Direct electron transfer, enzymes

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

Figure 17.12 Direct electrocatal3ftic oxidation of D-fnictose at a glassy carbon electrode painted with a paste of Ketjen black particles modified with D-fructose dehydrogenase from a Gluconobacter species. The enzyme incorporates an additional heme center allowing direct electron transfer from the electrode to the flavin active site. Cyclic voltammograms were recorded at a scan rate of 20 mV s and at 25 + 2 °C and pH 5.0. Reproduced by permission of the PCCP Owner Societies, from Kamitaka et al., 2007.

Kakehi N, Yamazaki T, Tsugawa W, Sode K. 2007. A novel wireless glucose sensor employing direct electron transfer principle based enzyme fuel cell. Biosens Bioelectron 22 2250-2255. 

Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based 02 biosensors with Cu, Zn-SOD confined onto cystein-modified Au electrode as an example. The presence of 02" in solution essentially increases both the cathodic and anodic peak currents of the SOD compared with its absence [150], Such a redox response was not observed at the bare Au or cysteine-modified Au electrodes in the presence of 02". The observed increase in the anodic and cathodic current response of the Cu, Zn-SOD/cysteine-modified Au electrode in the presence of 02 can be considered to result from the oxidation and reduction of 02, respectively, which are effectively mediated by the SOD confined on the electrode as shown in Scheme 3. Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modified Au electrode is essentially based on the inherent specificity of SOD for the dismutation of 02", i.e. SOD catalyzes both the reduction of 02 to H202 and the oxidation to 02 via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron transfer of SOD realized at the cysteine-modified Au electrode. Thus, this coupling between the electrode and enzyme reactions of SOD could facilitate the development of the third-generation biosensor for 02". ...

FIGURE 12.2 Schematic depicting a direct electron transfer process between an enzyme and the electrode, acting as an anode in this case. 

While direct electron transfer to laccases may help elucidate the mechanism of action of these enzymes it is unlikely that this process will supply sufficient power for a viable implantable biocatalytic fuel cell, because of difficulties associated with the correct orientation of the laccase and the two-dimensional nature of the biocatalytic layer on the surface. However, a recent attempt to immobilize laccase in a carbon dispersion, to provide electrodes with correctly oriented laccase for direct electron transfer, and a higher density of electrode material shows promise [53],... 

Direct electron transfer of proteins and enzymes on carbon nanotube electrodes... 

Direct electron transfer of other active enzymes... 

Biosensors based on direct electron transfer of enzymes... 

The electron-transfer rate between large redox protein and electrode surface is usually prohibitively slow, which is the major barricade of the electrochemical system. The way to achieve efficient electrical communication between redox protein and electrode has been among the most challenging objects in the field of bioelectrochemistry. In summary, two ways have been proposed. One is based on the so-called electrochemical mediators, both natural enzyme substrates and products, and artificial redox mediators, mostly dye molecules and conducted polymers. The other approach is based on the direct electron transfer of protein. With its inherited simplicity in either theoretical calculations or practical applications, the latter has received far greater interest despite its limited applications at the present stage. 

The enzyme-based biosensor has come through three steps (1) with oxygen for the media (2) with artificial intermediate for media and (3) without media and based for the direct electron transfer of redox proteins. The following is an example ... 

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