Electron transfer bioelectrocatalysis

Kamitaka Y, Tsujimura S, Setoyama N, Kajino T, Kano K. 2007. Fructose/dioxygen biofuel cell based on direct electron transfer-type bioelectrocatalysis. Phys Chem Chem Phys 9 1793-1801. 

C. Cai and J. Chen, Direct electron transfer and bioelectrocatalysis of hemoglobin at a carbon nanotube electrode. Anal. Biochem. 325, 285-292 (2004). 

X.H. Chen, C.M. Ruan, J.L. Kong, and J.Q. Deng, Characterization of the direct electron transfer and bioelectrocatalysis of horseradish peroxidase in DNA film at pyrolytic graphite electrode. Anal. Chim. 

The mechanism and theory of bioelectrocatalysis is still under development. Electron transfer and variation of potential in the electrodeenzyme-electrolyte system has therefore to be investigated. Whether the enzyme is soluble and the electron transfer process occurs through a mediator, or whether there is direct enzyme immobilization on the electrode surface, the homogeneous process in the enzyme active centre has to be described by the laws of enzyme catalysis, and the heterogeneous processes on the electrode surface by the laws of electrochemical kinetics. Besides this there are other aspects outside electrochemistry or... 

If the active site of the enzyme is located sufficiently close to the electrode surface electrons can be transferred directly from the enzyme to the electrode as depicted in Figure 5.3a. In the case of an anodic reaction, the electrode replaces the natural co-substrate (such as oxygen) as an electron acceptor. This process is known as direct electron transfer (DFT), often categorized as third-generation enzyme electrodes in the biosensor literature, and is the most elegant and simplest method of bioelectrocatalysis between an enzyme active site and an electrode. 

Coupling between a biologically catalyzed reaction and an electrochemical reaction, referred to as bioelectrocatalysis, is the constructional principle for enzyme-based electrochemical biosensors. This means that the flow of electrons from a donor through the enzyme to an acceptor must reach the electrode in order for the corresponding current to be detected. In case a direct electron transfer between the active site of an enzjane and an electrode is not possible, a small molecular redox active species, e.g. hydrophobic ferrocene, meldola blue and menadione as well as hydrophilic ferricyanide, can be used as an electron transfer mediator. This means that the electrons from the active site of the enzyme reduce the mediator molecule, which, in turn, can diffuse to the electrode, where it donates the electrons upon oxidation. When these mediator molecules are employed for coupling of an enzymatic redox reaction to an electrode at a constant potential, the resulting application can be referred to as mediated amperometry or mediated bioelectrocatalysis. 

FIG. 15 Schematic representation of direct bioelectrocatalysis. Electrons are transferred from substrate to an electrode through an intramolecular electron transfer from redox center A to redox center B in the enzyme molecule adsorbed on the electrode surface. 

The most important problem of bioelectrocatalysis is the study of the mechanism of electron transfer between the active center of the enzyme and the electrode, and realization of effective pathways for conjugation of enzy-... 

Another aspect of bioelectrocatalysis is the application of electrochemical methods to study aspects of the mechanism of the enzymatic effect and, in particular, of the relationship between conformational transformations of proteins, the redox potential value at the active center, and the electron transfer rate. Understanding the specificities of biocatalysts opens up a possibility for developing synthetic models of enzymes on the basis of complex compounds of a nonprotein nature. 

Nonetheless, the concepts of tunneling for electron transfer are quite promising for the interpretation of certain effects of bioelectrocatalysis. 

Structural and molecular understanding of mediated electron transfer enables a paradigm shift from a mediator acceptance screening to a rational mediator design which considers only stability and electron transfer performance parameters. The rational mediator design would employ a subsequent enzyme engineering step to ensure an efficient electron transfer between mediator and enzyme which would open novel and exciting opportunities for enzymes in bioelectrocatalysis. 

In third generation biosensors, electrons are transferred directly from the active site of the biocatalyst to the electrode surface, i.e. no mediator or oxygen is required (see Fig. 1C). This is termed direct electron transfer (DET) or direct bioelectrocatalysis. This is the ideal mechanism for an electrochemical biosensor because it reduces the number of necessary components for the device. However, most biological catalysts are unable to do direct electron transfer. In order to do DET, the biological active site must be close enough to the electrode surface for electron tunneling to... 

It is frequently the case that enzymes undergo slow heterogeneous electron transfer with an electrode surface. Such slow rates of electron transfer can largely be attributed to insulation of the redox active site by the bulk of the protein matrfac. This is especially true for en mes that have a non-dissociable redox cofactor, such as the flavin adenine dinucleotide (FAD) within the active site of glucose oxidase. In cases where an enzyme active site is physically inaccessible to the electrode surface, an artificial intermediate redox species is used to shuttle electrons between the enzyme and electrode. It should be noted at the outset that much of the early research in mediated bioelectrocatalysis was performed on homogeneous mixtures of substrate, enzyme and mediator. However, the vast majority of literature since the late 1980s has dealt exclusively with systems in which the enzyme and mediator are immobilized in some capacity at the electrode surface. Thus the majority of the topies eovered here are focused on immobilized systems to reflect the dominant trend in bioeleetrocatalysis. 

E ) bioelectrocatalysis, respectively. In the process of shuttling charge between the redox-center and the electrode, the mediator is cycled between its oxidized and reduced states. The mediator should be stable in both the reduced and oxidized forms and any side reactions between the mediator redox states and the enzyme or the environment should be eliminated. To be effective in its role, the mediator must often compete with the enzyme s natural substrate (e.g. molecular oxygen in case of oxidases), effectively and efficiently diverting the flow of electrons to and from the electrode. An efficient mediator should provide rapid reaction with the redox enzyme, effectively oxidizing or reducing the enzyme-active center. A mediator should also exhibit reversible electrochemistry (a large rate constant (fcet) for the interfacial electron-transfer at the electrode surface). 

As already discussed, microorganisms extract a certain share of energy for their living from the maximum theoretically exploitable energetic difference. In case of anodic bioelectrocatalysis, the energy difference is situated between the microbial substrate, that is, the fuel, and the potential of the terminal electron transfer site. Furthermore, and like in conventional electrocatalytic systems [5], several energetic losses at the bioelectrocatalyst-electrode interface occur (Figure 8.2). 

Specific redox characteristics of a catalyst derived from CV scans are also used to confirm an enzyme s ability for bioelectrocatalysis by either direct electron transfer (DET) or mediated electron transfer (MET) to the electrode. DET and MET are two distinct mechanisms of bioelectrocatalysis. MET has the advantage of being compatible with almost all naturally occurring oxidoreductase enzymes and coenzymes, but it requires additional components (either smaU-molecule redox mediators or redox polymers) because the enzymes cannot efficiently transfer electrons to the electrode. These additional components make the system more complex and less stable [8]. The vast majority of oxidoreductase enzymes that require MET to an electrode are nicotinamide adenine dinucleotide (NAD" ) dependent. Two of the most commonly encountered NAD -dependent enzymes in BFC anodes are glucose dehydrogenase (GDH) and alcohol dehydrogenase (ADH). These enzymes have been thoroughly characterized in respect to half-cell electrochemistry and have been demonstrated for operation in BFC. More information about MET can be found in Chapter 9. 

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