title: The PSII calcium site revisited.
abstract: Oxidation of H2O by photosystem II is a unique redox reaction in that it requires Ca2+ as well as Cl- as obligatory activators/cofactors of the reaction, which is catalyzed by Mn atoms. The properties of the binding site for Ca2+ in this reaction resemble those of other Ca2+ binding proteins, and recent X-ray structural data confirm that the metal is in fact ligated at least in part by amino acid side chain oxo anions. Removal of Ca2+ blocks water oxidation chemistry at an early stage in the cycle of redox reactions that result in O-O bond formation, and the intimate involvement of Ca2+ in this reaction that is implied by this result is confirmed by an ever-improving set of crystal structures of the cyanobacterial enzyme. Here, we revisit the photosystem II Ca2+ site, in part to discuss the additional information that has appeared since our earlier review of this subject (van Gorkom HJ, Yocum CF In: Wydrzynski TJ, Satoh K (eds) Photosystem II: the light-driven water:plastoquinone oxidoreductase), and also to reexamine earlier data, which lead us to conclude that all S-state transitions require Ca2+.
authors.affiliation: Department of Biophysics, Huygens Laboratory, Leiden University, P.O. Box 9504, Leiden, RA 2300, The Netherlands.
authors.forename: M
authors.initials: M
authors.lastname: Miqyass
authors.forename: H J
authors.initials: HJ
authors.lastname: van Gorkom
authors.forename: C F
authors.initials: CF
authors.lastname: Yocum
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding
chemical_list: D045332:Photosystem II Protein Complex; D008345:Manganese; D002118:Calcium