Electrochemistry of cytochrome c immobilized on cardiolipin-modified electrodes: a probe for protein-lipid interactions

Tripeptide Self-Assembled Monolayers as Biocompatible Surfaces for Cytochrome c Electrochemistry

Biocompatible tripeptide self-assembled monolayers (SAMs) are designed with a carboxylate group on the terminal amino acid (glutamate, aspartate, or amino adipate) to electrostatically attract the lysine groups around the heme crevice in horse heart cytochrome c (cyt c), creating an electroactive protein/tripeptide/Au interfacial structure. Exposing the peptide/Au electrode to cyt c resulted in an 11 ± 3 pmol/cm² electroactive protein surface coverage. Topographical images of the interfacial structure are obtained down to single-protein resolution by atomic force microscopy. Uniform protein monolayer assemblies are formed on the Au electrode with no major surface roughness changes. The cyt c/peptide/Au electrode systems were examined electrochemically to probe surface charge effects on the redox thermodynamics and kinetics of cyt c. Neutralization of protein surface charge due to adsorption on anionic COOH-terminated SAMs was found to change the formal potential, as determined by cyclic voltammetry. The cyt c/peptide/Au electrodes exhibit formal potentials shifted to more positive values, have a surface carboxylic acid pK_a of 6 or higher, and produce effective cyt c surface charges (Z_ox) of -6 to -14. The Marcus theory is utilized to determine the protein electron transfer rates, which are ∼5 times faster for cyt c/tripeptide/Au compared to cyt c/11-mercaptoundecanoic acid SAMs of similar chain lengths.

Electrochemical Detection of pH-Responsive Grafted Catechol and Immobilized Cytochrome c onto Lipid Deposit-Modified Glassy Carbon Surface

The electrochemical systems of both grafted catechol as a pH-responsive electrophore and immobilized cytochrome c as a model redox protein are detected by cyclic voltammetry at an optimized lipid deposit-modified glassy carbon electrode. The catechol covalent grafting is successfully performed by the one-pot/three-step electrochemical reduction of 3,4-dihydroxybenzenediazonium salts generated in situ from 4-nitrocatechol. The resulting glassy carbon electrode electrochemically modified by grafted catechol species is evaluated as an efficient electrochemical pH sensor. The optimized molar ratio for the lipid deposit, promoting cytochrome c electrochemical activity in solution onto glassy carbon electrode, is reached for the lipid mixture composed of 75% 1,2-dioleoyl-sn-glycero-3-phosphocholine and 25% cardiolipin. Cytochrome c immobilization into the optimized supported lipid deposit is efficiently achieved by cyclic voltammetry (10 cycles) recorded at the modified glassy carbon electrode in a cytochrome c solution. The pH-dependent redox response of the grafted catechol and that of the immobilized cytochrome c are finally detected at the same lipid-modified glassy carbon electrode without alteration of their structure and electrochemical properties in the pH range 5-9.

Cardiolipin-cytochrome c complex: Switching cytochrome c from an electron-transfer shuttle to a myoglobin- and a peroxidase-like heme-protein

Cytochrome c (cytc) is a small heme-protein located in the space between the inner and the outer membrane of the mitochondrion that transfers electrons from cytc-reductase to cytc-oxidase. The hexa-coordinated heme-Fe atom of cytc displays a very low reactivity toward ligands and does not exhibit significant catalytic properties. However, upon cardiolipin (CL) binding, cytc achieves ligand binding and catalytic properties reminiscent of those of myoglobin and peroxidase. In particular, the peroxidase activity of the cardiolipin-cytochrome c complex (CL-cytc) is critical for the redistribution of CL from the inner to the outer mitochondrial membranes and is essential for the execution and completion of the apoptotic program. On the other hand, the capability of CL-cytc to bind NO and CO and the heme-Fe-based scavenging of reactive nitrogen and oxygen species may affect apoptosis. Here, the ligand binding and catalytic properties of CL-cytc are analyzed in parallel with those of CL-free cytc, myoglobin, and peroxidase to dissect the potential mechanisms of CL in modulating the pro- and anti-apoptotic actions of cytc.

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.

Structural transformations of cytochrome c upon interaction with cardiolipin

Interactions of cytochrome c (cyt c) with cardiolipin (CL) play a critical role in early stages of apoptosis. Upon binding to CL, cyt c undergoes changes in secondary and tertiary structure that lead to a dramatic increase in its peroxidase activity. Insertion of the protein into membranes, insertion of CL acyl chains into the protein interior, and extensive unfolding of cyt c after adsorption to the membrane have been proposed as possible modes for interaction of cyt c with CL. Dissociation of Met80 is accompanied by opening of the heme crevice and binding of another heme ligand. Fluorescence studies have revealed conformational heterogeneity of the lipid-bound protein ensemble with distinct polypeptide conformations that vary in the degree of protein unfolding. We correlate these recent findings to other biophysical observations and rationalize the role of experimental conditions in defining conformational properties and peroxidase activity of the cyt c ensemble. Latest time-resolved studies propose the trigger and the sequence of cardiolipin-induced structural transitions of cyt c.