Manganese and Calcium Requirements for Reconstitution of Oxygen-Evolution Activity in Manganese-Depleted Photosystem II Membranes

The Mn complex of photosystem II and O₂-evolution activity are reconstituted in Mn-depleted photosystem II membranes in a light-dependent process called photoactivation. Recovery of O₂-evolution activity requires both Mn²⁺ and Ca²⁺ in the photoactivation medium. The Mn²⁺ and Ca²⁺ dependences of both the effective rate constant and yield of photoactivation have been determined. A comparison of these data with the predictions of mathematical models for photoactivation leads to the conclusion that photoactivation occurs in two stages. The first stage, photoligation of Mn, requires light and depends primarily on Mn²⁺. The second stage, binding of Ca²⁺, is required for expression of O₂-evolution activity. This two-stage model affords an excellent fit to the data and provides dissociation constants and binding stoichiometrics for Ca²⁺ and Mn²⁺. We conclude that one Mn²⁺ ion is bound and photooxidized in the rate-determining step(s) of photoactivation. On the basis of these results and data already in the literature, the molecular details of the elementary steps in photoactivation are discussed and a mechanism of photoactivation is proposed.