We have recently developed polypeptide modified and aminosilane modified electrodes for rapid direct electron transfer of ferredoxins from various origins. In the present study, by using mutated ferredoxin molecules, amino acid residues of ferredoxin have clearly shown electrochemically, for the first time, to have their own distinguished roles for biological functions, such as controlling the redox potential and the complex formation with FNR.Maize ferredoxins from photosynthetic (FdI) and non-photosynthetic (FdIII) organs and those of which amino acid residues had been modified by site-directed mutagenesis were used. All mutated ferredoxin prepared showed very similar circular dichroism spectra to that of wild type (WT), and showed well-defined cyclic voltammograms at the modified electrodes.For WT maize ferredoxin, the formal redox potential (E O ), diffusion coefficient (D), and the heterogeneous electron transfer rate constant (k O ) were obtained to be - 545 (± 5) mV (vs. Ag/AgCl), 1.0 (± 0.1) 10 - 6 cm - 2 s - 1 , and 3 (± 0.5) 10 - 3 cm s - 1 , respectively, at the modified In 2 O 3 electrode in a 50 mM tris-HCl buffer solution containing 0.33 M NaCl (pH=7.5) at 10 °C. Mutated ferredoxins, of which amino acid residue (even at evolutionary invariant residue such as Ser-39 and Ser-46 near the cluster) was changed to alanine, did not show significant change in redox potential. On the other hand, when Ser-46 for FdIII or Ser-45 for FdI was modified to glycine (Gly), (S46G-FdIII or S45G-FdI), the redox potential showed a large positive-shift by ca. 180 mV compared to those of WT. Computer calculation for the structure of these mutated ferredoxins showed that the modification of Ser to Gly introduce rather large distortion of the iron sulfur cluster, which may cause a large positive shift in redox potential.By using ferredoxin as an electron transfer mediator, in the presence of ferredoxin-NADP + -reductase (FNR, E.C. 1.18.1.2), NADP + was effectively reduced to give NADPH through the reduction of ferredoxin at the electrode. However, when the S46G-FdIII (or S45G-FdI) was used no catalytic current due to the enzyme reaction was observed. This is reasonably explained in terms of the diagram of the redox potentials of the components of ferredoxin/FNR/NADP + system.The redox potentials of D66K/D67K and D66N/D67N, where negatively charged aspartic acid (D) was converted to positively charged lysine (K) or neutral asparagine (N), did not change at all, but again the no catalytic current for the reduction of NADP + was observed. Gel chromatography showed that D66K/D67K and D66N/D67N did not bind significantly with FNR. These results suggest that D66 and D67 are the binding sites with FNR to form the ferredoxin-FNR complex.