Direct Electron Transfer from the FAD Cofactor of Cellobiose Dehydrogenase to Electrodes

Cellobiose dehydrogenase (CDH) is employed in the construction of biosensors and biofuel cells. The flavin adenine dinucleotide (FAD) containing, catalytic dehydrogenase domain (DH) of the enzyme oxidizes carbohydrates, while the cytochrome b containing domain (CYT) acts as an electron mediator and shuttles the electrons to the electrode. Here we demonstrate for the first time in an unequivocal manner direct electron transfer (DET) between the FAD and electrodes by showing clear nonturnover voltammetric waves in the absence and turnover waves in the presence of substrate by using cyclic voltammetry and square wave voltammetry. Results were obtained by entrapping CDH under a dialysis membrane on alkanethiol-modified, polycrystalline gold electrodes. DET from the FAD cofactor occurs at potentials 130 mV more negative than those previously reported and established DET from the electron-mediating CYT domain. However, direct electrochemistry was only observed for two types of basidiomycete class I CDHs from Trametes villosa and Phanerochaete sordida at pH values below 5 and not for ascomycete class II CDHs investigated under the same experimental conditions. The present findings are of high interest for the development of biosensors and biofuel cells featuring a lower substrate oxidation potential, which decreases the occurrence of interfering reactions and increases the cell voltage in biofuel cells. Furthermore, these findings may also be transferrable to structurally related enzymes such as glucose oxidase and glucose dehydrogenase.