Enzymatic Iron Oxidation and Reduction in Magnetite Synthesizing Magnetospirillum Magnetotacticum

Abstract
We investigated the enzymatic reduction and oxidation of iron in M.magnetotacticum which synthesizes magnetite at room temperature. NADH-Fe(III) reductase with the molecular mass of 36kDa was purified from the bacterium. The enzyme was located in cytoplasm and utilized NADH and NADPH in the presence of FMN as reductant and showed maximum activity at pH 7.0. The Km for NADH and NADPH were about 4.3µM and 119µM, respectively. The enzymatic activity was strongly inhibited by Zn²⁺. On the other hand, the dissimilatory nitrite reductase of M.magnetotacticum showed high Fe(II)-nitrite oxidoreductase activity. The enzyme was located in periplasmic space and could be isolated from the magnetite-containing cells but not from the non-magnetic cells. The enzyme composed of two identical subunits with a molecular mass of 54 kDa, each containing a c - and d₁-type heme. The activity was about 0.57 mol ferrous iron/mol of enzyme/sec at pH 8.0. The oxidized ferrous iron/reduced nitrite ratio was about 1.4, indicating that nitrite was reduced to NO. Furthemore, M.magnetotacticum synthesized much more magnetites when the bacterium grew using denitrification, the dissimilatory reduction of nitrate to dinitrogen via nitrite, nitric oxide and nitrous oxide. These results propose that the dissimilatory nitrite reductase of M.magnetotacticum may participate as Fe(II) oxidizing enzyme in magnetite synthesis under microaerobic conditions.