Studies on the inhibitory binding site of a new phenolic inhibitor of electron transfer in Photosystem II

Abstract

Inhibitor K-23 is a new highly efficient inhibitor of electron transfer in PSII of higher plants. Its inhibitory effect is based on the redox interaction with reaction center components of PSII and on the formation of a short cyclic electron transfer which leads to the reversible separation of photoinduced charge in PSII. However, its inhibitory site is not clear. In this report, we investigated the possible binding site of this inhibitor in PSII by the following aspects. 1) The effects of K-23 and DCMU on the oxygen evolution of trypsin-treated thylakoid were compared using ferricyanide as electron acceptor. It is suggested that even though inhibitory sites of K-23 and DCMU are at the acceptor side, their binding sites are different. 2) The effects of inhibitor K-23 on the content of plastoquinone in core complex of PSII were investigated by using HPLC method to identify the possible replacement of the primary electron acceptor of PSII, plastoquinone, at its binding site by the inhibitor K-23. It is found that inhibitor K-23 did not affect elution times of plastoquinone, but it affected the profiles of the absorption spectra. Inhibitor K-23 also results in the decrease of peak area of plastoquinone. 3) The absorption spectra showed that the absorption peak of plastoquinone at 262nm was disappeared in the presence of K-23. Analyzing the above results, we suggest that the K-23 binding at the QA site by reacting with it other than replacement of plastoquinone.