Abstract
The ferrous ion associated with the electron acceptors in Photosystem II can be oxidized by the unstable semiquinone form of certain high-potential quinones (phenyl- p-benzoquinone, dimethylbenzoquinone and benzoquinone) which are used as electron acceptors. In a flash sequence, alternating oxidation of the iron by the photoreduced semiquinone on odd-numbered flashes is followed by photoreduction of the iron on even-numbered flashes. These reactions are detected by monitoring EPR signals arising from Fe 3+. The oxidation of the iron can also occur in the frozen state (−30°C) indicating that the high-potential quinone can occupy the Q B site. The reaction also takes place when the exogenous quinone is added in the dark to samples in which Q B is already in the semiquinone form. The inhibitors of electron transfer between Q A − and Q B, DCMU and sodium formate, block the photoreductant-induced iron oxidation. It is suggested that the iron oxidation takes place through the Q B site. This unexpected photochemistry occurs under experimental conditions routinely used in studies of Photosystem II. Some previously reported phenomena can be reinterpreted on the basis of these new data.
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Schedule a callMore From: Biochimica et Biophysica Acta (BBA) - Bioenergetics
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