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°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. © 2004 Elsevier B.V. All rights reserved.
Vander Meulen, K. A., Hobson, A., & Yocum, C. F. (2004, April 12). Reconstitution of the photosystem II Ca 2+ binding site. Biochimica et Biophysica Acta - Bioenergetics. https://doi.org/10.1016/j.bbabio.2003.08.012