Oxidation‐reduction potentials of ferredoxin‐NADP+ reductase and flavodoxin from Anabaena PCC 7119 and their electrostatic and covalent complexes

Abstract

The oxidation‐reduction potentials of ferredoxin‐NADP⁺ reductase and flavodoxin from the cyanobacterium Anabaena PCC 7119 were determined by potentiometry. The potentials at pH 7 for the oxidized flavodoxin/flavodoxin semiquinone couple (E₂) and the flavodoxin semiquinone/hydroquinone couple (E₁) were – 212 mV and –436 mV, respectively. E₁ was independent of pH above about pH 7, but changed by approximately –60 mV/pH below about pH 6, suggesting that the fully reduced protein has a redox‐linked pK_a at about 6.1, similar to those of certain other flavodoxins. E₂ varied by – 50 mV/pH in the range pH 5–8. The redox potential for the two‐electron reduction of ferredoxin‐NADP⁺ reductase was –344 mV at pH 7 (ΔE_m=– 30 mV/pH). In the 1:1 electrostatic complex of the two proteins titrated at pH 7, E₂ was shifted by + 8 mV and E₁ was shifted by – 25 mV; the shift in potential for the reductase was + 4 mV. The potentials again shifted following treatment of the electrostatic complex with a carbodiimide, to covalently link the two proteins. By comparison with the separate proteins at pH 7, E₂ for flavodoxin shifted by – 21 mV and E₁ shifted by + 20 mV; the reductase potential shifted by + 2 mV. The potentials of the proteins in the electrostatic and covalent complexes showed similar pH dependencies to those of the individual proteins. Qualitatively similar changes occurred when ferredoxin‐NADP⁺ reductase from Anabena variabilis was complexed with flavodoxin from Azotobacter vinelandii. The shifts in redox potential for the complexes were used with previously determined values for the dissociation constant (K_d) of the electrostatic complex of the two oxidised proteins, in order to estimate K_d values for the interaction of the different redox forms of the proteins. The calculations showed that the electrostatic complexes, formed when the proteins differ in their redox states, are stronger than those formed when both proteins are fully oxidized or fully reduced.