Electron transfer between cytochrome c and copper enzymes

Electroactive nanobiomolecular architectures of laccase and cytochrome c on electrodes: applying silica nanoparticles as artificial matrix

Fully electroactive multilayer architectures combining the redox protein cytochrome c and the enzyme laccase by the use of silica nanoparticles as artificial matrix have been constructed on gold electrodes capable of direct dioxygen reduction. Laccase form Trametes versicolor and cytochrome c from horse heart were electrostatically coimmobilized by alternate deposition with interlayers of silica nanoparticles in a multilayer fashion. The layer formation has been monitored by quartz crystal microbalance. The electrochemical properties and performance of the nanobiomolecular entities were investigated by cyclic voltammetry, indicating, that a multistep electron transfer cascade, from the electrode via cytochrome c in the layered system toward the enzyme laccase, and here to molecular dioxygen was achieved. The response of the novel architecture is based on direct electron exchange between immobilized proteins and can be tuned by the assembly process.

Nanobiomolecular multiprotein clusters on electrodes for the formation of a switchable cascadic reaction scheme

A supramolecular multicomponent protein architecture on electrodes is developed that allows the establishment of bidirectional electron transfer cascades based on interprotein electron exchange. The architecture is formed by embedding two different enzymes (laccase and cellobiose dehydrogenase) and a redox protein (cytochrome c) by means of carboxy-modified silica nanoparticles in a multiple layer format. The construct is designed as a switchable dual analyte detection device allowing the measurement of lactose and oxygen, respectively. As the switching force we apply the electrode potential, which ensures control of the redox state of cytochrome c. The two signal chains are operating in a non-separated matrix and are not disturbed by the other biocatalyst.

Small electron-transfer proteins as mediators in enzymatic electrochemical biosensors

Electrochemical mediators transfer redox equivalents between the active sites of enzymes and electrodes and, in this way, initiate bioelectrocatalytic redox processes. This has been very useful in the development of the so-called second-generation biosensors, in which they transduce a catalyzed reaction into an electrical signal. Among other pre-requisites, redox mediators must be readily oxidized and/or reduced at the electrode surface and readily interact with the biorecognition component. Small chemical compounds (e.g. ferrocene derivatives, ruthenium, or osmium complexes and viologens) are frequently used for this purpose but, lately, small redox proteins (e.g. horse heart cytochrome c) have also been used as redox partners in biosensing applications. In general, docking between two complementary proteins introduces a second level of selectivity to the biosensor and enlarges the list of compounds analyzed. Moreover, electrochemical interferences are frequently minimized owing to the small overpotentials achieved. This paper provides an overview of enzyme biosensors that are mediated by electron-transfer proteins. The paper begins with a brief discussion of mediated electrochemistry in biosensing systems and proceeds with a detailed description of relevant work on the cooperative use of redox enzymes and biological electron donors and/or acceptors.

Investigation of the mediated electron transfer mechanism of cellobiose dehydrogenase at cytochrome c-modified gold electrodes

The present study reports on the comparison of direct and mediated electron transfer pathways in the interaction of the fungal enzyme cellobiose dehydrogenase (CDH) with the redox protein cytochrome c (cyt c) immobilised at a modified gold electrode surface. Two types of CDHs were chosen for this investigation: a basidiomycete (white rot) CDH from Trametes villosa and a recently discovered ascomycete from the thermophilic fungus Corynascus thermophilus. The choice was based on the pH-dependent interaction of these enzymes with cyt c in solution containing the substrate cellobiose (CB). Both enzymes show rather similar catalytic behaviour at lower pH, dominated by a direct electron exchange with the electrode. With increasing pH, however, also cyt c-mediated electron transfer becomes possible. The pH-dependent behaviour in the presence and in the absence of cyt c is analysed and the potential reaction mechanism for the two enzymes with a different pH-behaviour is discussed.

Protein electrodes with direct electrochemical communication

Electrochemistry using direct electron transfer between an electrode and a protein or an enzyme has developed into a means for studying biological redox reactions and for bioanalytics, biosynthesis and bioenergetics. This review summarizes recent work on direct protein electrochemistry with special emphasis on our results in bioelectrocatalysis using isolated enzymes and enzyme-protein couples.

Direct Electrochemical Behavior of Cytochrome c on DNA-modified Glassy Carbon Electrode and Its Application to Cytochrome c Biosensor

DNA was immobilized on glassy carbon electrodes to fabricate DNA-modified electrodes. The direct electron transfer of horse heart cytochrome c on DNA-modified glassy carbon electrode was achieved. A pair of well-defined redox peaks of cytochrome c appeared at Epc = -0.017 V and Epa = 0.009 V (vs. Ag/AgCl) in 10 mM phosphate buffer solution (pH 7.0) at a scan rate of 50 mV/s. The electron transfer coefficient (alpha) and the standard rate constant of the surface reaction (Ks) of cytochrome c on DNA-modified electrodes could be estimated to be 0.87 and 34.52 s(-1), respectively. The DNA-modified glassy carbon electrode could be applied to detect cytochrome c by means of differential pulse voltammetry (DPV). The cathodic peak current was proportional to the quantity of cytochrome c in the range of 4.0 x 10(-6) M to 1.2 x 10(-5) M. The correlation coefficient is 0.996, and with the detection limit was 1.0 x 10(-6) M (three times the ratio of signal to noise, S/N = 3).

Direct heterogeneous electron transfer of theophylline oxidase

Direct electron transfer (DET) was shown between the heme containing enzyme theophylline oxidase (ThO) and the surface of both graphite and gold electrodes. As proof on graphite a steady state current for theophylline was recorded using the electrode modified with adsorbed ThO. The electrode showed a Michaelis-Menten-like response to theophylline with a detection limit of 0.2 mM and a Michaelis-Menten constant equal to 3.2 mM. These initial results open up a possibility for the development of reagentless third generation biosensor based on heterogeneous DET between ThO and an electrode. On gold DET between ThO and the surface of aldrithiol modified gold was studied with spectroelectrochemical measurements. DET was observed for soluble ThO as a change of its spectrum in a gold capillary responding to a change in the applied potential. It was shown that the redox conversion of the heme domain of the enzyme is directly (mediatorlessly) driven by the potential applied at the gold electrode. The measurements enabled an estimation of the formal potential (E degrees ') of the redox process equal to -275 +/- 50 mV versus Ag|AgClsat at pH 7.0. The experimentally determined number of the electrons involved in this heterogeneous electron transfer process was estimated to be equal to 0.53. The low precision in determination of the E degrees ' and the value of the number of electrons lower than one indicate that kinetic restrictions disturbed the evaluation of the true thermodynamic values from relatively fast spectroelectrochemical measurements.

Electrochemistry of cytochrome c incorporated in Langmuir–Blodgett films of Nafion® and Eastman AQ 55®

Ultrathin films of Nafion and Eastman-AQ 55 loaded with cytochrome c (cyt c) were obtained and transferred on indium tin oxide (ITO) electrodes via the Langmuir-Blodgett (LB) technique. The pressure-area isotherms for mixed ionomer-protein films indicate that the miscibility of cyt c in the interfacial layer is better for Nafion than for AQ 55. Interestingly, these composite films maintain the electroactivity of cyt c without requiring the addition of promoters or mediators. Both for AQ 55-cyt c and Nafion-cyt c films, the half-wave potential for the reversible reduction of ferricytochrome c corresponds to the value expected for the weakly adsorbed protein. The modified electrodes show electrocatalytic reaction with ascorbate anion. Comparison with previous literature reports indicate that for Nafion the LB coating procedure is unique in keeping the electroactivity of cyt c.

Bioelectrocatalysis by redox enzymes at modified electrodes

Self-assembled monolayers of thiolated compounds are used as promoters for protein-electrode reactions. They provide an anchor group based on thiol chemisorptions and also a functional group for effective interaction with the protein. These interactions are often governed by electrostatic attraction. For example, for positively charged proteins, such as cytochrome c and the selenoprotein glutathione peroxidase, mercaptoalkanoic acids have been used. Clay modification of the electrode surface has been found to facilitate the heterogeneous electron transfer process for heme proteins, e.g. cytochrome c, cytochrome P450 and myoglobin. Interestingly, nucleic acids at carbon electrodes and thiol-modified double stranded oligonucleotides act as promoters of the redox communication to proteins, whereas the mechanism is still subject to controversy interpretations. By interacting the protein immobilised at the electrode with species in solution, signal chains have been constructed. The interaction can result in a simple co-ordination or redox reaction, depending on the nature of the reaction partners. For analytical purposes, e.g. biosensors, the electrochemical redox conversion of the immobilised protein is evaluated.

Phenol-oxidizing enzymes: mechanisms and applications in biosensors

Phenolic compounds are widely distributed in nature. Enzymes which catalyze their oxidation are monophenol monooxygenases, such as tyrosinases and laccases, and peroxidases. Their metabolic role includes the decomposition of natural complex aromatic polymers as well as polymerization of the oxidation products and the degradation of xenobiotics. Their catalytic properties and broad availability gained impact on the development of biosenors for both environmentally important pollutants and clinically relevant metabolites. Mechanisms for the phenol-oxidizine enzymes tyrosinases, laccases, and peroxidases are reviewed and some examples for their use in the construction of phenol selective biosenors are given.