How does reorganization energy change upon protein unfolding? Monitoring the structural perturbations in the heme cavity of cytochrome c

Mechano-control of Extracellular Electron Transport Rate via Modification of Inter-heme Coupling in Bacterial Surface Cytochrome

Bacterial outer-membrane multi-heme cytochromes (OMCs) mediate extracellular electron transport (EET). While heme alignment dictates the rate of EET, control of inter-heme coupling in a single OMC remains challenging, especially in intact cells. Given that OMCs diffuse and collide without aggregation on the cell surface, the overexpression of OMCs could increase such mechanical stress to impact the OMCs' protein structure. Here, the heme coupling is modified via mechanical interactions among OMCs by controlling their concentrations. Employment of whole-cell circular dichroism (CD) spectra of genetically engineered Escherichia coli reveals that the OMC concentration significantly impacts the molar CD and redox property of OMCs, resulting in a 4-fold change of microbial current production. The overexpression of OMCs increased the conductive current across the biofilm on an interdigitated electrode, indicating that a higher concentration of OMCs causes more lateral inter-protein electron hopping via collision on the cell surface. The present study would open a novel strategy to increase microbial current production by mechanically enhancing the inter-heme coupling.

Contributions to cytochrome c inner- and outer-sphere reorganization energy

Cytochromes c are small water-soluble proteins that catalyze electron transfer in metabolism and energy conversion processes. Hydrogenobacter thermophilus cytochrome c₅₅₂ presents a curious case in displaying fluxionality of its heme axial methionine ligand; this behavior is altered by single point mutation of the Q64 residue to N64 or V64, which fixes the ligand in a single configuration. The reorganization energy (λ) of these cytochrome c₅₅₂ variants is experimentally determined using a combination of rotating disc electrochemistry, chronoamperometry and cyclic voltammetry. The differences between the λ determined from these complementary techniques helps to deconvolute the contribution of the active site and its immediate environment to the overall λ (λ_Total). The experimentally determined λ values in conjunction with DFT calculations indicate that the differences in λ among the protein variants are mainly due to the differences in contributions from the protein environment and not just inner-sphere λ. DFT calculations indicate that the position of residue 64, responsible for the orientation of the axial methionine, determines the geometric relaxation of the redox active molecular orbital (RAMO). The orientation of the RAMO with respect to the heme is key to determining electron transfer coupling (H_AB) which results in higher ET rates in the wild-type protein relative to the Q64V mutant despite a 150 mV higher λ_Total in the former.

Efficient delocalization of the redox-active molecular orbital (RAMO) in HtWT results in an increase in H_AB value which in turn accelerates the electron transfer (ET) rate in spite of the higher reorganization energy (λ) than the HtQ64V mutant.

The role of protein dynamics and thermal fluctuations in regulating cytochrome c/cytochrome c oxidase electron transfer

In this overview we present recent combined electrochemical, spectroelectrochemical, spectroscopic and computational studies from our group on the electron transfer reactions of cytochrome c and of the primary electron acceptor of cytochrome c oxidase, the CuA site, in biomimetic complexes. Based on these results, we discuss how protein dynamics and thermal fluctuations may impact on protein ET reactions, comment on the possible physiological relevance of these results, and finally propose a regulatory mechanism that may operate in the Cyt/CcO electron transfer reaction in vivo. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C(60)). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.