(Invited) Current Understanding and Future Aspects of Direct Electron Transfer Mechanism of Fad Dependent Glucose Dehydrogenase Complex

Direct electron transfer (DET) between enzyme molecule and electrode provides the ideal principle for the enzyme based sensing system. In DET principle the electrons are transferred from enzyme active site to electrode directly in the absence of any additional electron acceptors, or mediators. This intriguing reaction is useful for various biodevices such as biosensors and biofuel-cells, especially by focusing anodic reactions, such as the oxidation of glucose, fructose, and ethanol. The absence of redox mediators in the system enables the operation of the sensor in redox potential closer to the co-factor in the enzyme, reducing the effect of interfering reactions by electroactive compounds in the sample. However, only a limited number of redox enzymes are capable of DET-reaction with electrodes. Among these, several dehydrogenases harboring a redox center in the catalytic subunit/domain and haem which is not involved in catalysis but act as a built-in mediator for electron transfer between enzyme and electrode are reported. One of the representative groups of DET enzymes is the oligomeric dehydrogenases composed of catalytic subunit harboring FAD as a co-factor, and an electron transfer subunit with haem c, the FAD dependent dehydrogenase complexes. Our research group have been engaged in the isolation, characterization, application and engineering of one of the representative FAD-dependent dehydrogenase complexes. FAD glucose dehydrogenase complex (FADGDH). FADGDH is comprised the three distinct subunits: the catalytic subunit (α subunit) that has an FAD cofactor in its redox center, shows catalytic activity, and oxidizes the first hydroxyl group of glucose; the small subunit (γ subunit), a hitch-hiker protein of the bacterial TAT secretion system, which is necessary for the proper folding and secretion of the α subunit; and the three heme-cytochrome c subunit (β subunit) that is in responsible for the transfer of electrons between the active-site cofactor and external electron acceptors. Thanks to the presence of β subunit, FADGDH is capable to transfer electrons directly to an electrode, making FADGDH as the ideal molecule for the 3rd Gen glucose sensors. This paper focuses on biomolecular aspects on DET mechanism of FAD dependent dehydrogenase complexes, considering current increasing number of applicational researches, and recent remarkable progress in understanding their DET mechanism, especially about the recent progress in understanding inter- and intra-molecular electron transfer of FADGDH are introduced. These studies included protein engineering approaches of these molecules, thereby led to construct engineered GDH. Finally, the future prospect of the FAD dependent dehydrogenase complexes will be addressed.