Electron and Proton Transfers around the b/f Complex in Chloroplasts: Modelling the Constraints on Q-cycle Activity

Abstract

The requirements for the operation of a Q-cycle in thylakoids are discussed. A computer model is described in which the state of reduction of components of the b/f complex is followed, single turnover at a time. Standard reduction potentials from the literature were assigned to the cytochrome b563 molecules; those for the second electron oxidation of plastoquinol at p-sites, and for the reduction of plastoquone at n-sites, were found by optimising the predicted stimulation of proton uptake by valinomycin. The stimulation has been attributed to uptake by doubly reduced plastoquinone at b/f complexes, a process thought to continue only if the membrane potential (ΔV) is kept low by ionophores. ΔV was simulated in the model via the electrochromic signal; its effect on electron transfers in the b/f complex was incorporated by modifying the reduction potentials. The extent of valinomycin stimulation of proton uptake, its dependence on [valinomycin] and flash frequency, the slow phase of the electrochromic signal and the extent of cytochrome b reduction were predicted by the model when the standard reduction potential for the p-site was set at -0.05 to -0.08 V. with that for the n-site at about 0 V.