PHOTOSYSTEM-II ENERGY COUPLING IN CHLOROPLASTS WITH H2O2 AS ELECTRON-DONOR

NH2OH-treated, non-water-splitting chloroplasts can oxidize H2O2 to O2 through Photosystem II at substantial rates (100-250 μequiv · h-1 · mg-1 chlorophyll with 5 mM H2O2) using 2,5-dimethyl-p-benzoquinone as an electron acceptor in the presence of the plastoquinone antagonist dibromothymoquinone. This H2O2 → Photosystem II → dimethylquinone reaction supports phosphorylation with a P e2 ratio of 0.25-0.35 and proton uptake with H+ e values of 0.67 (pH 8)-0.85 (pH 6). These are close to the P e2 value of 0.3-0.38 and the H+ e values of 0.7-0.93 found in parallel experiments for the H2O → Photosystem II → dimethylquinone reaction in untreated chloroplasts. Semi-quantitative data are also presented which show that the donor → Photosystem II → dibromothymoquinone (→O2) reaction can support phosphorylation when the donor used is a proton-releasing reductant (benzidine, catechol) but not when it is a non-proton carrier (I-, ferrocyanide). © 1979.