RETARDATION OF PHOTOSYNTHETIC WATER OXIDATION AND RESPIRATORY ELECTRON-TRANSFER BY PROTONOPHORIC UNCOUPLERS - A REVIEW

Many of the protonophoric uncouplers widely used in bioenergetics rese arch are ADRY reagents. These substances are oxidized by the Mn cluste r of the O-2-evolving complex of Photosystem II in chloroplasts and cy anobacteria, and they inhibit photosynthetic water oxidation. Oxidized ADRY reagents can be reduced by the membrane pool of plastoquinone, l eading to formation of an artificial cyclic electron transfer chain ar ound Photosystem II, A hypothesis is proposed for the mechanism of inh ibition of mitochondrial respiration under the influence of relatively high concentrations of protonophoric uncouplers. It is supposed that the uncouplers can be oxidized by the peroxide and oxoferryl intermedi ates of cytochrome oxidase and be reduced by ubiquinone. This shunts t he cytochrome bc(1) complex and reduces the sensitivity of respiration to inhibitors of this complex. Under conditions of respiratory substr ate deficiency, the uncoupler radicals recombine with ubisemiquinone a nion-radicals and apparently form stable complexes resulting in suppre ssion of respiration. Phenolic uncouplers also exhibit properties of u biquinone antagonists at the level of the ubiquinol oxidizing site Q(0 ) of the cytochrome bc(1) complex in mitochondria.