Binding of ferredoxin to ferredoxin:NADP+ oxidoreductase: the role of carboxyl groups, electrostatic surface potential, and molecular dipole moment.

The small, soluble, (2Fe-2S)-containing protein ferredoxin (Fd) mediates electron transfer from the chloroplast photosystem I to ferredoxin: NADP+ oxidoreductase (FNR), a flavoenzyme located on the stromal side of the thylakoid membrane. Ferredoxin and FNR form a 1:1 complex, which is stabilized by electrostatic interactions between acidic residues of Fd and basic residues of FNR. We have used differential chemical modification of Fd to locate aspartic and glutamic acid residues at the intermolecular interface of the Fd:FNR complex (both proteins from spinach). Carboxyl groups of free and FNR-bound Fd were amidated with carbodiimide/2-aminoethane sulfonic acid (taurine). The differential reactivity of carboxyl groups was assessed by double isotope labeling. Residues protected in the Fd:FNR complex were D-26, E-29, E-30, D-34, D-65, and D-66. The protected residues belong to two domains of negative electrostatic surface potential on either side of the iron-sulfur cluster. The negative end of the molecular dipole moment vector of Fd (377 Debye) is close to the iron-sulfur cluster, in the center of the area demarcated by the protected carboxyl groups. The molecular dipole moment and the asymmetric surface potential may help to orient Fd in the reaction with FNR. In support, we find complementary domains of positive electrostatic potential on either side of the FAD redox center of FNR. The results allow a binding model for the Fd:FNR complex to be constructed.