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 describing reactions occurring in biological systems, but the relevant parameters in the case of iron-sulfur proteins remain unknown. The recent discovery that 2[4Fe-4S] proteins homologous to Chromatium vinosum ferredoxin contain clusters with different reduction potentials now gives the opportunity to study the dependence of the intramolecular electron transfer rate between these clusters as a function of the driving force. This work shows how decreasing the reduction potential difference between the clusters by site-directed mutagenesis of C. vinosum ferredoxin modifies the rate of electron hopping between the two redox sites of the protein by measuring the line broadening of selected 1H NMR signals. Beside the shifts of the reduction potentials, no signs of large structural changes or of significant alterations of the intrinsic kinstic parameters among the different variants of C. vinosum 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 reduction potential difference. This small value is associated with a weak electronic coupling between the two closely spaced clusters. This set of parameters, determined for the first time in an iron-sulfur protein, may help to explain how efficient vectorial electron transfer occurs with a small driving force in the many enzymatic systems containing a 2[4Fe-4S] domain.