Oxidase electrochemical reactions

Bioelectrocatalysis involves the coupling of redox enzymes with electrochemical reactions [44]. Thus, oxidizing enzymes can be incorporated into redox systems applied in bioreactors, biosensors and biofuel cells. While biosensors and enzyme electrodes are not synthetic systems, they are, essentially, biocatalytic in nature (Scheme 3.5) and are therefore worthy of mention here. Oxidases are frequently used as the biological agent in biosensors, in combinations designed to detect specific target molecules. Enzyme electrodes are possibly one of the more common applications of oxidase biocatalysts. Enzymes such as glucose oxidase or cholesterol oxidase can be combined with a peroxidase such as horseradish peroxidase. 

Thus, the oxidase-substrate reaction yields oxidized substrate and inactive reduced enzyme the enzyme is returned to its active state by electron transfer, producing hydrogen peroxide. The peroxidase converts the peroxide, generating an electrochemical signal in the electrode. 

Figure 5.1 Electrochemical reaction of glucose with glucose oxidase immobilized on a working electrode. Copyright 2008 Abbott. Used with permission.

Association between enzymatic and electrochemical reactions has provided efficient tools not only for analytical but also for synthetic purposes. In the latter field, the possibilities of enzymatic electrocatalysis, e.g., the coupling of glucose oxidation (catalyzed either by glucose oxidase or glucose dehydrogenase) to the electrochemical regeneration of a co-substrate (benzoquinone or NAD+) have been demonstrated [171, 172]. An electroenzymatic reactor has also been developed ]172] to demonstrate how the enzyme-electrode association can be used to produce biochemicals on a laboratory scale. 

This enzyme-based biosensor uses glucose oxidase (GO) as a chemical recognition element, and an amperometric graphite foil electrode as the transducer. It differs from the first reported glucose biosensor discussed in the introduction to this chapter in that a mediator, 1,1 -dimethylferricinium, replaces molecular oxygen as the oxidant that regenerates active enzyme. The enzymatic reaction is given in Eq. 7.15, and the electrochemical reaction that provides the measured current is shown in Eq. 7.16. 

Figure 4-12 Design of amperometric enzyme electrode based on anodic detection of hydrogen peroxide generated from oxidase enzymatic reaction (e.g., glucose oxidase) (A), and expanded view of the sensing surface showing the different membranes and electrochemical process that yield the anodic current proportional to the substrate concentration in the sample (B). (From Meyerhoff N, New in vitro analytical approaches for clinical chemistry measurements in critical care. Clin Chem I990 36 I570.)...

Thus, any oxidase-substrate reaction that consumes O2 (ferricenium ion) in a well-defined process is amendable to the development of electrochemical sensors and assays for substrates and enzyme activity. 

Electrochemical analyzers based on the amperometric measurement of oxygen are used to measure the rate of oxidase enzyme reactions. For example, the substrate glucose is determined by measuring the rate of oxygen consumption in the presence of glucose oxidase. Results are obtained in less than a minute. Similarly,... 

Some special methods of enantioselective electrochemical reactions should be mentioned. D-Alanine was prepared with an ee close to 100% using the electrochemical reduction of pyruvic acid using an electrode on which amino acid oxidase and electron mediator were immobilized... 

The overall electrochemical oxidation pathway of 2,6-diaminopurine (I) and 2,6-diamino-8-purinol (ID which was proposed based on the results of these studies is shown in Figure 1. 2,6-Diamino-8-purinol was identified as an intermediate in the oxidation of 2,6-diaminopurine. 2,6-Diamino-8-purinol also forms in the xanthine oxidase catalyzed reaction of the drug 3. There is clearly a chemical similarity between the electrochemical and xanthine oxidase catalyzed reaction. 

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