Heterogeneous Electron Transfer of a Two-Centered Heme Protein: Redox and Electrocatalytic Properties of Surface-Immobilized Cytochrome c4

Assessing the Functional and Structural Stability of the Met80Ala Mutant of Cytochrome c in Dimethylsulfoxide

The Met80Ala variant of yeast cytochrome c is known to possess electrocatalytic properties that are absent in the wild type form and that make it a promising candidate for biocatalysis and biosensing. The versatility of an enzyme is enhanced by the stability in mixed aqueous/organic solvents that would allow poorly water-soluble substrates to be targeted. In this work, we have evaluated the effect of dimethylsulfoxide (DMSO) on the functionality of the Met80Ala cytochrome c mutant, by investigating the thermodynamics and kinetics of electron transfer in mixed water/DMSO solutions up to 50% DMSO v/v. In parallel, we have monitored spectroscopically the retention of the main structural features in the same medium, focusing on both the overall protein structure and the heme center. We found that the organic solvent exerts only minor effects on the redox and structural properties of the mutant mostly as a result of the modification of the dielectric constant of the solvent. This would warrant proper functionality of this variant also under these potentially hostile experimental conditions, that differ from the physiological milieu of cytochrome c.

How to Turn an Electron Transfer Protein into a Redox Enzyme for Biosensing

Cytochrome c is a small globular protein whose main physiological role is to shuttle electrons within the mitochondrial electron transport chain. This protein has been widely investigated, especially as a paradigmatic system for understanding the fundamental aspects of biological electron transfer and protein folding. Nevertheless, cytochrome c can also be endowed with a non-native catalytic activity and be immobilized on an electrode surface for the development of third generation biosensors. Here, an overview is offered of the most significant examples of such a functional transformation, carried out by either point mutation(s) or controlled unfolding. The latter can be induced chemically or upon protein immobilization on hydrophobic self-assembled monolayers. We critically discuss the potential held by these systems as core constituents of amperometric biosensors, along with the issues that need to be addressed to optimize their applicability and response.

Adsorbing surface strongly influences the pseudoperoxidase and nitrite reductase activity of electrode-bound yeast cytochrome c. The effect of hydrophobic immobilization

The Met80Ala and Met80Ala/Tyr67Ala variants of S. cerevisiae iso-1 cytochrome c (ycc) and their adducts with cardiolipin immobilized onto a gold electrode coated with a hydrophobic self-assembled monolayer (SAM) of decane-1-thiol were studied through cyclic voltammetry and surface-enhanced resonance Raman spectroscopy (SERRS). The electroactive species - containing a six-coordinate His/His axially ligated heme and a five-coordinate His/- heme stable in the oxidized and reduced state, respectively - and the pseudoperoxidase activity match those found previously for the wt species and are only slightly affected by CL binding. Most importantly, the reduced His/- ligated form of these variants is able to catalytically reduce the nitrite ion, while electrode-immobilized wt ycc and other His/Met heme ligated variants under a variety of conditions are not. Besides the pseudoperoxidase and nitrite reductase functions, which are the most physiologically relevant abilities of these constructs, also axial heme ligation and the equilibria between conformers are strongly affected by the nature - hydrophobic vs. electrostatic - of the non-covalent interactions determining protein immobilization. Also affected are the catalytic activity changes induced by a given mutation as well as those due to partial unfolding due to CL binding. It follows that under the same solution conditions the structural and functional properties of immobilized ycc are surface-specific and therefore cannot be transferred from an immobilized system to another involving different interfacial protein-SAM interactions.

Met80 and Tyr67 affect the chemical unfolding of yeast cytochromec: comparing the solutionvs.immobilized state

The motional regime affects the unfolding propensity and axial heme coordination of the Met80Ala and Met80Ala/Tyr67Ala variants of yeast iso-1 cytochromec.

Bacterial Respiratory Chain Diversity Reveals a Cytochrome c Oxidase Reducing O2 at Low Overpotentials

Cytochrome c oxidases (CcOs) are the terminal enzymes in energy-converting chains of microorganisms, where they reduce oxygen into water. Their affinity for O₂ makes them attractive biocatalysts for technological devices in which O₂ concentration is limited, but the high overpotentials they display on electrodes severely limit their applicative use. Here, the CcO of the acidophilic bacterium Acidithiobacillus ferrooxidans is studied on various carbon materials by direct protein electrochemistry and mediated one with redox mediators either diffusing or co-immobilized at the electrode surface. The entrapment of the CcO in a network of hydrophobic carbon nanofibers permits a direct electrochemical communication between the enzyme and the electrode. We demonstrate that the CcO displays a μM affinity for O₂ and reduces O₂ at exceptionally high electrode potentials in the range of +700 to +540 mV vs NHE over a pH range of 4-6. The kinetics of interactions between the enzyme and its physiological partners are fully quantified. Based on these results, an electron transfer pathway allowing O₂ reduction in the acidic metabolic chain is proposed.

Immobilized cytochrome c bound to cardiolipin exhibits peculiar oxidation state-dependent axial heme ligation and catalytically reduces dioxygen

Mitochondrial cytochrome c (cytc) plays an important role in programmed cell death upon binding to cardiolipin (CL), a negatively charged phospholipid of the inner mitochondrial membrane (IMM). Although this binding has been thoroughly investigated in solution, little is known on the nature and reactivity of the adduct (cytc-CL) immobilized at IMM. In this work, we have studied electrochemically cytc-CL immobilized on a hydrophobic self-assembled monolayer (SAM) of decane-1-thiol. This construct would reproduce the motional restriction and the nonpolar environment experienced by cytc-CL at IMM. Surface-enhanced resonance Raman (SERR) studies allowed the axial heme iron ligands to be identified, which were found to be oxidation state dependent and differ from those of cytc-CL in solution. In particular, immobilized cytc-CL experiences an equilibrium between a low-spin (LS) 6c His/His and a high-spin (HS) 5c His/- coordination states. The former prevails in the oxidized and the latter in the reduced form. Axial coordination of the ferric heme thus differs from the (LS) 6c His/Lys and (LS) 6c His/OH(-) states observed in solution. Moreover, a relevant finding is that the immobilized ferrous cytc-CL is able to catalytically reduce dioxygen, likely to superoxide ion. These findings indicate that restriction of motional freedom due to interaction with the membrane is an additional factor playing in the mechanism of cytc unfolding and cytc-mediated peroxidation functional to the apoptosis cascade.

A simple method for the determination of reduction potentials in heme proteins

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A simple method for determination of heme protein reduction potentials is described.

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We use the method to determine reduction potentials for human NPAS2 and human CLOCK.

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The method can be easily applied to other heme proteins.

We describe a simple method for the determination of heme protein reduction potentials. We use the method to determine the reduction potentials for the PAS-A domains of the regulatory heme proteins human NPAS2 (E_m = −115 mV ± 2 mV, pH 7.0) and human CLOCK (E_m = −111 mV ± 2 mV, pH 7.0). We suggest that the method can be easily and routinely applied to the determination of reduction potentials across the family of heme proteins.

Modeling and computations of the intramolecular electron transfer process in the two-heme protein cytochrome c(4)

The di-heme protein Pseudomonas stutzeri cytochrome c(4) (cyt c(4)) has emerged as a useful model for studying long-range protein electron transfer (ET). Recent experimental observations have shown a dramatically different pattern of intramolecular ET between the two heme groups in different local environments. Intramolecular ET in homogeneous solution is too slow (>10 s) to be detected but fast (ms-μs) intramolecular ET in an electrochemical environment has recently been achieved by controlling the molecular orientation of the protein assembled on a gold electrode surface. In this work we have performed computational modeling of the intramolecular ET process by a combination of density functional theory (DFT) and quantum mechanical charge transfer theory to disclose reasons for this difference. We first address the electronic structures of the model heme core with histidine and methionine axial ligands in both low- and high-spin states by structure-optimized DFT. The computations enable estimating the intramolecular reorganization energy of the ET process for different combinations of low- and high-spin heme couples. Environmental reorganization free energies, work terms ("gating") and driving force were determined using dielectric continuum models. We then calculated the electronic transmission coefficient of the intramolecular ET rate using perturbation theory combined with the electronic wave functions determined by the DFT calculations for different heme group orientations and Fe-Fe separations. The reactivity of low- and high-spin heme groups was notably different. The ET rate is exceedingly low for the crystallographic equilibrium orientation but increases by several orders of magnitude for thermally accessible non-equilibrium configurations. Deprotonation of the propionate carboxyl group was also found to enhance the ET rate significantly. The results are discussed in relation to the observed surface immobilization effect and support the notion of conformationally gated ET.