ENZYMATIC IRON OXIDATION AND REDUCTION IN MAGNETITE SYNTHESIZING MAGNETOSPIRILLUM-MAGNETOTACTICUM

We investigated the enzymatic reduction and oxidation of iron in M. ma gnetotacticum 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 4.3 mu M and 119 mu M, respectively. The enzymatic activity was strongly inhibited by Zn2+. On the other hand, the dissimilatory nitrite reductase of M. magnetota cticum showed high Fe(II)-nitrite oxidoreductase activity. The enzyme was located in periplasmic space and could be isolated from the magnet ite-containing cells but not from the non-magnetic cells. The enzyme c omposed of two identical subunits with a molecular mass of 54 kDa, eac h containing a c - and d(1)-type heme. The activity was about 0.57 mol ferrous iron/mol of enzyme/sec at pH 8.0. The oxidized ferrous iron/r educed nitrite ratio was about 1.4, indicating that nitrite was reduce d to NO. Furthermore, M. magnetotacticum synthesized much more magneti tes when the bacterium grew using denitrification, the dissimilatory r eduction of nitrate to dinitrogen via nitrite, nitric oxide and nitrou s oxide. These results propose that the dissimilatory nitrite reductas e of M. magnetotactium may participate as Fe(II) oxidizing enzyme in m agnetite synthesis under microaerobic conditions.