Oxidation of [2Fe–2S] isoferredoxins with [Co(NH3)6]3+ and [Cr(phen)3]3+(phen = 1,10-phenanthroline): mechanism of electron transfer

Abstract

The reactivity of different isoferredoxins, Fdl and Fdll, has been investigated for the first time. Similar saturation kinetic behaviour is observed in the oxidation of Fdl and Fdll, from spinach, parsley and Anabaena variabilis, with [Co(NH₃)₆]³⁺, highlighting the importance of conserved residues. Studies with the oxidant [Cr(phen)₃]³⁺(phen = 1,10-phenanthroline) have also been made for the first time. Both proteins are blocked by redox-inactive [Cr(NH₃)₆]³⁺(K_B= 525 for spinach Fdll and 512 M^–1 for parsley Fdl), in agreement with previously determined values of ≈470 M^–1. Use of the same site on the protein surface, influenced by favourable electrostatic interactions, but independent of ligand type on the complex (NH₃ or phen), is indicated. A site for reaction incorporating surface aromatic residues Tyr25/Tyr82, an acidic path, and a surface contour (hollow) region is implicated on the basis of kinetic and previous NMR studies. Factors affecting electron transfer to the oxidant at this site are considered.