title: Reconstitution of the photosystem II Ca2+ binding site.
abstract: The roles of Ca(2+) in H(2)O oxidation may be as a site of substrate binding, and as a structural component of the photosystem II O(2)-evolving complex. One indication of this dual role of the metal is revealed by probing the Mn cluster in the Ca(2+) depleted O(2) evolving complex that retains extrinsic 23- and 17-kDa polypeptides with reductants (NH(2)OH and hydroquinone) [Biochemistry 41 (2002) 958]. Calcium appears to bind to photosystem II at a site where it could bind substrate H(2)O. Equilibration of Ca(2+) with this binding site is facilitated by increased ionic strength, and incubation of Ca(2+) reconstitution mixtures at 22 degrees C accelerates equilibration of Ca(2+) with the site. The Ca(2+) reconstituted enzyme system regains properties of unperturbed photosystem II: Sensitivity to NH(2)OH inhibition is decreased, and Cl(-) binding with increased affinity can be detected. The ability of ionic strength and temperature to facilitate rebinding of Ca(2+) to the intact O(2) evolving complex suggests that the structural environment of the oxidizing side of photosystem II may be flexible, rather than rigid.
authors.affiliation: Department of Molecular, Cellular and Developmental Biology, University of Michigan, Ann Arbor, MI 48109-1048, USA.
authors.forename: Kirk A
authors.initials: KA
authors.lastname: Vander Meulen
authors.forename: Amanda
authors.initials: A
authors.lastname: Hobson
authors.forename: Charles F
authors.initials: CF
authors.lastname: Yocum
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D020134:Catalytic Domain; D007700:Kinetics; D008345:Manganese / Q000737:chemistry; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D014867:Water / Q000378:metabolism
chemical_list: D045332:Photosystem II Protein Complex; D014867:Water; D008345:Manganese; D002118:Calcium