title: A switch in heme axial ligation prepares Paracoccus pantotrophus cytochrome cd1 for catalysis.
abstract: 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.
authors.affiliation: Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom.
authors.forename: J W
authors.initials: JW
authors.lastname: Allen
authors.forename: N J
authors.initials: NJ
authors.lastname: Watmough
authors.forename: S J
authors.initials: SJ
authors.lastname: Ferguson
mesh_terms: D002384:Catalysis; D003574:Cytochrome c Group; D003580:Cytochromes / Q000378:metabolism*; D006418:Heme / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D009572:Nitrite Reductases / Q000378:metabolism*; D010084:Oxidation-Reduction; D010230:Paracoccus / Q000201:enzymology*; D011487:Protein Conformation
chemical_list: D003574:Cytochrome c Group; D003580:Cytochromes; D006418:Heme; C019315:cytochrome cd1; D009572:Nitrite Reductases