Flavocytochrome b2-cytochrome c interactions: the electron transfer reaction revisited

This review is concerned with the kinetics and mechanism of electron transfer processes which occur intermolecularly between reduced flavocytochrome b2 and cytochrome c molecules within an encounter complex. Analyses are given of previous reports which aimed at describing the formation of stable complexes obtained at low ionic strength in solution and in the crystalline state with a binding stoichiometry of 1 to 1 heme ratio. Relevant data allow to define the respective role of flavin and heme b2 in the electron transfer towards cytochrome c and give a description of the recognition areas on the two redox partners. The paper also refers to a recent computer model of their postulated interactions as based on the three-dimensional structure of the Saccharomyces cerevisiae single molecules. Special emphasis is given to rapid kinetic investigations of the electron transfer reaction between Hansenula anomala flavocytochrome b2 and cytochrome c studied as a function of concentration, ionic strength and temperature. Data showed that reaction rates were modulated by ionic strength, reaching a saturation behaviour at low ionic strength. In the present paper the temperature effects on Kd and kET have been re-examined. Thermodynamic analysis of the dissociation constant points out the importance of hydrophobic interactions in the complex formation. Analysis of the variations of rate constants in terms of semiclassical theory of electron-transfer reaction yields values of 1.12 eV for the reorganization energy and 0.05 cm-1 for the electronic coupling factor. Interpretation of the electronic coupling in terms of through-bond and/or through-space pathways takes into account the hypothetical model proposed for the binary complex. The functional implications of this model in the electron transfer reaction are discussed. Finally the existence of a conformational equilibrium between the initial binding complex and the complex from which electron transfer occurs is considered.