Direct and indirect electron transfer between electrodes and redox proteins

Use of 'solid-state' promoters in the electrochemistry of cytochrome c at a gold electrode

The direct electrochemistry of cytochrome c at a gold electrode was investigated by cyclic voltammetry using, as promoters, microperoxidase (the haem-undecapeptide obtained by hydrolysis of cytochrome c), Fe(III)-protoporphyrin IX or protoporphyrin-IX, all entrapped in a cellulose triacetate membrane. The results indicate that these immobilized systems strongly enhance the rate of electron transfer between the protein in solution and the electrode surface, and thus behave as 'solid-state' promoters, though with differing efficiencies. These results are of interest because they raise the possibility of engineering an efficient and versatile promoter active also at inert electrode surfaces.

Evidence for fast and discriminatory electron transfer of proteins at modified gold electrodes

The electrochemistry of the redox proteins, cytochrome c, cytochrome b5, plastocyanin and ferredoxin at modified gold electrodes has been examined on the basis that electron transfer takes place at electroactive sites which are microscopic in size. Using this model, it is now proposed that electrochemistry of these proteins occurs at suitably modified sites with fast rates at potentials near the standard redox potential. The microscopic model implies that redox proteins and enzymes take part in fast electron transfer at specific sites on the electrode, other sites being completely ineffective. This form of molecular recognition, i.e. the ability to discriminate between the different sites on an electrode surface, mimics homogeneous redox reactions wherein redox active proteins 'recognize' their biological partners in a very specific sense. Previously, protein electrochemistry has been interpreted via use of a macroscopic model in which the proteins are transported to the electrode surface by linear diffusion followed by quasi-reversible or irreversible electron transfer to the electrode surface. The microscopic model, which assumes that the movement of the protein occurs predominantly by radial diffusion to very small sites, would appear to explain the data more satisfactorily and be consistent with biologically important, homogeneous redox reactions which are known to be fast.

A new procedure for fast isolation and purification of plastocyanin from the cyanobacterium Anabaena variabilis

Methods are described for growing the cyanobacterium A. variabilis and for the isolation and purification of plastocyanin from the grown culture. Cell paste which had been stored at -35°C was suspended in 1 mM MES buffer, pH 6.5 and centrifuged. The supernatant was diluted to a conductivity of 0.12 mS, [Fe(CN)6](3-) added to a concentration of 0.5 mM and the solution loaded on a S Sepharose Fast Flow column. After elution and ultrafiltration, the plastocyanin containing fractions were reloaded on a S Sepharose Fast Flow column for final purification. A typical yield in three days from cells harvested from 3×20 l of medium was 32 mg plastocyanin with a minimum absorbance ratio A278/A597=1.14. This procedure is faster and the yield higher than for previous procedures.

Electron transfer reactions of metalloproteins at peptide-modified gold electrodes

The electron transfer reactions of four small redox proteins, cytochrome c. ferredoxin, plastocyanin and azurin, have been investigated at novel peptide-modified gold electrodes. These proved to be effective and selective in facilitating electron transfer. Good, quasi-reversible electron transfer was achieved selectively at different peptide-protein configurations by changing the pH or the ionic strength of the solution. The use of peptides as promoters for protein electrochemistry opens up the possibility of designing very specific electrode surfaces for larger molecules like enzymes.

Direct electrochemistry of protein-protein complexes involving cytochrome c, cytochrome b5, and plastocyanin

The direct electrochemistry of the cytochrome c/cytochrome b5 and cytochrome c/plastocyanin complexes has been investigated at edge-plane graphite and modified gold electrode surfaces, which are selective for one of the two components of the complex. Electrochemical response of one protein at an otherwise electrostatically unfavorable electrode surface was achieved in the presence of the other protein, and the calculated heterogeneous electron-transfer rate constant and diffusion coefficient were found to be in good agreement with the values determined previously from the electrochemistry of the individual proteins [Armstrong, F. A., Hill, H. A. O., & Walton, N. J. (1988) Acc. Chem. Res. 21, 407 and references therein]. A dynamic model of the protein-protein-electrode ternary complex is proposed to explain the promotion effect, and this model is supported by a study comparing the electrochemical responses of covalent and electrostatic cytochrome c/plastocyanin complexes. It is also suggested that the behavior of protein-protein complexes at electrode surfaces could be related to that of the complexes associated with biological membranes.

Direct electron transfer of redox proteins at the bare glassy carbon electrode

A simple system is presented for the microscale, direct voltammetry of redox proteins, typically 25 micrograms, in the absence of mediators and/or modifiers. The sample consists of a droplet of anaerobic solution laid onto an oversized disc of nitric-acid-pretreated glassy carbon as the working electrode. Very reproducible, Nernstian responses are obtained with horse heart cytochrome c. The midpoint potential Em (pH 7.0) is dependent on the ionic strength, ranging from $293 mV in 1 mM potassium phosphate to $266 mV in 0.1 M phosphate. At fixed buffer and cytochrome concentrations the magnitude of the voltammetric response is found to be independent of pH over six pH units around neutrality. It is suggested that the response of the present system is not complicated by pH-dependent properties of the electrode surface around physiological pH and, therefore, that the use of this system is practical in biochemically oriented studies. Direct, quasi-reversible responses have also been obtained at pH 7.0 (5 mM phosphate) from Desulfovibrio vulgaris. Hildenborough strain, tetraheme cytochrome c3 (pI = 10.0 at 4 C; 3 X Em = -0.32 mV, Em = -0.26 V), and cytochrome c553 (pI = 9.3; Em = +60 mV), and from Megasphaera elsdenii rubredoxin (pI congruent to 3; Em = -353 mV). The latter protein absorbs onto the glassy carbon surface, thus forming a system with possible applications in the electrochemical study of ferredoxin-linked enzymes.