MOA-stilbene: a new effector of the Qi site of the chloroplast cytochrome bf complex

Abstract

The chloroplast cytochrome bf complex is unaffected by micromolar concentrations of almost all the reported antibiotic inhibitors of the bcl complexes [l], with the notable exception of stigmatellin which has been shown to have an analogous action in both systems [2,3]. As for the Qi site, only the rather unspecific inhibitor NQNO (and HQNO) has been shown to affect the Qi site of the bf complex [4,5]. This effector produces changes of the transient kinetics of the enzyme consistent with a block of electron transfer through the Qi site, and yet does not significantly alter the steady state turnover number of the enzyme [4] or its protonmotive action [6]. This is in contrast to the action of NQNO on the mammalian enzyme where a full block in the Qi site, together with a cessation of electron and proton transfer activity, is evident. Studies with NQNO in thylakoids are considerably hampered by its strong inhibitory action on photosystem 11 [7].