Electrochemical Comparison of Heme Proteins by Insulated Electrode Voltammetry

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.

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.

Termination of Biological Function at Low Temperatures: Glass or Structural Transition?

Energy of life is produced by electron transfer in energy chains of respiration or photosynthesis. A small input of free energy available to biology puts significant restrictions on how much free energy can be lost in each electron-transfer reaction. We advocate the view that breaking ergodicity, leading to violation of the fluctuation-dissipation theorem (FDT), is how proteins achieve high reaction rates without sacrificing the reaction free energy. Here we show that a significant level of nonergodicity, represented by a large extent of the configurational temperature over the kinetic temperature, is maintained in the entire physiological range for the cytochrome c electron transfer protein. The protein returns to the state consistent with the FDT below the crossover temperature close to the temperature of the protein glass transition. This crossover leads to a sharp increase in the activation barrier of electron transfer and is displayed by a kink in the Arrhenius plot for the reaction rate constant.

Theory and Electrochemistry of Cytochrome c

Extensive simulations of cytochrome c in solution are performed to address the apparent contradiction between large reorganization energies of protein electron transfer typically reported by atomistic simulations and much smaller values produced by protein electrochemistry. The two sets of data are reconciled by deriving the activation barrier for electrochemical reaction in terms of an effective reorganization energy composed of half the Stokes shift (characterizing the medium polarization in response to electron transfer) and the variance reorganization energy (characterizing the breadth of electrostatic fluctuations). This effective reorganization energy is much smaller than each of the two components contributing to it and is fully consistent with electrochemical measurements. Calculations in the range of temperatures between 280 and 360 K combine long, classical molecular dynamics simulations with quantum calculations of the protein active site. The results agree with the Arrhenius plots for the reaction rates and with cyclic voltammetry of cytochrome c immobilized on self-assembled monolayers. Small effective reorganization energy, and the resulting small activation barrier, is a general phenomenology of protein electron transfer allowing fast electron transport within biological energy chains.

Surface-enhanced vibrational spectroscopy for probing transient interactions of proteins with biomimetic interfaces: electric field effects on structure, dynamics and function of cytochrome c

Most of the biochemical and biophysical processes of proteins take place at membranes, and are thus under the influence of strong local electric fields, which are likely to affect the structure as well as the reaction mechanism and dynamics. To analyse such electric field effects, biomimetic interfaces may be employed that consist of membrane models deposited on nanostructured metal electrodes. For such devices, surface-enhanced resonance Raman and IR absorption spectroscopy are powerful techniques to disentangle the complex interfacial processes of proteins in terms of rotational diffusion, electron transfer, and protein and cofactor structural changes. The present article reviews the results obtained for the haem protein cytochrome c, which is widely used as a model protein for studying the various reaction steps of interfacial redox processes in general. In addition, it is shown that electric field effects may be functional for the natural redox processes of cytochrome c in the respiratory chain, as well as for the switch from the redox to the peroxidase function, one of the key events preceding apoptosis.

Impact of self-assembly composition on the alternate interfacial electron transfer for electrostatically immobilized cytochromec

We report on the effects of self-assembled monolayer (SAM) dilution and thickness on the electron transfer (ET) event for cytochrome c (CytC) electrostatically immobilized on carboxyl terminated groups. We observed biphasic kinetic behavior for a logarithmic dependence of the rate constant on the SAM carbon number (ET distance) within the series of mixed SAMs of C(5)COOH/C(2)OH, C(10)COOH/C(6)OH, and C(15)COOH/C(11)OH that is in overall similar to that found earlier for the undiluted SAM assemblies. However, in the case of C(15)COOH/C(11)OH and C(10)COOH/C(6)OH mixed SAMs a notable increase of the ET standard rate constant was observed, in comparison with the corresponding unicomponent (omega-COOH) SAMs. In the case of the C(5)COOH/C(2)OH composite SAM a decrease of the rate constant versus the unicomponent analogue was observed. The value of the reorganization free energy deduced through the Marcus-like data analysis did not change throughout the series; this fact along with the other observations indicates uncomplicated rate-determining unimolecular ET in all cases. Our results are consistent with a model that considers a changeover between the alternate, tunneling and adiabatic intrinsic ET mechanisms. The physical mechanism behind the observed fine kinetic effects in terms of the protein-rigidifying omega-COOH/CytC interactions arising in the case of mixed SAMs are also discussed.

Kinetic, Thermodynamic, and Mechanistic Patterns for Free (Unbound) Cytochromec at Au/SAM Junctions: Impact of Electronic Coupling, Hydrostatic Pressure, and Stabilizing/Denaturing Additives

Combined kinetic (electrochemical) and thermodynamic (calorimetric) investigations were performed for an unbound (intact native-like) cytochrome c (CytC) freely diffusing to and from gold electrodes modified by hydroxyl-terminated self-assembled monolayer films (SAMs), under a unique broad range of experimental conditions. Our approach included: 1) fine-tuning of the charge-transfer (CT) distance by using the extended set of Au-deposited hydroxyl-terminated alkanethiol SAMs [-S-(CH(2))(n)-OH] of variable thickness (n=2, 3, 4, 6, 11); 2) application of a high-pressure (up to 150 MPa) kinetic strategy toward the representative Au/SAM/CytC assemblies (n=3, 4, 6); 3) complementary electrochemical and microcalorimetric studies on the impact of some stabilizing and denaturing additives. We report for the first time a mechanistic changeover detected for "free" CytC by three independent kinetic methods, manifested through 1) the abrupt change in the dependence of the shape of the electron exchange standard rate constant (k(o)) versus the SAM thickness (resulting in a variation of estimated actual CT range within ca. 15 to 25 A including ca. 11 A of an "effective" heme-to-omega-hydroxyl distance). The corresponding values of the electronic coupling matrix element vary within the range from ca. 3 to 0.02 cm(-1); 2) the change in activation volume from +6.7 (n=3), to approximately 0 (n=4), and -5.5 (n=6) cm(3) mol(-1) (disclosing at n=3 a direct pressure effect on the protein's internal viscosity); 3) a "full" Kramers-type viscosity dependence for k(o) at n=2 and 3 (demonstrating control of an intraglobular friction through the external dynamic properties), and its gradual transformation to the viscosity independent (nonadiabatic) regime at n=6 and 11. Multilateral cross-testing of "free" CytC in a native-like, glucose-stabilized and urea-destabilized (molten-globule-like) states revealed novel intrinsic links between local/global structural and functional characteristics. Importantly, our results on the high-pressure and solution-viscosity effects, together with matching literature data, strongly support the concept of "dynamic slaving", which implies that fluctuations involving "small" solution components control the proteins' intrinsic dynamics and function in a highly cooperative manner as far as CT processes under adiabatic conditions are concerned.

Electrochemistry of unfolded cytochrome c in neutral and acidic urea solutions

The present investigation reports the first experimental measurements of the reorganization energy of unfolded metalloprotein in urea solution. Horse heart cytochrome c (cyt c) has been found to undergo reversible one-electron transfer reactions at pH 2 in the presence of 9 M urea. In contrast, the protein is electrochemically inactive at pH 2 under low-ionic strength conditions in the absence of urea. Urea is shown to induce ligation changes at the heme iron and lead to practically complete loss of the alpha-helical content of the protein. Despite being unfolded, the electron-transfer (ET) kinetics of cyt c on a 2-mercaptoethanol-modified Ag(111) electrode remain unusually fast and diffusion controlled. Acid titration of ferric cyt c in 9 M urea down to pH 2 is accompanied by protonation of one of the axial ligands, water binding to the heme iron (pK(a) = 5.2), and a sudden protein collapse (pH < 4). The formal redox potential of the urea-unfolded six-coordinate His18-Fe(III)-H(2)O/five-coordinate His18-Fe(II) couple at pH 2 is estimated to be -0.083 V vs NHE, about 130 mV more positive than seen for bis-His-ligated urea-denatured cyt c at pH 7. The unusually fast ET kinetics are assigned to low reorganization energy of acid/urea-unfolded cyt c at pH 2 (0.41 +/- 0.01 eV), which is actually lower than that of the native cyt c at pH 7 (0.6 +/- 0.02 eV), but closer to that of native bis-His-ligated cyt b(5) (0.44 +/- 0.02 eV). The roles of electronic coupling and heme-flattening on the rate of heterogeneous ET reactions are discussed.

Electrochemical and Spectroscopic Investigations of Immobilized De Novo Designed Heme Proteins on Metal Electrodes

On the basis of rational design principles, template-assisted four-helix-bundle proteins that include two histidines for coordinative binding of a heme were synthesized. Spectroscopic and thermodynamic characterization of the proteins in solution reveals the expected bis-histidine coordinated heme configuration. The proteins possess different binding domains on the top surfaces of the bundles to allow for electrostatic, covalent, and hydrophobic binding to metal electrodes. Electrostatic immobilization was achieved for proteins with lysine-rich binding domains (MOP-P) that adsorb to electrodes covered by self-assembled monolayers of mercaptopropionic acid, whereas cysteamine-based monolayers were employed for covalent attachment of proteins with cysteine residues in the binding domain (MOP-C). Immobilized proteins were studied by surface-enhanced resonance Raman (SERR) spectroscopy and electrochemical methods. For all proteins, immobilization causes a decrease in protein stability and a loosening of the helix packing, as reflected by a partial dissociation of a histidine ligand in the ferrous state and very low redox potentials. For the covalently attached MOP-C, the overall interfacial redox process involves the coupling of electron transfer and heme ligand dissociation, which was analyzed by time-resolved SERR spectroscopy. Electron transfer was found to be significantly slower for the mono-histidine-coordinated than for the bis-histidine-coordinated heme. For the latter, the formal heterogeneous electron-transfer rate constant of 13 s(-1) is similar to those reported for natural heme proteins with comparable electron-transfer distances, which indicates that covalently bound synthetic heme proteins provide efficient electronic communication with a metal electrode as a prerequisite for potential biotechnological applications.

Redox and redox-coupled processes of heme proteins and enzymes at electrochemical interfaces

Modern bioelectrochemical methods rely upon the immobilisation of redox proteins and enzymes on electrodes coated with biocompatible materials to prevent denaturation. However, even when protein denaturation is effectively avoided, heterogeneous protein electron transfer is often coupled to non-Faradaic processes like reorientation, conformational transitions or acid-base equilibria. Disentangling these processes requires methods capable of probing simultaneously the structure and reaction dynamics of the adsorbed species. Here we provide an overview of the recent developments in Raman and infrared surface-enhanced spectroelectrochemical techniques applied to the study of soluble and membrane bound redox heme proteins and enzymes. Possible biological implications of the findings are critically discussed.

Electron-transfer processes of cytochrome C at interfaces. New insights by surface-enhanced resonance Raman spectroscopy

The heme protein cytochrome c acts as an electron carrier at the mitochondrial-membrane interface and thus exerts its function under the influence of strong electric fields. To assess possible consequences of electric fields on the redox processes of cytochrome c, the protein can be immobilized to self-assembled monolayers on electrodes and studied by surface-enhanced resonance Raman spectroscopy. Such model systems may mimic some essential features of biological interfaces including local electric field strengths. It is shown that physiologically relevant electric field strengths can effectively modulate the electron-transfer dynamics and induce conformational transitions.

Electron transfer dynamics of cytochrome c bound to self-assembled monolayers on silver electrodes

Cytochrome c (Cyt-c) was electrostatically immobilised on Ag electrodes coated with self-assembled monolayers (SAM) that are formed by omega-carboxyl alkanethiols with different alkyl chain lengths (C(x)). Surface enhanced resonance Raman (SERR) spectroscopy demonstrated that electrostatic binding does not lead to conformational changes of the heme protein under the conditions of the present experiments. Employing time-resolved SERR spectroscopy, the rate constants of the heterogeneous electron transfer (ET) between the adsorbed Cyt-c and the Ag electrode were determined for a driving force of zero electronvolts. For SAMs with long alkyl chains (C(16), C(11)), the rate constants display a normal exponential distance dependence, whereas for shorter chain lengths (C(6), C(3), C(3)), the ET rate constant approaches a constant value (ca. 130 s(-1)). The onset of the non-exponential distance-dependence is paralleled by an increasing kinetic H/D effect, indicating a coupling of the redox reaction with proton transfer (PT) steps. This unusual kinetic behaviour is attributed to the effect of the electric field at the Ag/SAM interface that increasingly raises the energy barrier for the PT processes with decreasing distance of the adsorbed Cyt-c from the electrode. The distance-dependence of the electric field strength is estimated on the basis of a simple electrostatic model that can consistently describe the redox potential shifts of Cyt-c as determined by stationary SERR spectroscopy for the various SAMs. At low electric fields, PT is sufficiently fast so that rate constants, determined as a function of the driving force, yield the reorganisation energy (0.217 electronvolts) of the heterogeneous ET.

Proton-coupled electron transfer of cytochrome c

Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolayers (SAM) on an Ag electrode, which are formed by omega-carboxyl alkanethiols of different chain lengths (C(x)). The dynamics of the electron-transfer (ET) reaction of the adsorbed heme protein, initiated by a rapid potential jump to the redox potential, was monitored by time-resolved surface enhanced resonance Raman (SERR) spectroscopy. Under conditions of the present experiments, only the reduced and oxidized forms of the native protein state contribute to the SERR spectra. Thus, the data obtained from the spectra were described by a one-step relaxation process yielding the rate constants of the ET between the adsorbed Cyt-c and the electrode for a driving force of zero electronvolts. For C(16)- and C(11)-SAMs, the respective rate constants of 0.073 and 43 s(-1) correspond to an exponential distance dependence of the ET (beta = 1.28 A(-1)), very similar to that observed for long-range intramolecular ET of redox proteins. Upon further decreasing the chain length, the rate constant only slightly increases to 134 s(-1) at C(6)- and remains essentially unchanged at C(3)- and C(2)-SAMs. The onset of the nonexponential distance dependence is paralleled by a kinetic H/D effect that increases from 1.2 at C(6)- to 4.0 at C(2)-coatings, indicating a coupling of the redox reaction with proton-transfer (PT) steps. These PT processes are attributed to the rearrangement of the hydrogen-bonding network of the protein associated with the transition between the oxidized and reduced state of Cyt-c. Since this unusual kinetic behavior has not been observed for electron-transferring proteins in solution, it is concluded that at the Ag/SAM interface the energy barrier for the PT processes of the adsorbed Cyt-c is raised by the electric field. This effect increases upon reducing the distance to the electrode, until nuclear tunneling becomes the rate-limiting step of the redox process. The electric field dependence of the proton-coupled ET may represent a possible mechanism for controlling biological redox reactions via changes of the transmembrane potential.

Voltammetric probes of cytochrome electroreactivity: the effect of the protein matrix on outer-sphere reorganization energy and electronic coupling probed through comparisons with the behavior of porphyrin complexes

Using surface-modified electrodes composed of omega-hydroxyalkanethiols, an experimentally based value for the inner-sphere reorganization energy of the bis(imidazole)iron porphyrin system has been obtained by examining the solvent dependence of the reorganization energy of bis(N-methylimidazole)meso-tetraphenyl iron porphyrin. The value obtained (0.41 +/- 0.06 eV) is remarkably similar to values we have recently reported for the reorganization energy of cytochrome b(5) (0.43 +/- 0.02 eV) and cytochrome c (0.58 +/- 0.06 eV). This strongly suggests that the protein matrix mimics the behavior of a low dielectric solvent and effectively shields the heme from the solvent. The effect of the orientation of the heme relative to the electrode was also explored by sytematically varying the steric bulk of the axial ligands. On the basis of a good linear correlation between the electronic coupling and the cosine of the angle between the heme plane and the surface of the electrode, it is suggested that a parallel orientation of the heme yields a maximum in the electronic coupling. Relevance to interheme protein electron transfer is discussed.

Direct voltammetric investigation of the electrochemical properties of human hemoglobin: relevance to physiological redox chemistry

Voltammetric measurements on solutions of human hemoglobin using gold electrodes modified with omega-hydroxyalkanethiols have yielded the first direct measure of the reorganization energy of the protein. The value obtained based on extrapolation of the experimentally measured currents, 0.76 eV, is independent of pH (i.e., over the physiologically relevant rage, pH 6.8-7.4) and is remarkably similar to values obtained for myoglobin. This result is perhaps surprising given the marked dependence of the measured reduction potential of hemoglobin on pH (i.e., the redox Bohr effect). Electron transfer rates from the electrode to hemoglobin were also measured. Using similarly measured heterogeneous electron-transfer rates for cytochrome b(5), it is possible to predict the magnitude of the homogeneous electron-transfer rate from cytochrome b(5) to methemoglobin using a formalism developed by Marcus. These predicted rates are in reasonable agreement with reported rates of this physiological reaction based on stopped-flow kinetics experiments. These results suggest that the intrinsic electroreactivity of these heme proteins is sufficient to account for physiologically observed rates. Residual differences between homogeneous phase kinetics and those predicted by heterogeneous phase reactions are suggested to be due to small reductions in the outer-sphere reorganization energy of both component proteins which arise due to solvent exclusion at the interface between the two proteins in complex.