Structural studies on the interaction between ferredoxin and ferredoxin-NADP+ reductase

Structural studies carried out on a cross-linked complex between spinach ferredoxin-NADP⁺ reductase (EC 1.18.1.2) and its protein substrate ferredoxin allowed the identification of peptide regions involved in the interaction between the two proteins. Carboxyl groups of ferredoxin were shown to interact with amino groups of the reductase. Two types of cross-links could be located within a few residues. The major one was found between the peptide segment 72-91 of the reductase, where Lys-85 and/or Lys-88 were identified as the amino donors in the carbodiimide-promoted linkage, and the ferredoxin region 76-97, which contains Asp-84, Glu-88, Glu-92, Glu-93, and Glu-94. Carboxypeptidase Y digestions would suggest an involvement of a glutamic residue of the acidic cluster 92-94. The minor cross-link was found between the α-NH₂ of the N-terminal residue of the reductase and the peptide region 5-37 of ferredoxin, which contains Glu-15, Asp-20, Asp-21, Asp-26, Glu-29, Glu-30, Glu-31, and Asp-34. Glu-15, Glu-31, and Asp-34 could be ruled out. Lack of chymotrypsin digestion at Tyr-23 of the cross-linked peptide would indicate Asp-20, -21, or -26 as the most likely COOH donor in the linkage. These results point out the importance of the N-terminal moiety of the reductase, where the flavin binding domain is located, for the interaction with ferredoxin.