Interaction of Halogenated 1,4-Benzoquinones with System II of Photosynthesis

Abstract

The interaction of halogenated p-benzoquinones with PS II has been analyzed by measurements of fluorescence induction curves and the average oxygen yield per flash in isolated class II chloroplasts. It was found: 1)The normalized area over the fluorescence induction curve in the presence of DCMU. A/Fmax, markedly increases if halogenated p-benzoquinones are added before DCMU. The effect is eliminated by DCMU addition prior to that of the quinones. 2)The extent of A/Fmax increases with increasing dark time between the additions of 2,3.5-tri- bromo-6-methyl-1,4-benzoquinone (TBTO) and DCMU. respectively. 3) Some of the halogenated p-benzoquinones were found to act as efficient electron acceptors under repetitive excitation at low flash frequency (2 Hz). In the case of TBTO and 2,3-di-chloro-5-t-butyl-1,4-benzoquinone (TBU 13) the sensitivity of the average oxygen yield per flash was shifted towards higher concentrations of DCMU. 4) Halogenated p-benzoquinones can also affect the stability of oxidizing redox equivalents in the water oxidizing enzyme system Y. This effect depends on the nature of the substituents. The present results are interpreted by the assumption of a covalent binding of halogenated p-benzoquinones in the vicinity of Qᴀ. This binding is prevented by DCMU. The possibility of allosteric interaction between the donor and acceptor side of PS II is discussed.