[Role of plastoquinone in photosystem 2 reactions]

The hexane treatment of the freeze-dried chloroplasts from bean leaves results in complete inhibition of electron transport from water to silicomolybdate, as a specific electron acceptor from photosystem 2 insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea. The photophosphorylation coupled to this electron flow is inhibited as well. The reconstruction with the concentrated hexane extract or with the coenzyme Q results in partial reactivation of the light-induced O2 evolution, silicomolybdate reduction and coupled phosphorylation. The electron transport model of the oxidizing side of photosystem 2 is put forward, according to which plastoquinone acts as an electron carrier between the manganese water-splitting complex and the photosystem 2 reaction center. The accumulation of positive charges during four subsequent photosystem 2 excitations occurs in the form of ion-pairs of plastosemiquinone radical anion with manganese (III) cation, separated from the photochemical center by a hydrophobic barrier. The two water molecules are oxidized by interaction with an electron acceptor of moderate redox potential near 0.8 beta without formation of toxic active species such as OH-radicals as intermediates.