[Estimation of the number of manganese atoms functioning in the donating side of photosystem 2].

Comparison of theoretically expected and experimental dependence of reactivation of the donor side of photosystem 2 (PS-2) on concentration of exogenic MnCl2 that was observed in PS-2 preparation after complete extraction of manganese was made. It is shown that in the absence of Mg2+ or bivalent cations of other metals (Me2+) the reactivation corresponds to the filling of a tetranuclear Mn-containing active center; in the presence of 3 microM Mg2+ or other Me2+ the reactivation takes place as a result of binding of two manganese atoms in the center. It is suggested that a tetranuclear manganese complex (which probably is responsible for the photosynthetic oxidation of water) functions in the donor side of PS-2; two of these four manganese atoms can be substituted by Mg2+ or other Me2+ without changing activity of the center.