Electrocatalysis and redox enzymes

As we demonstrate in this chapter, enzymes can be extremely active electrocatalysts at ambient temperatures and mild pH, and have significantly higher reaction selectivity than precious metals. The main disadvantage in applying redox enzymes for electrocatalysis arises from their large size, which means that the catalytic active site density is low. Enzymes also have a relatively short hfetime (usually not more than a few months), making them more suited to disposable applications. 

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.

An important application of CyD complexes is in the field of mediated electrocatalysis and bioelectrocatalysis. Detection of a target analyte using a biosensor based on a redox enzyme is well recognized as a more convenient solution than one based on the electrochemistry of reaction products. An ideal mediator for electrocatalysis should be soluble in water or easily anchored on an electrode surface. CyDs have often been employed in solubilizing hydrophobic molecules used either as mediators in catalysis or occurring as the products of catalytic reactions. In the latter case, their role was to avoid fouling of the electrode surface. 

The structure and physicochemical properties of the enzymes which have been used to date to promote electrochemical reactions are briefly outlined. Methods of their immobilization are described. The status of research on redox transformations of proteins and enzymes at the electrode-electrolyte interface is discussed. Current concepts on the ways of conjugation of enzymatic and electrochemical reactions are summarized. Examples of bioelectrocatalysis in some electrochemical reactions are described. Electrocatalysis by enzymes under conditions of direct mediatorless transport of electrons between the electrode and the enzyme active center is considered in detail. Lastly, an analysis of the status of work pertaining to the field of sensors with enzymatic electrodes and to biofuel cells is provided. 

The thing to be noted here is that the ° values of the 02/ 02" and 02" H202 redox couples are -0.35 and 0.68 V vs Ag/AgCl at pH 7.4 and thus the SODs, for example, Cu, Zn-SOD (Cu (I/II)) with ° = 65mV can mediate both the oxidation of 02 to 02 and the reduction of 02" to H202. Such a bi-directional electromediation (electrocatalysis) by the SOD/SAM electrode is essentially based on the inherent specificity of the SOD enzyme which catalyzes the dismutation of 02 to 02 and H202 via a redox cycle of their metal complex moiety (Scheme 3). 

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". ...

Fe 2S], a [4Fe-4S] and a [3Fe-4S] center. The enzyme catalyzes the reversible redox conversion of succinate to fumarate. Voltammetry of the enzyme on PGE electrodes in the presence of fumarate shows a catalytic wave for the reduction of fumarate to succinate (much more current than could be accounted for by the stoichiometric reduction of the protein active sites). Typical catalytic waves have a sigmoidal shape at a rotating disk electrode, but in the case of succinate dehydrogenase the catalytic wave shows a definite peak. This window of optimal potential for electrocatalysis seems to be a consequence of having multiple redox sites within the enzyme. Similar results were obtained with DMSO reductase, which contains a Mo-bis(pterin) active site and four [4Fe 4S] centers. 

Such a bidirectional electromediation (electrocatalysis) by SOD/Cys/Au is essentially based on the inherent specificity of SOD for the dismutation of O2 i.e. SOD catalyzes both the reduction of 02 to H2O2 and the oxidation to O2 via a redox cycle of its Cu complex moiety as well as the direct electron transfer of SOD we obtained at Cys/Au. Thus, this coupling between the electrode and enzyme... 

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