Iron Reductase for Magnetite Synthesis in the Magnetotactic Bacterium Magnetospirillum magnetotacticum

Ferric iron reductase was purified from magnetotactic bacterium Magnetospirillum (formerly Aquaspirillum) magnetotacticum (ATCC 31632) to an electrophoretically homogeneous state. The enzyme was loosely bound on the cytoplasmic face of the cytoplasmic membrane and was found more frequently in magnetic cells than in nonmagnetic cells. The molecular mass of the purified enzyme was calculated upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be about 36 kDa, almost the same as that calibrated by gel filtration analysis. The enzyme required NADH and flavin mononucleotide (FMN) as optimal electron donor and cofactor, respectively, and the activity was strongly inhibited by Zn(2+) acting as a partial mixed-type inhibitor. The K(m) values for NADH and FMN were 4.3 and 0.035 μM, respectively, and the K(i) values for Zn(2+) were 19.2 and 23.9 μM for NADH and FMN, respectively. When the bacterium was grown in the presence of ZnSO(4), the magnetosome number in the cells and the ferric iron reductase activity declined in parallel with an increase in the ZnSO(4) concentration of the medium, suggesting that the ferric iron reductase purified in the present study may participate in magnetite synthesis.