Site-specific mutagenesis demonstrates that the structural requirements for efficient electron transfer in Anabaena ferredoxin and flavodoxin are highly dependent on the reaction partner: kinetic studies with photosystem I, ferredoxin:NADP+ reductase, and cytochrome c.

Electron transfer reactions involving site-specific mutants of Anabaena ferredoxin (Fd) and flavodoxin (Fld) modified at surface residues close to the prosthetic groups, with photoexcited P700 in spinach photosystem I (PSI) particles, ferredoxin:NADP+ reductase (FNR), and horse cytochrome c (cytc), have been investigated by laser flash photolysis and stopped-flow spectrophotometry. Nonconservative mutations in Fd at F65 and E94, which have been shown to result in very large inhibitions of electron transfer to FNR, were found to yield wild-type behavior in reactions with PSI and cytc. In general, the effects of Fd mutagenesis on the PSI reactions were considerably smaller than those observed for the FNR reaction. In the case of Fld, mutagenesis was found to have only small effects on both the FNR and PSI reactions, although the specific sites whose mutation caused changes in electron transfer properties differed for the two systems. In contrast, several of the Fld mutants showed appreciably larger effects on the nonphysiological reaction with cytc. We conclude from these studies that the structural requirements for efficient electron transfer involving the Fd and Fld molecules differ, depending upon the reactant with which these redox proteins interact. This is consistent with the multiple roles that these proteins have in vivo in biological electron transfer and implies that different conserved residues in these proteins have evolved to satisfy varying requirements of particular reaction partners.