Discussion on Direct Electron Transfer-Type Bioelectrocatalysis of Downsized and Axial-Ligand Exchanged Variants of D-Fructose Dehydrogenase

D-Fructose dehydrogenase (FDH) gives a clear direct electron transfer (DET)-type bioelectrocatalytic wave even at planar gold (Au) electrodes. The recombinant (native) FDH (r_FDH) has three hemes c in subunit II (1c, 2c, and 3c from N-terminus). With a view to downsize the enzyme and shorten the distance between an electrode-active site and an electrode, we constructed a variant that lacked 143 amino acid residues involving the heme 1c moiety (Delta 1cFDH) and a variant that lacked 199 amino acid residues involving the heme 1c and 2c moieties (Delta 1c2cFDH). In order to shift the redox potential of heme 2c of Delta 1cFDH to the negative direction, the M450 residue as the axial 6th ligand of heme 2c was also replaced with glutamine (M450Q Delta 1cFDH). The DET-type catalytic properties of r_FDH and the three variants at planar Au electrodes were compared with each other, and the steady-state waves were analyzed on a random orientation model. The orientation of the enzymes on the electrode was also discussed. In addition, in order to examine the electron transfer pathway in the DET-type reaction of Delta 1c2cFDH, ESR measurements and inhibition of DET-type reaction by cyanide ion were performed. (C) The Author(s) 2020. Published by ECSJ.