The electron transfer pathway in direct electrochemical communication of fructose dehydrogenase with electrodes

•Subunit I/III subcomplex (ΔcFDH) without heme C subunit is constructed.•ΔcFDH shows a high activity to oxidize d-fructose, but loses DET activity.•The formal potentials of the three heme C moieties in FDH have been determined.•FDH has electron acceptor specificity.•The heme C with the highest E°′ does not seem to be involved in DET reaction.

A heterotrimeric membrane-bound fructose dehydrogenase (FDH) complex from Gluconobacter japonicus NBRC3260 catalyzes oxidation of d-fructose into 2-keto-d-fructose and is one of typical enzymes allowing a direct electron transfer (DET)-type bioelectrocatalysis. Subunits I and II have a covalently bound flavin adenine dinucleotide and three heme C moieties, respectively. We have constructed subunit I/III subcomplex (ΔcFDH) lacking of the heme C subunit. ΔcFDH catalyzes the oxidation of d-fructose with several artificial electron acceptors, but loses the DET ability. The formal potentials (E°′) of the three heme C moieties of FDH have been determined to be − 10 ± 4, 60 ± 8 and 150 ± 4 mV (vs. Ag|AgCl|sat. KCl) at pH 5.0, while the onset potential of FDH-catalyzed DET-type bioelectrocatalytic wave is − 100 mV. Judging from these results, we conclude that FDH communicates electrochemically with electrodes via the heme C, and discuss the pathway of the electron transfer in the catalytic process.

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