MOA-stilbene: A new tool for investigation of the reactions of the chloroplast cytochrome bf complex

Abstract

MOA-stilbene is known to be a specific inhibitor of the Qo site of mammalian cytochrome bc 1 complex. We show that it also binds to the chloroplast cytochrome bf complex. Binding to the reduced enzyme induces a red-shift of the Soret and visible absorption bands of the haems b. Steady state and single turnover experiments with thylakoid membranes show that MOA-stilbene promotes additional 'oxidant-induced reduction' of the b haems and slows their subsequent dark reoxidation. In single turnover experiments, the associated slow phase of the carotenoid bandshift at 518 nm is only partially decreased in apparent extent and rate. These and other effects are similar to those produced by NQNO, a Qi site effector, and by analogy indicate that MOA-stilbene should also be primarily a Qi-site effector of the cytochrome bf complex. MOA-stilbene has less effect on other parts of the photosynthetic chain. This confers an important advantage on MOA-stilbene in that its effects on the cytochrome bf complex can be studied by using Photosystem II to activate turnover. Myxothiazol displays effects on the cytochrome bf complex which are similar to, but much weaker than, those of MOA-stilbene.A Q cycle-based model of turnover of the cytochrome bf complex is presented, which can account for several unusual features of kinetic behaviour.

Selective interaction of valinomycin/K+with the cytochromebfcomplex of chloroplasts

“…The observed valinomycin/K+ effect is almost identical to that recently reported for MOA stilbene on a reduced purified cytochrome bf preparation [12]. However, with valinomycin the concentration required for the spectral shift is distinctly lower than with MOA stilbene.…”

“…This experiment was carried out in the presence of nonactin, which caused full dissipation of the light-induced membrane potential, as confirmed by P515 measurements (not shown) (see [18]). The observed effects seem to be in agreement with an inhibitory effect of valinomycin/K+ at the Q,-site, in analogy to the interpretation of the MOA stilbene effect [12]. However, other interpretations appear possible.…”

“…This mutual synergism indicates that despite of the phenomenological similarities expressed in Fig. 3 and apparent by comparison with the data in [12], the modes of action of valinomycin/K+ and MOA stilbene are not identical. In the experiment of Fig.…”

“…So far, two basic types of bf inhibitors have been characterised, the DBMIB-type [7-91 at the quinol oxidation site (also called Q,:; Q,-or Qz-site) and the NQNO-type [lO,l l] at the qumone reduction site (also called Q,-; Qi-or Qc-site). Recently, with MOA stilbene a new inhibitor of the bf complex was introduced [12], with properties similar to NQNO, but displaying the advantage of lesser side-effect on PSI1 activity.…”

In Vitro Cyclic Electron Transport in Barley Thylakoids follows Two Independent Pathways

“…The common textbook view of cyclic electron transport shows donation of electrons by Fd to Cyt b. The fact that cyclic electron transport can be suppressed by antimycin A was thought to indicate the involvement of Cyt b, but unlike mitochondrial Cyt b, the chloroplast homolog has turned out to be insensitive to antimycin A (Moss and Bendall, 1984;Rich et al, 1992). Thus, the antimycin-sensitive component in thylakoid membranes has not been identified.…”