W5 is displaced in all S states with Sr(2+) as the heterocation, suggesting that this water may play a critical role during water oxidation.
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06511, United States.
authors.forename: Leslie
authors.initials: L
authors.lastname: Vogt
authors.forename: Mehmed Z
authors.initials: MZ
authors.lastname: Ertem
authors.forename: Rhitankar
authors.initials: R
authors.lastname: Pal
authors.forename: Gary W
authors.initials: GW
authors.lastname: Brudvig
authors.forename: Victor S
authors.initials: VS
authors.lastname: Batista
mesh_terms: D002118:Calcium / Q000378:metabolism*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D011789:Quantum Theory; D013324:Strontium / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D010100:Oxygen; D002118:Calcium; D013324:Strontium