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 FA and FB harbored by PsaC, with FB 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 FdGa. FdGa binding resulted in a faster recombination between P700+ and (FA, FB)-, a slower electron escape from (FA, FB)- to exogenous acceptors, and a decreased amount of intracomplex FdFe reduction, in accordance with competitive binding between FdFe and FdGa. [FdGa] titrations of these effects revealed that the dissociation constant for the PSI:FdGa complex is different whether (FA, FB) 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 (FA, FB), considered as a single electron acceptor, due to FdGa binding. This effect of FdGa binding, which can be extrapolated to FdFe 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 FA and FB is also discussed with respect to the possible consequences on intra-PSI electron transfer and on the escape process.