Direct and indirect electron transfer between electrodes and redox proteins

Abstract

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.Many of the fundamental processes in Nature rely upon the redox processes of constituent biomolecules. Cell respiration involves the stepwise oxidation of organic substrates via a cascade of redox reactions. In photosynthetic organisms, chains of electron carriers, for example, flavoproteins, ironsulphur proteins and cytochromes, participate in light-induced photosynthetic electron transport. The intrinsic mechanisms of electron transfer reactions, and the detailed role of the protein in aiding the electron transfer between redox centres, have become foci for extensive physical and biochemical investigations [l].Electrochemistry provides a powerful tool for examining electron transfer properties. The molecules of particular interest are redox-active proteins and enzymes. Although there have been many investigations of the electrochemical behaviour of the isolated and 'free' prosthetic groups [2, 31, the properties of the latter in an intact protein are so perturbed that to gain an understanding of their biological role, it is essential to have information on the behaviour of the proteins themselves.Only relatively recently has the feasibility of studying the electron transfer reactions of redox proteins at an electrode surface been viewed with any real optimism. Indeed, for many years it was thought that reversible, direct electron transfer (i. e. without mediation by small electron carriers) between electrodes and redox proteins was not possible. Several reasons were given: the slow rates of diffusion of these species were presumed to lead to greatly diminished faradaic currents; the 'buried' nature of the redox groups would make interaction with an electrode prohibitive for all but a few orientations of the protein at the electrode surface and proteins appeared to denature at the electrode-electrolyte interface. In fact, the early studies of protein electrochemistry were unlikely to succeed simply due to the type of electrodes used [4-61. Mercury was the favoured electrode material and, in such systems, adsorption phenomena play a predominant role with the possibility of confor...

The relationship between redox enzyme activity and electrochemical potential—cellular and mechanistic implications from protein film electrochemistry

“…ref. [6][7][8][9][10][11][12][13]. Consequently, this Perspective takes the opportunity to highlight some of the challenges and opportunities we see arising from the current state of the art in the dynamic electrochemistry of redox enzymes.…”

The purification of a cd1‐type nitrite reductase from, and the absence of a copper‐type nitrite reductase from, the aerobic denitrifier Thiosphaera pantotropha; the role of pseudoazurin as an electron donor

“…7) shows that pseudoazurin has a peak anodic current at a potential of +25 mV and a peak cathodic current at a potential of -55 mV relative to the standard calomel electrode and that the process is quasi-reversible (Frew and Hill, 1988). The standard electrode potential was estimated to be 230 mV versus normal hydrogen electrode and the difference between the potentials for the two peak currents was 80 mV which approximates to the value expected for a one-electron transfer (Frew and Hill, 1988). For comparison, the standard electrode potential of azurin from P. denitrificans estimated spectroscopically is also 230 mV versus the normal hydrogen electrode (Martinkus et al, 1980).…”

“…The electrode potential of pseudoazurin was measured by direct electrochemistry (Frew and Hill, 1988). The electrode potential of a 100 pM solution of pseudoazurin in 50 mM Hepes pH 7.00 was measured relative to the saturated calomel electrode (standard electrode potential, E" = +244 mV versus the normal hydrogen electrode using an edge-planeoriented pyrolytic graphite electrode, as described by Burrows et al (1991).…”

“…The effect of the copper-chelating agent diethyldithiocarbamate on pseudoazurin was investigated. Diethyldithiocarbamate was added in stoichiometric amounts to a 26 pM (Frew and Hill, 1988). solution of the pseudoazurin in 100 mM Tris/HCl pH 8 and spectra were taken immediately.…”

Biohydrogen for a New Generation of H2/O2 Biofuel Cells: A Sustainable Energy Perspective

“…In the 1980s, bioelectrochemistry proved to be a powerful tool with which to link electronic transfer data to the structure and function of low-weight redox proteins. [146][147][148][149][150] Huge redox enzymes incorporate redox cofactors during maturation, where conversion of the substrates takes place. Efficient intramolecular electron transfer is ensured by electron relays over distances of less than 14 , allowing electron tunneling from the active site to the surface of the protein.…”