Electrochemical reactions, promoted enzymes

In amperometry, the current produced by the oxidation or reduction of an electroactive analyte species at an electrode surface is monitored under controlled potential conditions. The magnitude of the current is then related to the quantity of analyte present. However, as both antibody and antigen are not intrinsically electroactive, a suitable label must be introduced to the immunocomplex to promote an electrochemical reaction at the immunosensors. In this respect, enzyme labels including the... 

Construction principles and the mechanism for biosensors derived from enzymes. Combined enzymatic and electrochemical reactions proceeding on electrodes from various materials in electrolyte solutions promote development of many biosensor types for detection of glucose, amino acids, lactose, urea, pyruvate and other metabolites. Biosensors are successfully applied to environmental contamination control, medical diagnostics and the food industry. 

Fig. 17.9. Schemes showing the application of enzymes to promote electrochemical reactions (from Ref. 36 with permission).

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. 

Much progress in the study of electrochemical properties of protein macromolecules has been achieved recently using another method, namely, the study of redox transformations of proteins, the carriers of electrons and enzymes, and their active groups at the electrode-electrolyte interface. This approach is intimately related to the use of enzymes to promote electrochemical reactions and pursues the purpose of elucidation of the mechanism of electron transport and the structural features of enzymes. 

Adsorption film formed by cellulose triacetate (TAC) can also be used for immobilization of a biological system at the surface of the glassy carbon or gold electrodes [222-224]. For instance, it is possible to dissolve microperoxidase (MPO) in this film to simulate a membrane-entrapped microperoxidase. A direct, reversible electron transfer between this enzyme and the electrode is observed in nonmodified electrodes without the TAC film [224] and also when MPO is dissolved in the TAC film [85]. Fast electrochemical reaction of cytochrome c and azurin at the electrodes can be observed if the electrode covered by the TAC film containing MPO is immersed in the solution of these proteins. Thus, the immobilized film containing microperoxidase behaves like a solid state promoter of protein electrochemistry [223, 224]. 

Layers of conducting polymers promote electron transfer. This is useful if the critical receptor process is a redox reaction. Such layers are utilized preferably in electrochemical biosensors with enzymes which catalyse biochemical redox reactions. Conducting polymers possess the advantages of classical polymers (solvent function and compatibility with organic substances) as well as of semiconductors or metals (conductance). Examples are discussed in the chapters dealing with biosensors. 

Enzymes are specific proteins performing the role of catalyst in living organisms. All enzymes can be subdivided into six classes. Until now the enzymes of two classes, viz. oxidoreductases catalyzing oxidation-reduction reactions and hydrolases promoting hydrolysis reactions, have been used in electrochemical systems. 

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