Mediated Electron Transfer MET

Thus, electrons can be directly transferred (DET) to the anode via the cell membrane (a) or via so-called nanowires (b) [vi.vii]. Alternatively, mediated electron transfer (MET) can take place via bacterial electron-shuttling compounds [viii] or reduced secondary metabolites like, e.g., hydrogen, formate, or ethanol [v,ix]. 

The electrical contacting of redox enzymes that defy direct electrical communication with electrodes can be established by mediated electron transfer using synthetic or biologically active charge carriers. Mediated electron transfer (MET) can be effected by a diffusional mechanism (Figure 2), where the electron relay is either oxidized or reduced at the electrode surface. Diffusional penetration of the oxidized or... 

Figure 5.3 Schematics of (a) direct electron transfer (DET) and (b) mediated electron transfer (MET) between an enzyme active site and an electrode to oxidize a substrate.

As an alternative to DET, small, artificial substrate/co-substrate electroactive molecules (mediators) can be used to shuttle electrons between the enzyme and the electrode (Figure 5.3b). This involves a process in which the enzyme takes part in the first redox reaction with the substrate and is re-oxidized or reduced by the mediator which in turn is regenerated, through a combination of physical diffusion and self-exchange, at the electrode surface. The mediator circulates continuously between the enzyme and the electrode, cycled between its oxidized and reduced forms, producing current. This process is known as mediated electron transfer (MET). 

In contrast to mediated electron transfer (MET), the ability for electrons to transfer between an enzyme s cofactor and an electrode (acting as an... 

Heme-containing peroxidases thus belong to the rather restricted group of the redox enzymes for which DET has been shown [11,12]. If, however, the electronic commnnication between the cofactor of the peroxidase and the electrode is slow, small redox molecnles can be exploited as mediators to carry the electrons between the enzyme and the electrode. This mediated electron transfer (MET) proceeds in accordance with the peroxidase catalytic cycle (reactions 1-3), where the role of the mediator can be carried by the reducing substrate AH. The differ-... 

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. 

Typically, the addition of redox mediators is used as a way to shuttle electrons from the active site to electrode surfaces [13]. This mechanism is referred to as mediated electron transfer (MET). Redox mediators are usually small, mobile molecules containing redox moieties that assist in transferring electrons between redox enzymes and electrodes by diffusing in and out of the enzyme active site. Despite helping overcome the tunnehng distance between the active site and electrode surface, the use of mediators also has certain disadvantages these include high costs, potential toxicity, and instability of the systems, since mediators can diffuse over time. 

When discussing the transfer of electrons from the enzyme active site to the electrode surface, thus generating catalytic current, there are two types of electron transfer mechanisms mediated electron transfer (MET) and direct electron transfer (DET) [13]. Most oxidoieductase enzymes that have been commonly used in BFC development are unable to promote the transfer of electrons themselves because of the long electron transfer distance between the enzyme active site and the electrode surface as a result, DET is slow. In such a case, a redox-active compound is incorporated to allow for MET. In this approach, a small molecule or redox-active polymer participates directly in the catalytic reaction by reacting with the enzyme or its cofactor to become oxidized or reduced and diffuses to the electrode surface, where rapid electron transfer takes place [14]. Frequently, this redox molecule is a diffusible coenzyme or cofactor for the enzyme. Characteristic requirements for mediator species include stability and selectivity of both the oxidized and reduced forms of the species. The redox chemistry for the chosen mediator is to be reversible and with minimal overpotential [15]. 

Electrons can be transferred between an enzyme and electrode directly, termed direct electron transfer (DET), or through a mediator, termed mediated electron transfer (MET). Enzymes capable of DET contain a reactive metal center or other redox center, such as flavin adenine dinucleotide (FAD), fixed in the active site. When the active site is located within a short distance of the electrode (less than —20 A), electrons are transferred between them through electron tunneling [16]. Only a few classes of enzymes have demonstrated DET, notably cytochrome c, some hydrogen-ases, peroxidases, oxidases, and laccases [17,18]. In BFCs, laccases and oxidases are the most commonly found enzymes capable of DET. 

Thus, to realize the full potential of BECs, investigators are now eonsidering mediated electron transfer (MET) to inerease the eleetronie assoeiation between the total enzyme on the eleetrode and, henee, the power density. By using a redox mediator to shuttle eleetrons to and from eleetrode to enzyme, it is possible to exploit the entire three-dimensional eleetrode area. These mediators, whether diffusion-based or wired [30], therefore eapitafize on a catalyst layer that is much thicker (100 pm) than the monolayers in DET. Although MET has been shown to produce power densities several orders of magnitude higher than DET, the presence of an added electron transfer step, however, introduces another avenue for possible ineffieieney. 

For an EFC to operate, the enzyme needs to be electrically connected to the electrode surface. The ET between enzymes and electrodes has been reviewed in detail, for example, by Habermuller et al. in 2000 and Katz et al. in 2002 [31,32]. Electronic connection can be achieved via direct electron transfer (DET) or mediated electron transfer (MET) mechanisms. 

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