Evidence for two cyclic photophosphorylation reactions concurrent with ferredoxin-catalyzed non-cyclic electron transport

Abstract

Addition of ferredoxin to isolated spinach chloroplast thylakoid membranes reconstitutes phosphorylating electron transfer characterized by elevated P/O ratios (1.6). When oxygen is the terminal electron acceptor for reduced ferredoxin, the P/O value is lowered by antimycin A or low concentrations (I0/~M) of heparin (an anionic macromolecule), but not by inhibition of the activity of membrane-bound ferredoxin-NADP reductase. When NADP is present as the terminal electron acceptor for reduced ferredoxin, the elevated P/O value (again 1.6) is unaffected either by antimycin A or low concentrations (I0/~M) of heparin. When ferredoxin-catalyzed cyclic or Q-loop activity is sensitive to antimycin A, the ferredoxin pool and P-700 are both present in a largely reduced state. The opposite result is obtained for antimycin-A-insensitive activity in the presence of NADP. Our results show that conditions exist whereby ferredoxin-catalyzed cyclic electron transport is insensitive to a classical inhibitor of the cytochrome b function. We suggest that the antimycin-A-insensitive pathway of ferredoxin-catalyzed cyclic electron transport may involve the activity of ferredoxin-NADP reductase.

Photoacoustic Measurements in Vivo of Energy Storage by Cyclic Electron Flow in Algae and Higher Plants

“…This result is contrary to experiments with preparations of thylakoids from C3-type higher plants which clearly exhibit photophosphorylation driven by cyclic electron flow through PSI in vitro (2,14). It is possible that some activity of PSII is required for cyclic electron flow to occur in C3 higher plants.…”

Abnormal Regulation of Photosynthetic Electron Transport in a Chloroplast ycf9 Inactivation Mutant

“…2A). It is known that in the presence of excess reductant (reduced Fd), the electron transport system may divert electrons to molecular oxygen instead of NADP ϩ (Mehler reaction) (36), at a rate proportional to the concentration of reduced Fd (37). Diversion of electrons to oxygen leads to a rapid formation of superoxide radicals, which have to be metabolized by the detoxification system of chloroplasts.…”

N-Terminal Structure of Maize Ferredoxin:NADP+ Reductase Determines Recruitment into Different Thylakoid Membrane Complexes

“…The final transfer of electrons to NADP + is catalyzed by the enzyme ferredoxin:NADP + reductase (FNR), which is both bound to the thylakoid as a peripheral protein and present as a soluble protein in the chloroplast stroma. In the alternative pathway of cyclic electron transport (CET), electrons are passed back to the plastoquinone pool from either ferredoxin or NADPH, which generates a proton gradient resulting in ATP synthesis, with minimal water splitting and no net reduction of NADP + (Arnon et al, 1963;Hosler and Yocum, 1985;Munekage et al, 2004). A role for FNR in CET has been proposed due to reported interactions between FNR and characterized components of the CET apparatus in higher plants and algae (Zhang et al, 2001;DalCorso et al, 2008;Iwai et al, 2010), perturbation of CET in cyanobacteria that lack membrane-bound FNR (van Thor et al, 2000), electron donation from FNR to quinones (Bojko et al, 2003), and the disruption of CET in spinach (Spinacia oleracea) thylakoids by FNR inhibitors (Shahak et al, 1981).…”