title: Unexpected weak magnetic exchange coupling between haem and non-haem iron in the catalytic site of nitric oxide reductase (NorBC) from Paracoccus denitrificans1.
abstract: Bacterial NOR (nitric oxide reductase) is a major source of the powerful greenhouse gas N2O. NorBC from Paracoccus denitrificans is a heterodimeric multi-haem transmembrane complex. The active site, in NorB, comprises high-spin haem b3 in close proximity with non-haem iron, FeB. In oxidized NorBC, the active site is EPR-silent owing to exchange coupling between FeIII haem b3 and FeBIII (both S=5/2). On the basis of resonance Raman studies [Moënne-Loccoz, Richter, Huang, Wasser, Ghiladi, Karlin and de Vries (2000) J. Am. Chem. Soc. 122, 9344-9345], it has been assumed that the coupling is mediated by an oxo-bridge and subsequent studies have been interpreted on the basis of this model. In the present study we report a VFVT (variable-field variable-temperature) MCD (magnetic circular dichroism) study that determines an isotropic value of J=-1.7 cm-1 for the coupling. This is two orders of magnitude smaller than that encountered for oxo-bridged diferric systems, thus ruling out this configuration. Instead, it is proposed that weak coupling is mediated by a conserved glutamate residue.
authors: School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK.
authors: Jessica H
authors: JH
authors: Van Wonderen
authors: Vasily S
authors: VS
authors: Oganesyan
authors: Nicholas J
authors: NJ
authors: Watmough
authors: David J
authors: DJ
authors: Richardson
authors: Andrew J
authors: AJ
authors: Thomson
authors: Myles R
authors: MR
authors: Cheesman
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D020134:Catalytic Domain; D002942:Circular Dichroism; D004578:Electron Spin Resonance Spectroscopy; D018698:Glutamic Acid / Q000737:chemistry / Q000378:metabolism; D006418:Heme / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D060328:Magnetic Phenomena; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D010231:Paracoccus denitrificans / Q000737:chemistry* / Q000201:enzymology; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D018698:Glutamic Acid; D006418:Heme; D007501:Iron; D010088:Oxidoreductases; C034747:nitric-oxide reductase