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Flavodoxin-mediated electron transfer from photosystem I to ferredoxin-NADP+ reductase in Anabaena: role of flavodoxin hydrophobic residues in protein-protein interactions.

Three surface hydrophobic residues located at the Anabaena flavodoxin (Fld) putative complex interface with its redox partners were replaced by site-directed mutagenesis. The effects of these replacements on Fld interaction with both its physiological electron donor, photosystem I (PSI), and its electron acceptor, ferredoxin-NADP+ reductase (FNR), were analyzed. Trp57, Ile59, and Ile92 contributed to the optimal orientation and tightening of the FNR:Fld and PSI:Fld complexes. However, these side chains did not appear to be involved in crucial specific interactions, but rather contributed to the obtainment of the optimal orientation and distance of the redox centers required for efficient electron transfer. This supports the idea that the interaction of Fld with its partners is less specific than that of ferredoxin and that more than one orientation is efficient for electron transfer in these transient complexes. Additionally, for some of the analyzed processes, WT Fld seems not to be the most optimized molecular species. Therefore, subtle changes at the isoalloxazine environment not only influence the Fld binding abilities, but also modulate the electron exchange processes by producing different orientations and distances between the redox centers. Finally, the weaker apoflavodoxin interaction with FNR suggests that the solvent-accessible region of FMN plays a role either in complex formation with FNR or in providing the adequate conformation of the FNR binding region in Fld.