Reconstitution of the photosystem IICa2+ binding site

The roles of Ca2+ in H2O 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 Ca2+ depleted O-2 evolving complex that retains extrinsic 23- and 17-kDa polypeptides with reductants (NHOH and hydroquinone) [Biochemistry 41 (2002) 958]. Calcium appears to bind to photosystem II at a site where it could bind substrate H2O. Equilibration of Ca2+ with this binding site is facilitated by increased ionic strength, and incubation of Ca2+ reconstitution mixtures at 22 degreesC accelerates equilibration of Ca2+ with the site. The Ca2+ reconstituted enzyme system regains properties of unperturbed photosystem II: Sensitivity to NH2OH inhibition is decreased, and Cl- binding with increased affinity can be detected. The ability of ionic strength and temperature to facilitate rebinding of Ca2+ 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. (C) 2004 Elsevier B.V. All tights reserved.