Laser-flash absorption spectroscopy study of the competition between ferredoxin and flavodoxin photoreduction by Photosystem I in Synechococcus sp PCC 7002: Evidence for a strong preference for ferredoxin

The competition between ferredoxin and flavodoxin for electrons from Photos ystem I was analyzed by flash absorption spectroscopy of the photoreduction processes that take place in the presence of both acceptor proteins in vit ro. Steady state photoreduction assays indicate a strong inhibition of the apparent flavodoxin photoreduction activities of Photosystem I in the prese nce of ferredoxin. Flash-absorption experiments carried out at 626 nm, a wa velength where the reduction of ferredoxin shows no spectral contribution, show that the photoreduction of oxidized flavodoxin and flavodoxin semiquin one are inhibited by ferredoxin in a quantitatively similar way. The experi mental data can be satisfactorily described by a reaction model that assume s that both redox states of flavodoxin do not compete with ferredoxin for b inding on PS I and that the binding equilibrium between ferredoxin and PS I is not changed in their presence. In contrast, a model which assumes that ferredoxin and flavodoxin actually compete for binding to PS I gives poor r esults. Similarly, experimental data observed in the presence of both redox states of flavodoxin can also be quantitatively described under the assump tion that the binding equilibrium between flavodoxin semiquinone and PS I i s not disturbed by oxidized flavodoxin. Taken together, this analysis shows that PS I favors ferredoxin over flavodoxin and flavodoxin semiquinone ove r oxidized flavodoxin. This behavior is in accordance with the values of th e dissociation constants for complexes between PS I and its acceptors. Howe ver, in case of ferredoxin the observed preference is stronger than expecte d from these values, indicating that ferredoxin is almost absolutely prefer red by PS I over flavodoxin and is always reduced first.