Mechanism of the Six-Electron Reduction of Nitrite to Ammonia by Cytochrome<i>c</i>Nitrite Reductase

Cytochrome c nitrite reductase catalyzes the six-electron reduction of nitrite to ammonia without the release of potential reaction intermediates, such as NO or hydroxylamine. On the basis of the crystallographic observation of reaction intermediates and of density functional calculations, we present a working hypothesis for the reaction mechanism of this multiheme enzyme which carries a novel lysine-coordinated heme group (Fe-Lys). It is proposed that nitrite reduction starts with a heterolytic cleavage of the N−O bond which is facilitated by a pronounced back-bonding interaction of nitrite coordinated through nitrogen to the reduced (Fe(II)) but not the oxidized (Fe(III)) active site iron. This step leads to the formation of an {FeNO}6 species and a water molecule and is further facilitated by a hydrogen bonding network that induces an electronic asymmetry in the nitrite molecule that weakens one N−O bond and strengthens the other. Subsequently, two rapid one-electron reductions lead to an {FeNO}8 form and, by protonation, to an Fe(II)−HNO adduct. Hereafter, hydroxylamine will be formed by a consecutive two-electron two-proton step which is dehydrated in the final two-electron reduction step to give ammonia and an additional water molecule. A single electron reduction of the active site closes the catalytic cycle.