Intramolecular electron transfer in [4Fe-4S] proteins: estimates of the reorganization energy and electronic coupling in Chromatium vinosum ferredoxin

The semi-classical electron transfer theory has been very successful in des cribing reactions occurring in biological systems, but the relevant paramet ers in the case of iron-sulfur proteins remain unknown. The recent discover y that 2[4Fe-4S] proteins homologous to Chromatium vinosum ferredoxin conta in clusters with different reduction potentials now gives the opportunity t o study the dependence of the intramolecular electron transfer rate between these clusters as a function of the driving force. This work shows how dec reasing the reduction potential difference between the clusters by site-dir ected mutagenesis of C. vinosum ferredoxin modifies the rate of electron ho pping between the two redox sites of the protein by measuring the line broa dening of selected H-1 NMR signals. Beside the shifts of the reduction pote ntials, no signs of large structural changes or of significant alterations of the intrinsic kinetic parameters among the different variants of C. vino sum ferredoxin have been found. A reorganization energy of less than 0.5 eV was deduced from the dependence of the electron transfer rates with the re duction potential difference, This small value is associated with a weak el ectronic coupling between the two closely spaced clusters. This set of para meters, determined for the first time in an iron-sulfur protein, may help t o explain how efficient vectorial electron transfer occurs with a small dri ving force in the many enzymatic systems containing a 2[4Fe-4S] domain.