Unique binding site for Mn2+ ion responsible for reducing an oxidized Yz tyrosine in manganese-depleted photosystem II membranes

Binding of Mn2+ to manganese-depleted photosystem II and electron donation from the bound Mn2+ to an oxidized Y-z tyrosine were studied under the same equilibrium conditions. Mn2+ associated with the depleted membranes in a nonsaturating manner when added alone, but only one Mn2+ ion per photosystem II (PS IT) was bound to the membranes in the presence of other divalent cations including Ca2+ and Mg2+. Mn2+-dependent electron donation to photosystem II studied by monitoring the decay kinetics of chlorophyll fluorescence and the electron paramagnetic resonance (EPR) signal of an oxidized Y-z tyrosine (Y-z(+)) after a single-turnover flash indicated that the binding of only one Mn2+ ion to the manganese-depleted PS II is sufficient for the complete reduction of Y-z(+) induced by Rash excitation. The results indicate that the manganese-depleted membranes have only one unique binding site, which has higher affinity and higher specificity for Mn2+ compared with Mg2+ and Ca2+, and that Mn2+ bound to this unique site can deliver an electron to Y-z(+) with high efficiency. The dissociation constant for Mn2+ of this site largely depended on pH, suggesting that a single amino acid residue with a pK(a) value around neutral pH is implicated in the binding of Mn2+. The results are discussed in relation to the photoactivation mechanism that forms the active manganese cluster.