BINDING OF FERREDOXIN TO FERREDOXIN-NADP- THE ROLE OF CARBOXYL GROUPS, ELECTROSTATIC SURFACE-POTENTIAL, AND MOLECULAR DIPOLE-MOMENT( OXIDOREDUCTASE )

The small, soluble, (2Fe-2S)-containing protein ferredoxin (Fd) mediat es 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, whic h is stabilized by electrostatic interactions between acidic residues of Fd and basic residues of FNR. We have used differential chemical mo dification of Fd to locate aspartic and glutamic acid residues at the intermolecular interface of the Fd:FNR complex (both proteins from spi nach). Carboxyl groups of free and FNR-bound Fd were amidated with car bodiimide/2-aminoethane sulfonic acid (taurine). The differential reac tivity of carboxyl groups was assessed by double isotope labeling. Res idues protected in the Fd:FNR complex were D-26, E-29, E-30, D-34, D-6 5, and D-66. The protected residues belong to two domains of negative electrostatic surface potential on either side of the iron-sulfur clus ter. 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 mom ent and the asymmetric surface potential may help to orient Fd in the reaction with FNR. In support, we find complementary domains of positi ve electrostatic potential on either side of the FAD redox center of F NR. The results allow a binding model for the Fd:FNR complex to be con structed.