Mapping the electron transfer interface between cytochrome b5 and cytochrome c.

To characterize the cytochrome b(5) (Cyt b(5))-cytochrome c (Cyt c) interactions during electron transfer, variants of Cyt b(5) have been employed to assess the contributions of electrostatic interactions (substitution of surface charged residues Glu44, Glu48, Glu56, and Asp60 and heme propionate), hydrophobic interactions, and the thermodynamic driving forces (substitutions for hydrophobic residues in heme pocket residues Phe35, Pro40, Val45, Phe58, and Val61). The electrostatic interactions play an important role in maintaining the stability and specificity of the Cyt b(5)-Cyt c complex that is formed. There is no essential effect on the intraprotein complex electron transfer even if most of the involved negatively charged residues on the surface of Cyt b(5) have been removed. The results support a dynamic docking paradigm for Cyt b(5)-Cyt c interactions. The orientation that is optimal for binding may not be optimal form for electron transfer. Substitution of hydrophobic residues does not have a significant effect on the binding between Cyt b(5) and Cyt c; rather, it regulates the electron transfer rates via changes in the driving force. Combining the electron transfer studies of the Cyt b(5)-Cyt c system and the Cyt b(5)-Zn-Cyt c system, we obtain the reorganization energy (0.6 eV) at an ionic strength of 150 mM.