Photosystem II does not convert nascent oxygen to the poisonous singlet form

Abstract

AbstractIn the light, the Mn4CaO5complex of Photosystem II (PSII) splits water producing O2and the triplet state of the primary donor (3P680) of PSII generates reactive singlet oxygen (1O2). We show that nascent O2is not converted to1O2, but originates exclusively from ambient O2, indicating that the sensitivity of PSII to oxidative damage is not a consequence of the water-splittingper se, and showing that the suggested oxygen channels function nearly perfectly, conveying nascent O2out of the reach of3P680. This may have been crucial during evolution of oxygenic photosynthesis, as3P680cannot be quenched by carotenoids that protect non- oxygenic photosystems. In addition, the data indicate that a1O2-independent mechanism contributes to the light-induced damage of PSII.