Energy Conservation in Photoreductions by Photosystem II. Reversal of Dibromothymoquinone Inhibition of Hill-Reactions by Phenylenediamines

Abstract

Abstract The plastoquinone antagonist dibromothymoquinone interrupts the photosynthetic electron transport chain between photosystem II and I by inhibiting the oxidation of plastohydroquinone by photosystem I. The effect of phenylenediamines on NADP and ferricyanide photoreduction inhibited by dibromothymoquinone is studied in isolated chloroplasts. IV-substituted phenylene­ diamines fully restore NADPH and ATP formation as well as oxygen evolution in DBMIB inhibited NADP reduction, but are practically inactive in stimulating ferricyanide reduction by photo­ system II. On the other hand phenylenediamine and C-substituted phenylenediamines are only weakly active in restoring NADP reduction but are very active in stimulating ferricyanide reduc­ tion. The P/e2 ratio in restoration of NADP reduction by phenylenediamines is close to 1, whereas the P/e2 ratio in ferricyanide reduction is about 0.4 (the ratio is dependent on the DBMIB con­ centration). The reversal of the DBMIB inhibition in NADP reduction by phenylenediamines is attributed to a bypass of the inhibition site: Reduced phenylenediamines are electron donors for photosystem I after and oxidized phenylenediamines are electron acceptors for photosystem II be­ fore the DBMIB inhibition site. To explain the different rates and P/e2 ratios in the various systems it is assumed that all phenylenediamines are reduced at the expense of water oxidation by photo­ system II but via plastoquinone and on the inside of the membrane. Therefore two energy con­ serving steps (i. e. two proton releasing sites on the inside of the membrane) are involved, N-sub­ stituted phenylenediamines like TMPD reduced by photosystem II inside remain inside the mem­ brane and are reoxidized inside by photosystem I. This way they connect photosystem II bade onto photosystem I, electron transport rates are high and the P/e2 ratio is one in the restored NADP photoreduction. Phenylenediamine and C-substituted phenylenediamines on the other hand travel back to the outside through the membrane, and by doing so carry some of the protons released on the inside of the membrane by the energy conserving steps back to the outside. This way they are very active in stimulating the rate of ferricyanide photoreduction but the P/e2 ratio is only about 0.4. These phenylenediamines being removed from the inside are less active in restoring the rate of NADP reduction