Electric-field-induced redox potential shifts of tetraheme cytochromes c3 immobilized on self-assembled monolayers: surface-enhanced resonance Raman spectroscopy and simulation studies

Midpoint potentials of hemes a and a3 in the quinol oxidase from Acidianus ambivalens are inverted

The aa3 type B oxygen reductase from the thermophilic archaeon Acidianus ambivalens (QO) was immobilized on silver electrodes and studied by potential-dependent surface-enhanced resonance Raman (SERR) spectroscopy. The immobilized enzyme retains the native structure at the level of the heme pockets and exhibits reversible electrochemistry. From the potential dependence of specific spectral marker bands, the midpoint potentials of hemes a and a3 were unambiguously determined for the first time, being 320 +/- 20 mV for the former and 390 +/- 20 mV for the latter. Both hemes could be treated as independent one-electron Nernstian redox couples, indicating that the interaction potential is smaller than 50 mV. The reversed order of the midpoint potentials compared to those of type A (mitochondrial-like) oxidases, as well as the lack of substantial Coulombic interactions, suggests a different mechanism of electroprotonic energy transduction. In contrast to type A enzymes, a-a3 intraprotein electron transfer in QO is already guaranteed by the order of the midpoint potentials at the onset of enzyme reduction and, therefore, does not require a complex network of cooperativities to ensure exergonicity. In the immobilized state, conformational transitions of the QO a3-CuB active site, which are believed to be essential for proton translocation, are drastically slowed compared to those in solution. We ascribe this finding to the effect of the interfacial electric field, which is of the same order of magnitude as in biological membranes. These results suggest that the membrane potential may play an active role in the regulation of the enzymatic activity of QO.

Modeling heme proteins using atomistic simulations

Heme proteins are found in all living organisms, and perform a wide variety of tasks ranging from electron transport, to the oxidation of organic compounds, to the sensing and transport of small molecules. In this work we review the application of classical and quantum-mechanical atomistic simulation tools to the investigation of several relevant issues in heme proteins chemistry: (i) conformational analysis, ligand migration, and solvation effects studied using classical molecular dynamics simulations; (ii) electronic structure and spin state energetics of the active sites explored using quantum-mechanics (QM) methods; (iii) the interaction of heme proteins with small ligands studied through hybrid quantum mechanics-molecular mechanics (QM-MM) techniques; (iv) and finally chemical reactivity and catalysis tackled by a combination of quantum and classical tools.

Long distance electron transfer in cytochrome c oxidase immobilised on electrodes. A surface enhanced resonance Raman spectroscopic study

Cytochrome c oxidase was tethered to a functionalised Ag electrode via a histidine-tag on the C-terminus of subunit I or II and embedded in a phospholipid bilayer. The uniformly oriented membrane-bound proteins were studied by surface enhanced resonance Raman spectroscopy (SERRS) that reveals preservation of the native structures of the heme a and heme a(3) sites. On the basis of time-dependent SERRS measurements, the rate constant for the heterogeneous electron transfer to heme a was determined to be 0.002 s(-1) independent of the enzyme orientation and the overpotential. Taking into account that the electrode-to-heme a distance is larger than 50 A, these findings suggest an electron hopping mechanism in which the Cu(A) center is not involved. Electrochemical reduction is restricted to heme a whereas electron transfer from heme a to heme a(3), which in solution occurs on the nanosecond time scale, is drastically slowed down. It may be that the network of cooperativities that links intramolecular electron transfer and proton translocation is perturbed in the immobilised enzyme, possibly due to the effect of the interfacial electric field.

Redox Processes of Cytochrome c Immobilized on Solid Supported Polyelectrolyte Multilayers

The heme protein cytochrome c (Cyt-c), immobilized on polyelectrolyte multilayers on a silver electrode, was studied by stationary and time-resolved surface-enhanced resonance Raman (SERR) spectroscopy to probe the redox site structure and the mechanism and dynamics of the potential-dependent interfacial processes. The layers were built up by sequential adsorption of polycations (poly[ethylene imine] (PEI); polyallylamine hydrochloride (PAH)) and polyanions (poly[styrene sulfonate] (PSS)). All multilayers terminated by PSS electrostatically bind Cyt-c. On PEI/PSS coatings, Cyt-c is peripherally bound and fully redox-active. Due to the interfacial potential drop, the apparent redox potential is lowered by 40 mV compared to that in solution. The rate constant for the heterogeneous electron transfer (ET) of ca. 0.1 s(-1) is consistent with electron tunneling through largely ordered PEI/PSS layers. ET is coupled to a reversible conformational transition of Cyt-c that involves a change of the coordination pattern of the heme. Additional (PAH/PSS) double layers cause a broadening of the redox transition and a drastic negative shift of the redox potential, which is attributed to the formation of PSS/Cyt-c complexes. It is concluded that Cyt-c can effectively compete with PAH for binding of PSS, resulting in a rearrangement of the layered structure and a penetration of the PSS-bound Cyt-c into the PAH/PSS double layers. This conclusion is consistent with SERR intensity and quartz microbalance measurements. ET was found to be overpotential-independent and faster than that for PEI/PSS coatings, which is interpreted in terms of specific PSS/Cyt-c complexes serving as gates for the heterogeneous ET.

Conformational transitions and redox potential shifts of cytochrome P450 induced by immobilization

Cytochrome P450 (P450) from Pseudomonas putida was immobilized on Ag electrodes coated with self-assembled monolayers (SAMs) via electrostatic and hydrophobic interactions as well as by covalent cross-linking. The redox and conformational equilibria of the immobilized protein were studied by potential-dependent surface-enhanced resonance Raman spectroscopy. All immobilization conditions lead to the formation of the cytochrome P420 (P420) form of the enzyme. The redox potential of the electrostatically adsorbed P420 is significantly more positive than in solution and shows a steady downshift upon shortening of the length of the carboxyl-terminated SAMs, i.e., upon increasing the strength of the local electric field. Thus, two opposing effects modulate the redox potential of the adsorbed enzyme. First, the increased hydrophobicity of the heme environment brought about by immobilization on the SAM tends to upshift the redox potential by stabilizing the formally neutral ferrous form. Second, increasing electric fields tend to stabilize the positively charged ferric form, producing the opposite effect. The results provide insight into the parameters that control the structure and redox properties of heme proteins and contribute to the understanding of the apparently anomalous behavior of P450 enzymes in bioelectronic devices.

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.