Direct Oxidation of Glucose Oxidase

Oxygen is the natural electron acceptor from the reduced form of GO, and for any other oxidase. The reduction of oxygen is not without problems. It leads to the degradation of the enzyme by the hydrogen peroxide and the dependency on available oxygen may create interferences in some applications, as has been discussed above. For these reasons, other electron acceptors have been investigated. The idea is to find some way by which the (GO)red could be reoxidized anaerobically, with the ultimate sink for the electrons being the electrode itself (Heller, 1999). 

the following reactions take place in the enzyme layer, 

Various fast redox couples such as ferrocene, ferro/ferri cyanide, and ruthenium hexamine have been used as mobile mediators. In order to be electron acceptors their standard potentials must be more positive than that of FADH2/FAD redox couple (E° = 0.05 V, at pH = 7). The requirement of mobility is, however, in conflict with the lifetime of the sensor. Because the mediator is of comparable size to the substrate, it cannot be confined to the electrode proper by, for example, a dialysis membrane. In fact, the only way this type of sensor can operate is in a sample containing a sufficient concentration of the mediator (Cass et al., 1984). Obviously, this requirement makes such sensors suitable only for in vitro applications. 

In principle, glucose oxidase could be oxidized directly at the electrode, which would be the ultimate electron acceptor. However, direct electron transfer between redox enzymes and electrodes is not possible because the FADH2/FAD redox centers are buried inside insulating protein chains (Heller, 1990). If it were not the case, various membrane redox enzymes with different standard potentials would equalize their potentials on contact, thus effectively shorting out the biological redox chains. The electron transfer rate is strongly dependent on the distance x between the electron donor and the electron acceptor. 

Substrate Enzyme Sensor Stability time Range (M) 

---