Photosystem‐II Electron Transport and Phosphorylation with Dibromothymoquinone as the Electron Acceptor

Abstract

Dibromothymoquinone has two effects on isolated chloroplasts. At very low concentrations it inhibits the reduction of conventional hydrophilic electron acceptors, probably by acting as a plastoquinone antagonist. At higher concentrations it acts as an electron acceptor, intercepting electrons either before or at the site of its inhibitory activity. Reduced dibromothymoquinone can be readily reoxidized by excess ferricyanide in the reaction mixture or by molecular oxygen. The transfer of electrons to this substance from water is coupled to phosphorylation. The pH optima for this reduction and associated phosphorylation are both at 7.3, considerably lower than the pH optimum of 8.4 observed with the normal Hill reaction. The ratio of molecules of ATP formed to pairs of electrons transported is relatively constant (0.3 –0.4) between pH 6 and 9. The rate of reduction is independent of the presence or absence of ADP and phosphate or uncouplers. The reduction of dibromothymoquinone resembles the reduction of other lipid‐soluble oxi‐dants such as oxidized p‐phenylenediamines and 2,5‐dimethylquinone in several respects. Both reactions are insensitive to the plastocyanin inhibitors KCN and polylysine, but are sensitive to 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea. They support phosphorylation with a similar efficieney. However, dibromothymoquinone reduction differs from the reduction of other lipophilic oxidants in that its sensitivity to dichlorophenyl‐dimethylurea decreases with increasing light intensities. This implies that it may be reduced via a pool (plastoquinone?) which is a common electron acceptor for independent photosystem II units whereas oxidized p‐phenylenediamines and 2,5‐dimethylquinone are reduced directly by these independent units. Copyright © 1973, Wiley Blackwell. All rights reserved