Heme/Copper Assembly Mediated Nitrite and Nitric Oxide Interconversion

The heme_a3/Cu_B active site of cytochrome c oxidase is responsible for cellular nitrite reduction to nitric oxide; the same center can return NO to the nitrite pool via oxidative chemistry. Here, we show that a partially reduced heme/Cu assembly reduces NO₂^– ion, producing nitric oxide. The heme serves as the reductant, but the Cu^II ion is also required. In turn, a μ-oxo heme-Fe^III–O–Cu^II complex facilitates NO oxidation to nitrite; the final products are the reduced heme and Cu^II–nitrito complexes.