The Role of Water-Soluble Polypeptides and Calcium in Photosynthetic Oxygen Evolution

  • Demetrios F. Ghanotakis
  • Charles F. Yocum


The oxidizing side of photosystem II operates at unusually high redox potentials (2H2O → O2 + 4H+ + 4e, EO' = + 0.82V), and this implies the presence of a structural environment which can contribute to the stabilization of the reactive chemical intermediates formed during water oxidation. The photocatalysts of PSII activity are thought to be associated with two hydrophobic proteins of molecular weights of 43 and 47 kDa found in the PSII “core” complex as described by Satoh, et al. (1). This complex, although photochemically active, does not possess the capacity to evolve oxygen. A substantial body of experimental evidence now supports the view that three water-soluble proteins (33, 23 and 17 kDa) form part of the structure of the oxygen evolving apparatus. These water soluble polypeptides are believed to be essential for productive binding of inorganic cofactors such as Mn, Cl and Ca2+ within the oxygen-evolving complex. The involvement of Mn in water oxidation is well-documented (2–5). The implication of Cl as a cofactor for oxygen evolution activity by Warburg and Luttgens (6) and later by Bovè, et al. (7) has been confirmed by Izawa and his colleagues (8,9). The presence of the anion stabilizes activity against inactivation by heat, elevated pH and amines (10–13); Cl probably binds to a site on, or very near, Mn (13,14). Recent work has established the existence of a high-affinity Ca2+ binding site within the oxygen-evolving complex (15–18). Our own observations indicate that the 17 and 23 kDa polypeptides are important for the structural integrity of the oxidizing side of PSII (19,20). The presence of the 17 and 23 kDa species forms an environment which favors tight binding of Ca2+ (19) and Cl (21,22), and also shields the manganese complex from destruction by bulky reductants (20). Many studies have provided data to indicate that the 33 kDa protein is somehow associated with manganese binding to the PSII complex, although there are reports in the literature which describe conditions for release of the 33 kDa polypeptide which produce minimal effects on Mn binding to the PSII complex (23,24). To further investigate the involvement of calcium in oxygen evolution activity, we have used various lanthanides which are known to substitute for calcium in other calcium-binding proteins isolated from biological systems (25). In this paper, we show that calcium and lanthanum compete for sites on the oxidizing side of photosystem II, and in addition we report conditions which allow release of the 33 kDa protein without extraction of manganese.


Water Oxidation PSII Complex Manganese Complex Photosynthetic Oxygen Evolution PSII Membrane 
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Copyright information

© Plenum Press, New York 1986

Authors and Affiliations

  • Demetrios F. Ghanotakis
    • 1
  • Charles F. Yocum
    • 1
  1. 1.Division of Biological Sciences and Department of ChemistryThe University of MichiganAnn ArborUSA

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