Direct heterogeneous electron transfer reactions and the molecular orientation Of D-fructose dehydrogenase (FDH, from Gluconobacter sp.) adsorbed onto electrodes were investigated. Catalytic oxidation currents based on the direct electron transfer reactions of FDH adsorbed onto basal-plane, highly oriented pyrolytic graphite (basal-plane HOPG), plastic formed carbon plate and tin-doped indium oxide electrodes were observed from a potential around -0.1 V (vs. Ag/AgCl/saturated KCl) in phosphate solution (pH 5.0) in the presence of fructose as a substrate for FDH. The catalytic current for the FDH adsorbed onto basal-plane HOPG electrodes indicated a pH dependence. The catalytic oxidation currents were only observed in acidic solutions (pH <= 6), and were not observed in neutral and alkaline solutions. The reason why the catalytic current was not obtained in neutral and alkaline solutions was because the FDH complex decomposed in neutral and alkaline solutions based on the AFM measurements. The differential pulse voltammetric investigations for FDH adsorbed onto basal-plane HOPG electrode together with the AFM and electrophoresis results indicated that the flavin-containing subunit in FDH accepts electrons from D-fructose, and transfers these electrons to the heme c-containing subunit, and then the direct electron transfer reaction of FDH occurred at the heme c site. (C) 2007 Published by Elsevier B.V.