Anabaena Flavodoxin as an Electron Carrier from Photosystem I to Ferredoxin-NADP~+ Reductase.Role of Flavodoxin Residues in Protein-Protein Interaction and Electron Transfer

摘要

Biochemical and structural studies indicate that electrostatic and hydrophobic interactions are critical in the formation of optimal complexes for efficient electron transfer (ET) between ferredoxin-NADP+ reductase (FNR) and ferredoxin (Fd).Moreover,it has been shown that several charged and hydrophobic residues on the FNR surface are also critical for the interaction with flavodoxin (Fid),although,so far,no key residue on Ihe Fid surface has been found to be the counterpart of such FNR side chains.In this study,negatively charged side chains on the Fid surface have been individually modified,either by the introduction of positive charges or by their neutralization.Our results indicate that although Glul6,Glu20,Glu61,Asp65,and Asp96 contribute to the orientation and optimization of the Fid interaction,either with FNR or with photosystem I (PSI) (presumably through the formation of salt bridges),for efficient ET,none of these side chains is invojved in the formation of crucial salt bridges for optimal interaction with FNR.These data support the idea that the FNR-Fld interaction is less specific than the FNR-Fd interaction.However,analysis of the reactivity of these mutated Fids toward the membrane-anchored PSI complex indicated that all mutants,except Glul6Gln,lack the ability to form a stable complex with PSI.Thrl2,Thr56,Asn58,and Asn97 are present in the close environment of the isoalloxazine ring of FMN in Anabaena Fid.Their roles in the interaction with and ET to FNR and PSI have also been studied.Mutants at these Fid positions indicate that residues in the close environment of the isoalloxazine ring modulate the ability of Fid to bind to and to exchange electrons with its physiological counterparts.