Applications of Quinoprotein GDHs as DGlucose Sensors

As our understanding of the mechanisms of catalysis and electron transfer of quinoproteins progresses, the idea of the application of these enzymes as biosensors becomes more attractive, and thus electrochemical studies aimed at the design of specific quinoprotein-based electrodes have increased. s-GDH is the first quinoprotein applied to such a biosensor, and a D-glucose sensor with s-GDH in a single-use electrochemical test strip containing ferricyanide as a mediator is already on the market.

Prior to the appearance of this s-GDH sensor, a fungal flavoprotein D-glucose oxidase and NAD-dependent GDH had been used as the enzyme for D-glucose biosensors. However, biosensors based on D-glucose oxidase are sensitive to oxygen fluctuations in blood samples, while those based on NAD GDH have poor stability due to the loss of the cofactor NAD. These problems are overcome by using PQQ-dependent GDH, which is oxygen insensitive and has a tightly bound PQQ. In addition, since the electron transfer rate of s-GDH is very much higher than that of D-glucose oxidase, the biosensor based on s-GDH has produced more than twice the current density of similar D-glucose oxidase-based electrodes when s-GDH is immobilized in the presence of an insoluble ferrocene [47].

Thus, several characteristics of the s-GDH-based D-glucose sensor, especially the high catalytic activity and oxygen insensitivity, make it suitable for in vivo blood D-glucose monitoring in the management of diabetes.

Although it has still not been applied to commercial sensors, m-GDH has been examined for its electrochemical reaction on electrodes. Attempts have been made to attach m-GDH from Erwinia sp. to an electrode surface using several different materials such as carbon paste, where no direct electron transfer from the enzyme is observed unless some insoluble quinone mediator is included in the paste [48]. Anodic current has been detected with Erwinia m-GDH depending on D-glucose concentrations, and is greatly increased in the presence of soluble electron mediators such as PMS. In order to improve the specificity and the response range toward D-glucose, the same type of m-GDH-based electrode has been prepared by immobilizing m-GDH from an E. coli mutant, in which His775 is substituted with Asp, on the carbon paste electrode [49]. The mutated m-GDH-entrapped electrode exhibits an expanded response range for D-glucose (370 mmol L-1) and a narrower substrate specificity.

The substrate specificity of m-GDH from G. suboxydans IFO 12528 is attractive for D-glucose biosensors. The enzyme is highly specific to D-glucose and maltose is oxidized by only 5% of that for D-glucose. Other aldohexoses and aldopentoses have no effects on m-GDH from acetic acid bacteria [40].

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