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Cytochrome cd1 nitrite reductase (cd1) from Paracoccus pantotrophus is a respiratory enzyme capable of using nitrite, hydroxylamine and oxygen as electron accepting substrates. Structural studies have shown that when the enzyme is reduced there is a change in the axial ligation of both hemes, which has been proposed to form part of the catalytic cycle. Here we report the use of a physiological electron donor, pseudoazurin, to investigate the relationship between heme ligation and catalysis. A combination of visible absorption and electron paramagnetic resonance spectroscopies reveals the formation of a catalytically competent state of oxidized cd1 with 'switched' axial ligands immediately after complete reoxidation of reduced cd1 with hydroxylamine. This activated conformer returns over 20 min at 25 degrees C to the state previously observed for oxidized 'as isolated' cd1, which is catalytically inactive towards the same substrates.

Original publication




Journal article


Nat Struct Biol

Publication Date





885 - 888


Catalysis, Cytochromes, Heme, Models, Molecular, Nitrite Reductases, Oxidation-Reduction, Paracoccus, Protein Conformation