Direct electron transfer (DET) mechanism of FAD dependent dehydrogenase complexes ∼from the elucidation of intra- and inter-molecular electron transfer pathway to the construction of engineered DET enzyme complexes∼

Summary Direct electron transfer (DET) reaction is intriguing for application to biodevices such as biosensors and biofuel-cells. However, for improvement of DET for application to electrochemical devices, the understanding of its mechanism is essential. This review focuses on the recent progress on studies of biomolecular aspects of DET mechanism of flavin adenine dinucleotide (FAD) dependent dehydrogenase complex, especially, glucose dehydrogenase (GDH) and fructose dehydrogenase (FDH). These enzymes are hetero-trimeric membrane bound dehydrogenases, comprised of three distinctive subunits: a catalytic subunit, a small subunit, and an three haem c containing electron transfer subunit. The catalytic subunit was recently elucidated to harbor 3Fe-4S cluster as the FAD primary electron acceptor, and presumably to transfer electron from catalytic subunit to electron transfer subunit. The small subunit is a hitch-hiker protein, which is essential for proper folding and secretion of catalytic subunit into the periplasmic space. The electron transfer subunit has three haem binding motif, which make these enzyme complexes possible for DET. First, application studies of these enzymes to DET-based electrochemical devices are overviewed to address the challenges faced. Next, the recent progress in elucidation of inter- and intra-molecular electron transfer pathway of these enzymes were summarized. Finally, the future prospect of the application of FAD dependent dehydrogenase complexes was addressed.