Studies on the ADRY agent-induced mechanism of the discharge of the holes trapped in the photosynthetic waterspliting enzyme system Y

Abstract

The effect of 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene (ANT 2p) on the oxygen evolution, fluorescence and delayed light emission of spinach chloroplasts has been investigated. It was found that; 1. 1. ANT 2p strongly accelerates the deactivation of states S 2 and S 3 of the water-splitting enzyme system Y. 2. 2. In DCMU-poisoned chloroplasts ANT 2p prevents the back reaction of the electrons located at the primary acceptor, Q, with the holes (positive charges) stored in the water-splitting enzyme system Y. 3. 3. In chloroplast suspensions without artificial electron acceptors, the fluorescence rise in weak actinic light vanishes in the presence of ANT 2p. The fluorescence yield in DCMU-inhibited chloroplasts is not significantly changed by ANT 2p. 4. 4. The intensity of the delayed light emitted after excitation with one short flash is remarkably decreased by ANT 2p. 5. 5. In weak actinic light the reduction rate of the artificial electron acceptor methyl viologen is suppressed in the presence of ANT 2p. From these experimental results it is concluded that ANT 2p induces a cycle within the electron transport chain, leading to a dissipative recombination of the holes stored in the water-splitting enzyme Y with the electrons of an as yet unknown donor. Two possibilities for the mode of action of this cycle are discussed.