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

Reduction of ferredoxin by photosystem I (PSI) involves the [4Fe-4S] clusters F_A and F_B harbored by PsaC, with F_B 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_Ga. Fd_Ga binding resulted in a faster recombination between P700⁺ and (F_A, F_B)^-, a slower electron escape from (F_A, F_B)^- to exogenous acceptors, and a decreased amount of intracomplex Fd_Fe reduction, in accordance with competitive binding between Fd_Fe and Fd_Ga. [Fd_Ga] titrations of these effects revealed that the dissociation constant for the PSI:Fd_Ga complex is different whether (F_A, F_B) 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_A, F_B), considered as a single electron acceptor, due to Fd_Ga binding. This effect of Fd_Ga binding, which can be extrapolated to Fd_Fe 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_A and F_B is also discussed with respect to the possible consequences on intra-PSI electron transfer and on the escape process.