Gallium ferredoxin as a tool to study the effects of ferredoxin binding to photosystem I without ferredoxin reduction

Abstract

Reduction of ferredoxin by photosystem I (PSI) involves the [4Fe-4S] clusters F and F harbored by PsaC, with F being the direct electron transfer partner of ferredoxin (Fd). Binding of the redox-inactive gallium ferredoxin to PSI was investigated by flash-absorption spectroscopy, studying both the P700 decay and the reduction of the native iron Fd in the presence of Fd. Fd binding resulted in a faster recombination between P700 and (F, F), a slower electron escape from (F, F) to exogenous acceptors, and a decreased amount of intracomplex Fd reduction, in accordance with competitive binding between Fd and Fd. [Fd] titrations of these effects revealed that the dissociation constant for the PSI:Fd complex is different whether (F, F) is oxidized or singly reduced. This difference in binding, together with the increase in the recombination rate, could both be attributed to a c. -30 mV shift of the midpoint potential of (F, F), considered as a single electron acceptor, due to Fd binding. This effect of Fd binding, which can be extrapolated to Fd because of the highly similar structure and the identical charge of the two Fds, should help irreversibility of electron transfer within the PSI:Fd complex. The effect of Fd binding on the individual midpoint potentials of F and F is also discussed with respect to the possible consequences on intra-PSI electron transfer and on the escape process.

Role of the two PsaE isoforms on O2 reduction at photosystem I in Arabidopsis thaliana

“…4), which can be almost perfectly fitted (see red curve and residuals) with 2 exponential components. As previously described [25], this behavior is fully consistent with a simple kinetic scheme, where P700 + decays by recombination with (F A , F B ) -(rate k r ) and slow reduction by the exogenous donor DCPIPH 2 (rate k don. ) and where (F A , F B )decays by recombination with P700 + and by electron escape to oxygen (rate k e ).…”

“…leaves were dark-adapted for 15 min to allow relaxation of nonphotochemical quenching. In vitro flash absorption spectroscopy P700 + decay kinetics following a single turnover saturating laser flash was measured at 800 nm in PSI preparations as previously described[25].Measurements were made in Tricine 20 mM pH 8, in the presence of 125 mM sucrose, 5 mM MgCl 2 , 30 mM NaCl, 0.5 mM ascorbate and 5 µM DCPIP. The P700 + decay kinetics were nicely fitted with two exponential components, from which the rates k r and k e of recombination and electron escape from the terminal electron acceptor (F A , F B )were determined as explained in Mignée et al (2017).…”

Electron transport pathways in isolated chromoplasts from Narcissus pseudonarcissus L.

“…To show the participation of Fd in the chromorespiratory pathway, a redox‐inactive Fd from Thermosynechoccus elongatus in which the FeS cluster iron is replaced by gallium (Fd‐Ga; Mutoh et al ., ) was added to the chromoplasts. Redox‐inactive Fd has recently been used to study Fd binding to photosystem I blocking Fd reduction (Mignée et al ., ). It can be used as a general tool for the study of Fd‐dependent electron transport reactions.…”

Functional basis of electron transport within photosynthetic complex I

“…In the cell, the very negative reduction potential of N0 would limit reduction of PS-CI until the Fd pool is in a highly reduced state (at least 100 times more abundant than PS-CI based on copy per cell estimates) 59,60 , with the close proximity of Fd to N0 providing an electron tunnelling pathway from Fd to N1/N2 29,53 . It remains possible that the potential is altered by the binding of subunits as is observed in R-CI 61 , or the substrate itself, as seen in photosystem I 62,63 . It has been suggested that electron transfer may occur in reverse in PS-CI if the proton motive force is high or the PQ pool is predominantly reduced, such as under decreased CO 2 or fluctuating light intensities 2 .…”