Surface modifiers for the promotion of direct electrochemistry of cytochrome c

Redox-linked conformational changes in proteins detected by a combination of infrared spectroscopy and protein electrochemistry. Evaluation of the technique with cytochrome c

We have developed a new technique for the study of redox-linked conformational changes in proteins, by the combination of two established techniques. Fourier-transform infrared spectroscopy has been used together with direct electrochemistry of the protein at a modified metal electrode surface. The technique has been evaluated with cytochrome c, because of its well-characterized electrochemistry and because the availability of X-ray crystallographic and NMR studies of both redox states of the protein provides a reference against which our data can be compared. In electrochemical control experiments, it was confirmed that the spectroelectrochemical cell design allows fast, accurate and reproducible control of the redox poise of the protein. The resulting reduced-minus-oxidized infrared difference spectra show the changes in the frequencies and intensities of molecular vibrations which arise from the redox-linked conformational change. In contrast to the absolute infrared spectra of proteins, such difference spectra can be sufficiently straightforward to allow interpretation at the level of individual bonds. A complete interpretation of the spectra is beyond the scope of the present paper: however, on the basis of the data presented, we are able to suggest assignments for all except one of the major bands between 1500 cm-1 and 1800 cm-1.

Inhibited enzyme electrodes. Part 2: The kinetics of the cytochrome oxidase system

An inhibition enzyme electrode to measure toxic gases can be constructed using the respiratory enzyme cytochrome oxidase. The rate of enzyme turnover is followed by reducing cytochrome c on a gold electrode modified with the mediator bis(4-pyridyl) disulphate. The kinetics and mechanism of the system have been measured. The electrochemical kinetics for the oxidation of cytochrome c have been studied by rotating disc voltammetry and are shown to obey the Koutecky-Levich equation. The standard electrochemical rate constant is found to be 3 x 10(-3) cms-1. At ambient oxygen concentration the orders of the current with respect to the concentration of cytochrome oxidase, cytochrome c and oxygen are found to be 1/2, 1/2 and zero respectively. These orders are consistent with the rate limiting step being the turnover of the enzyme under saturated conditions in a thin reaction layer close to the electrode. At lower oxygen concentrations a good fit between the experimental results and a theoretical model further confirms the assignation of the mechanism. The rate constants describing the oxidation and reduction of the enzyme have been measured. The pH dependence of the current has been studied.

Electron transfer between the hydrogenase from Desulfovibrio vulgaris (Hildenborough) and viologens. 1. Investigations by cyclic voltammetry

The electron transfer kinetics between the hydrogenase from Desulvovibrio vulgaris (strain Hildenborough) and three different viologen mediators has been investigated by cyclic voltammetry. The mediators methyl viologen, di(n-aminopropyl) viologen and propyl viologen sulfonate differ in redox potential and in net charge. Dependent on the pH both the one- and two-electron-reduced forms or only the two-electron-reduced form of the viologens are effective in electron exchange with hydrogenase. Calculations of the second-order rate constant k for the reaction between reduced viologen and hydrogenase are based on the theory of the simplest electrocatalytic mechanism. Values for k are in the range of 10(6)-10(7) M-1 s-1 and increase in the direction propyl viologen sulfonate----methyl viologen----di(n-aminopropyl) viologen. An explanation is based on electrostatic interactions. It is proposed that the electron transfer reaction is the rate-determining step in the catalytic mechanism.

Direct and indirect electron transfer between electrodes and redox proteins

The direct electrochemistry of redox proteins has been achieved at a variety of electrodes, including modified gold, pyrolytic graphite and metal oxides. Careful design of electrode surfaces and electrolyte conditions are required for the attainment of rapid and reversible protein-electrode interaction. The electron transfer reactions of more complex systems, such as redox enzymes, are now being examined. The 'well-behaved' electrochemistry of redox proteins can be usefully exploited by coupling the electrode reaction to enzymes for which the redox proteins act as cofactors. In systems where direct electron transfer is very slow, small electron carriers, or mediators, may be employed to enhance the rate of electron exchange with the electrode. The organometallic compound ferrocene and its derivatives have proved particularly effective in this role. A new generation of electrochemical biosensors employs ferrocene derivatives as mediators.

Liquid chromatography-electrochemical detection of ferro- and ferricytochrome c at a chemically modified gold electrode

Chemically modified electrodes, constructed by adsorption of 4,4'-dithiodipyridine onto a polyvinylferrocene-treated gold surface, were employed for the amperometric detection of cytochrome c following size-exclusion chromatography. The electrode response was nearly reversible, permitting quantitation both of the ferro-form of the protein at +0.15 V vs. Ag/AgCl and of the ferri-form at -0.15 V. The limit of detection for the reduced species was 3 pmol injected, and the response was linear over three orders of magnitude. Using the chemically modified electrode approach, cytochrome c monitoring was sufficiently selective that the compound could be determined in human plasma pretreated only by dilution and particulate filtration.

Application of hydrogenase in biotechnological conversions

Evidence will be presented in this review article that the application of hydrogenase has large biotechnological possibilities. Our investigations show: Fast reaction of hydrogenase at an electrode surface to reduce H+; Photochemical production of H2 by hydrogenase by photosensitized Ru-complexes dissolved in reversed micellar membranes and vectorial H+ transport through the membrane to the water phase; The production of fine chemicals in reversed micelles by a system containing specific enzymes, hydrogenase and H2. The rules to obtain maximal conversion rates with this system will be presented.

Reactions of electron-transfer proteins at electrodes

Studies of electron-transfer reactions of redox proteins have, in recent years, attracted widespread interest and attention. Progress has been evident from both physical and biological standpoints, with the increasing availability of three-dimensional structural data for many small electron-transfer proteins prompting a variety of systematic investigations (Isied, 1985). Most recently, attention has been directed towards questions concerning the elementary transfer of electrons between spatially remote redox sites, and the nature of protein–protein interactions which, for intermolecular processes, stabilize specific precursor complexes which may be optimally juxtaposed for electron-transfer. These and other issues, including the necessary reversibility of protein interfacial interactions and the dynamic properties of proteins as carriers of electrons in biological electron-transport systems, are now being addressed in the rapidly emerging field of direct (unmediated) protein electrochemistry. It is our intention in this article to discuss developments made in this area and highlight points which we believe to have the most bearing on our current understanding of diffusion-dominated, protein-mediated electron transport at electrode surfaces. First we shall outline some basic considerations which are best considered with reference to homogeneous systems.

The measurement of changes in pH associated with electrochemically driven respiration in rat liver mitochondria

Using an optically transparant thin layer electrode, it has been possible to measure the pH changes associated with the electrochemical turnover of horse heart cytochrome c in the presence of rat liver mitochondria and oxygen. Direct electrochemistry of cytochrome c at a gold electrode modified with bis(4-pyridyl)bisulfide allowed electron flux (current) to be measured simultaneously with the differential change in absorbance associated with phenol red, a pH-sensitive dye. Although the alkalinization due to the reduction of oxygen to water was readily observed, any initial acidification associated with proton pumping was not detected. It is suggested that at the high ratios of oxidized-to-reduced cytochrome c present during the steady state attained, proton pumping may be absent or more localized.