Effect of electrostatic immobilization on the electrochemistry of human and horse cytochrome c

Abstract

Protein film voltammetry is a sensitive tool to characterize the electron transfer properties of redox proteins in a variety of environments and conformational states. Here, a detailed voltammetric study aimed to explore the effect of electrostatic immobilization on the electron transfer thermodynamics and kinetics of adsorbed human- and horse- cytochrome c was carried out. For this purpose, the two cytochromes were adsorbed on thiol monolayers (SAM) with different immobilization strengths and donor–acceptor distances. While thermodynamic redox parameters do not seem to be affected by the monolayer thickness and charge density, electron transfer kinetics are significantly modulated by the protein immobilization strength. Stronger protein–SAM electrostatic interactions result in lower electron transfer rates in both non-adiabatic and friction kinetic regimes. This behavior is further characterized by smaller pre-exponential factors and activation enthalpies in Arrhenius type plots. These kinetic results in the physiologically relevant non-adiabatic electron transfer regime are shown to be consistent with the recently developed Matyushov’s theoretical formulation of protein electron transfer. Moreover, a comparison between the kinetic parameters of the two cytochrome variants supports the hypothesis that differences between their electron transfer rates originate in their structural flexibility to accommodate the conformational changes required to form the precursor complex between cytochrome and a negatively charged redox partner.