A heme-C-containing enzyme complex that exhibits nitrate and nitrite reductase activity from the dissimilatory iron-reducing bacterium Geobacter metallireducens

Nitrate reduction in the dissimilatory iron-reducing bacterium Geobacter me tallireducens was investigated, Nitrate reductase and nitrite reductase act ivities in nitrate-grown cells were detected only in the membrane fraction. The apparent K-m values for nitrate and nitrite were determined to be 32 a nd 10 mu M, respectively. Growth on nitrate was not inhibited by either tun gstate or molybdate at concentrations of 1 mM or less, but was inhibited by both at 10 and 20 mM. Nitrate and nitrite reductase activity in the membra ne fraction was not, however, affected by dialysis with 20 mM tungstate. An enzyme complex that exhibited both nitrate and nitrite reductase activity was solubilized from membrane fractions with CHAPS and was partially purifi ed by preparative gel electrophoresis. It was found to be composed of four different polypeptides with molecular masses of 62, 52, 36, and 16 kDa, The 62-kDa polypeptide [a low-midpoint potential (-207 mV), multiheme cytochro me c] exhibited ni trite reductase activity under denaturing conditions. No molybdenum was detected in the complex by plasma-emission mass spectrometr y.