Electron transfer and conformational transitions of cytochrome c are modulated by the same dynamical features

Abstract

Cytochrome c is a prototypical multifunctional protein that is implicated in a variety of processes that are essential both for sustaining and for terminating cellular life. Typically, alternative functions other than canonical electron transport in the respiratory chain are associated to alternative conformations. In this work we apply a combined experimental and computational study of Cyt c variants to assess whether the parameters that regulate the canonical electron transport function of Cyt c are correlated with those that determine the transition to alternative conformations, using the alkaline transition as a model conformational change. The results show that pKa values of the alkaline transition correlate with the activation energies of the frictionally-controlled electron transfer reaction, and that both parameters are mainly modulated by the flexibility of the Ω-loop 70–85. Reduction potentials and non-adiabatic ET reorganization energies, on the other hand, are both modulated by the flexibilities of the Ω-loops 40–57 and 70–85. Finally, all the measured thermodynamic and kinetic parameters that characterize both types of processes exhibit systematic variations with the dynamics of the hydrogen bond between the axial ligand Met80 and the second sphere ligand Tyr67, thus highlighting the critical role of Tyr67 in controlling canonical and alternative functions of Cyt c.