A switch in heme axial ligation prepares Paracoccus pantotrophus cytochrome cd(1) for catalysis

摘要

Cytochrome cd(1) nitrite reductase (cd(1)) 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 ed, with 'switched' axial ligands immediately after complete reoxidation of reduced ed, with hydroxylamine. This activated conformer returns over 20 min at 25 degrees C to the state previously observed for oxidized 'as isolated' cd(1), which is catalytically inactive towards the same substrates. [References: 27]