Interpretation of ionic strength dependencies of the electron transfer in the Cytochrome c-Cytochrome b 5 system for the wild-type and lysine-mutated yeast Cytochrome c

Ionic strength dependencies of electron transfer between Cytochrome b ₅ and variants of yeast Cytochrome c were analyzed by curve fitting to the simple model of the electrostatic interaction between the two proteins assuming the process to be non-diffusion-controlled. Mutagenesis of Lys79, but not Lys72, leads to an increase of effective radius of the interacting charged species, suggesting that the mutation effects of the two residues on the electrostatic field distribution near the contact site are different, even within the crude electrostatic model used. Extrapolation of the ionic strength dependencies to infinite ionic strength resulted in similar values, around (2–3)×10^-6 for all Cyt c variants considered thus showing the lysine residue mutations to primarily affect protein association rather than the electron transfer directly. Based on the ionic strength dependencies of binding constants of the two proteins into an electrostatically stabilized complex, the monomolecular electron transfer rate constant was estimated to be 1.1×10⁴–1.6×10⁵ s^-1. The electrostatic part of the binding energy of the complex at I=0.19 was estimated to be-2.4 kcal/mol, strongly at variance with the values-13.0 and-6.4 kcal/mol reported for the two types of complexes identified using Brownian dynamics techniques. Possible reasons for this discrepancy are discussed.