Glucose oxidase reduced form

Figure 11.39 summarizes the reactions taking place in this amperometric sensor. FAD is the oxidized form of flavin adenine nucleotide (the active site of the enzyme glucose oxidase), and FAD1T2 is the active site s reduced form. Note that O2 serves as a mediator, carrying electrons to the electrode. Other mediators, such as Fe(CN)6 , can be used in place of O2. 

Electron transfer of the glucose oxidase/polypyrrole on the electrode surface was confirmed by differential pulse voltammetiy and cyclic voltammetry. The glucose oxidase clearly exhibited both reductive and oxidative current peaks in the absence of dissolved oxygen in these voltammograms. These results indicate that electron transfer takes place from the electrode to the oxidized form of glucose oxidase and the reduced form is oxidized by electron transfer to the electrode through polypyrrole. It may be concluded that polypyrrole works as a molecular wire between the adsorbed glucose oxidase and the platinum electrode. 

Examples of this class of enzymes are glucose oxidase and D-amino acid oxidase The classification of the flavoproteins used here is that originally suggested which has been modified recently . In the author s own view the original classification has the advantage of being simple and yet quite useful whereas the new classification does not add to simplify and classify the rather complex picture of flavoprotein catalysis. Nevertheless, in flavoprotein oxidases, the 1,5-dihydroflavin is very reactive towards Oj. On the other hand, the two-electron reduced form of flavoprotein oxidases reacts slowly with pure one-electron acceptors, e.g. ferricyanide. That the two-electron transition is biologically favoured in these enzymes explains why they can react easily with sulfite... 

There are several species in this reaction that can be used for electrochemical sensing. Detection of proton released from the gluconic acid was used in the poten-tiometric glucose electrode (Section 6.2.1). The amperometric sensor can be based on oxidation of hydrogen peroxide, on reduction of oxygen, or on the oxidation of the reduced form of glucose oxidase itself. 

Figure 19. Feedback and GC mode SECM measurements of enzyme kinetics, (a) Feedback mode locally electrogenerated mediator is reduced by glucose ( ) at the substrate surface the reaction is catalyzed by glucose oxidase, (b) the GC mode the reduced form of the mediator is continuously produced by the enzyme-catalyzed reaction at the substrate and collected at the tip. The tip probes the concentration profile of reduced mediator species.

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