Reconstruction of the water-oxidizing complex in manganese-depleted photosystem II preparations using mononuclear manganese complexes

The water-oxidizing complex of chloroplast photosystem II is composed of a cluster of four manganese atoms that can accumulate four oxidizing redox eq uivalents. Depletion of manganese from the water-oxidizing complex fully in hibits oxygen evolution, However, the complex can be reconstituted in the p resence of exogenous manganese in a process called photoactivation. In the present study, mononuclear manganese complexes with ligands derived from ei ther nitrosonaphthol and ethylenediamine (Niten) or from diaminohexane and salicylaldehyde (Salhxn) are used in photoactivation experiments. Measureme nts of photoinduced changes of chlorophyll fluorescence yield, thermal diss ipation using photoacoustic spectroscopy, photoreduction of 2,6-dichorophen olindophenol and oxygen evolution in manganese-depleted and in reconstitute d photosystem II preparations demonstrate that photoactivation is more effi cient when Niten and Salhxn complexes are used instead of MnC2, It is infer red that the aromatic ligands facilitate the interaction of the manganese a toms with photosystem II. The addition of CaCl2 and of the extrinsic polype ptide of 33 kDa known as the manganese-stabilizing protein during photoacti vation further enhances the recovery of electron transport and oxygen evolu tion activities. It is proposed that mononuclear manganese complexes are ab le to contribute to reconstitution of the water-oxidizing complex by sequen tial addition of single ions similarly to the current model for assembly of the tetranuclear manganese cluster and that these complexes constitute sui table model systems to study the assembly of the water-oxidizing complex.