Inhibition of electron transfer and the electrogenic reaction in the cytochrome [formula omitted] complex by 2- n-nonyl-4-hydroxyquinoline N-oxide (NQNO) and 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB)

Abstract

We investigated the interaction between the cytochrome b f complex and plastoquinone by determining the effect of the electron transport inhibitors 2- n-nonyl-4-hydroxy-quinoline N-oxide (NQNO) and 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB) on the flash-induced turnover of cytochromes b 6 and f, on the slow phase of the electrogenic reaction (515 s) and on steady-state electron transport. The experiments were performed using thylakoid membranes in the presence of duroquinol, methyl viologen, and diuron, conditions in which electron transport is driven by Photosystem I and includes the cytochrome b f complex. The data from these experiments indicate that NQNO and DBMIB inhibit at two different sites on the cytochrome b f complex. The data can be accommodated by a modified Q-cycle in which the primary site of inhibition by NQNO is the quinone reductase site (Q c) and the primary site of DBMIB inhibition is the quinol oxidase site (Q z). NQNO is envisioned to inhibit the oxidation of the cytochrome b 6-heme located nearest the outer aqueous phase, as well as to slow electron transfer between the two b-hemes. The effect of NQNO on the 515 s supports the notion that the slow electrogenic reaction is due to electron transfer between the two cytochrome b 6-hemes, followed by a reaction associated with plastoquinone reduction at the Q c-site (Jones and Whitmarsh (1985) Photobiochem. Photobiophys. 9, 119–127). The results indicate that the Q z-site and Q c-site are likely separated by 70% of the dielectrically weighted distance across the membrane. At low concentrations of DBMIB the kinetics of the 515 s and cytochrome b 6 reduction are monophasic, indicating that DBMIB moves from one inhibitory site to another within the turnover time of the cytochrome b f complex. This observation suggests that the debinding rate of DBMIB from the Q z-site is greater than 200 s −1. Lastly, comparing the effect of DBMIB on the extent of cytochrome f oxidation and the rate of cytochrome f reduction, raises the possibility that DBMIB has a second lower affinity binding site on the cytochrome b f complex, possibly interrupting electron flow between the FeS center and cytochrome f.