A novel, kinetically stable, catalytically active, all-ferric, nitrite-bound complex of Paracoccus pantotrophus cytochrome cd1.
The oxidized form of Paracoccus pantotrophus cytochrome cd(1) nitrite reductase, as isolated, has bis-histidinyl co-ordination of the c haem and His/Tyr co-ordination of the d(1) haem. On reduction, the haem co-ordinations change to His/Met and His/vacant respectively. If the latter form of the enzyme is reoxidized, a conformer is generated in which the ferric c haem is His/Met co-ordinated; this can revert to the 'as isolated' state of the enzyme over approx. 20 min at room temperature. However, addition of nitrite to the enzyme after a cycle of reduction and reoxidation produces a kinetically stable, all-ferric complex with nitrite bound to the d(1) haem and His/Met co-ordination of the c haem. This complex is catalytically active with the physiological electron donor protein pseudoazurin. The effective dissociation constant for nitrite is 2 mM. Evidence is presented that d(1) haem is optimized to bind nitrite, as opposed to other anions that are commonly good ligands to ferric haem. The all-ferric nitrite bound state of the enzyme could not be generated stoichiometrically by mixing nitrite with the 'as isolated' conformer of cytochrome cd(1) without redox cycling.
Allen
James W A
JW
Department of Biochemistry, University of Oxford, UK.
Higham
Christopher W
CW
Zajicek
Richard S
RS
Watmough
Nicholas J
NJ
Ferguson
Stuart J
SJ
D002384:Catalysis; D003574:Cytochrome c Group; D003580:Cytochromes / Q000737:chemistry* / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000737:chemistry; D007501:Iron / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D009572:Nitrite Reductases / Q000737:chemistry* / Q000378:metabolism; D009573:Nitrites / Q000737:chemistry*; D010084:Oxidation-Reduction; D010230:Paracoccus / Q000201:enzymology*; D011485:Protein Binding; D013053:Spectrophotometry
D003574:Cytochrome c Group; D003580:Cytochromes; D008024:Ligands; D009573:Nitrites; D006418:Heme; C019315:cytochrome cd1; D007501:Iron; D009572:Nitrite Reductases