Electric field-induced functional changes in electrode-immobilized mutant species of human cytochrome c

Post-translational modifications and naturally occurring mutations of cytochrome c have been recognized as a regulatory mechanism to control its biology. In this work, we investigate the effect of such in vivo chemical modifications of human cytochrome c on its redox properties in the adsorbed state onto an electrode. In particular, tyrosines 48 and 97 have been replaced by the non-canonical amino acid p-carboxymethyl-L-phenylalanine (pCMF), thus mimicking tyrosine phosphorylation. Additionally, tyrosine 48 has been replaced by a histidine producing the natural Y48H pathogenic mutant. Thermodynamics and kinetics of the interfacial electron transfer of wild-type cytochrome c and herein produced variants, adsorbed electrostatically under different local interfacial electric fields, were determined by means of variable temperature cyclic film voltammetry. It is shown that non-native cytochrome c variants immobilized under a low interfacial electric field display redox thermodynamics and kinetics similar to those of wild-type cytochrome c. However, upon increasing the strength of the electric field, the redox thermodynamics and kinetics of the modified proteins markedly differ from those of the wild-type species. The mutations promote stabilization of the oxidized form and a significant increase in the activation enthalpy values that can be ascribed to a subtle distortion of the heme cofactor and/or difference of the amino acid rearrangements rather than to a coarse protein structural change. Overall, these results point to a combined effect of the single point mutations at positions 48 and 97 and the strength of electrostatic binding on the regulatory mechanism of mitochondrial membrane activity, when acting as a redox shuttle protein.

Probing Structure and Reaction Dynamics of Proteins Using Time-Resolved Resonance Raman Spectroscopy

The mechanistic understanding of protein functions requires insight into the structural and reaction dynamics. To elucidate these processes, a variety of experimental approaches are employed. Among them, time-resolved (TR) resonance Raman (RR) is a particularly versatile tool to probe processes of proteins harboring cofactors with electronic transitions in the visible range, such as retinal or heme proteins. TR RR spectroscopy offers the advantage of simultaneously providing molecular structure and kinetic information. The various TR RR spectroscopic methods can cover a wide dynamic range down to the femtosecond time regime and have been employed in monitoring photoinduced reaction cascades, ligand binding and dissociation, electron transfer, enzymatic reactions, and protein un- and refolding. In this account, we review the achievements of TR RR spectroscopy of nearly 50 years of research in this field, which also illustrates how the role of TR RR spectroscopy in molecular life science has changed from the beginning until now. We outline the various methodological approaches and developments and point out current limitations and potential perspectives.

Temperature-Driven Changeover in the Electron-Transfer Mechanism of a Thermophilic Plastocyanin

Electron-transfer kinetics of the thermophilic protein Plastocyanin from Phormidium laminosum adsorbed on 1,ω-alkanedithiol self-assembled monolayers (SAMs) deposited on gold have been investigated. The standard electron-transfer rate constant has been determined as a function of electrode-protein distance and solution viscosity over a broad temperature range (0-90 °C). For either thin or thick SAMs, the electron-transfer regime remains invariant with temperature, whereas for the 1,11-undecanethiol SAM of intermediate chain length, a kinetic regime changeover from a gated or friction-controlled mechanism at low temperature (0-30 °C) to a nonadiabatic mechanism above 40 °C is observed. To the best of our knowledge, this is the first time a thermal-induced transition between these two kinetic regimes is reported for a metalloprotein.

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.

Analyzing the catalytic processes of immobilized redox enzymes by vibrational spectroscopies

Analyzing the structure and function of redox enzymes attached to electrodes is a central challenge in many fields of fundamental and applied life science. Electrochemical techniques such as cyclic voltammetry which are routinely used do not provide insight into the molecular structure and reaction mechanisms of the immobilized proteins. Surface-enhanced infrared absorption (SEIRA) and surface-enhanced resonance Raman (SERR) spectroscopy may fill this gap, if nanostructured Au or Ag are used as conductive support materials. In this account, we will first outline the principles of the methodology including a description of the most important strategies for biocompatible protein immobilization. Subsequently, we will critically review SERR and SEIRA spectroscopic approaches to characterize the protein and active site structure of the immobilized enzymes. Special emphasis is laid on the combination of surface-enhanced vibrational spectroscopies with electrochemical methods to analyze equilibria and dynamics of the interfacial redox processes. Finally, we will assess the potential of SERR and SEIRA spectroscopy for in situ investigations on the basis of the first promising studies on human sulfite oxidase and hydrogenases under turnover conditions.

Spectroscopic analysis of immobilised redox enzymes under direct electrochemical control

This article reviews recent developments in spectroscopic analysis of electrode-immobilised enzymes under direct, unmediated electrochemical control. These methods unite the suite of spectroscopic methods available for characterisation of structural, electronic and coordination changes in proteins with the exquisite control over complex redox enzymes that can be achieved in protein film electrochemistry in which immobilised protein molecules exchange electrons directly with an electrode. This combination is particularly powerful in studies of highly active enzymes where redox states can be controlled even under fast electrocatalytic turnover. We examine examples in which UV-visible, IR, Raman and MCD spectroscopy have been combined with direct electrochemistry to probe redox-dependent chemistry, and consider future opportunities for 'direct' spectroelectrochemistry of immobilised enzymes.