title: The MCD and EPR of the heme centers of nitric oxide reductase from Pseudomonas stutzeri: evidence that the enzyme is structurally related to the heme-copper oxidases.
abstract: EPR spectra at liquid helium temperatures and MCD spectra at room temperature and 4.2 K are presented for fully oxidized nitric oxide reductase (NOR) from Pseudomonas stutzeri. The MCD spectra show that the enzyme contains three heme groups at equivalent concentrations but distinctive in their axial coordination. Two, in the low-spin ferric state at all temperatures, give rise to infrared charge-transfer transitions which show the hemes to have bis-histidine and histidine-methionine ligation, respectively. The EPR spectra show them to be magnetically isolated. The third heme has an unusual temperature-dependent spin state and spectroscopic features which are consistent with histidine-hydroxide coordination. No EPR signals have been detected from this heme. Together with its unusual near-infrared MCD, this suggests a spin-spin interaction between this heme and another paramagnet. The three hemes account for only 75% of the iron content, and it is concluded that the additional paramagnet is a mononuclear ferric ion. These results provide further evidence that NOR is indeed structurally related to heme-copper oxidases and that it contains a heme/non-heme iron spin-coupled pair at the active site.
authors.affiliation: Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences, University of East Anglia, Norwich, U.K. m.cheesman@uea.ac.uk
authors.forename: M R
authors.initials: MR
authors.lastname: Cheesman
authors.forename: W G
authors.initials: WG
authors.lastname: Zumft
authors.forename: A J
authors.initials: AJ
authors.lastname: Thomson
mesh_terms: D001665:Binding Sites; D002942:Circular Dichroism; D003080:Cold Temperature; D003300:Copper / Q000737:chemistry*; D003573:Cytochrome b Group; D003580:Cytochromes / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D029968:Escherichia coli Proteins; D006418:Heme / Q000737:chemistry*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry*; D011549:Pseudomonas / Q000201:enzymology*; D013696:Temperature
chemical_list: D003573:Cytochrome b Group; D003580:Cytochromes; D029968:Escherichia coli Proteins; C083850:cytochrome bo3, E coli; D006418:Heme; D003300:Copper; D010088:Oxidoreductases; C024748:copper oxidase; C034747:nitric-oxide reductase