title: Spectroscopic characterization and assignment of reduction potentials in the tetraheme cytochrome C554 from Nitrosomonas europaea.
abstract: The tetraheme cytochrome c(554) (cyt c(554)) from Nitrosomonas europaea is an essential electron transfer component in the biological oxidation of ammonia. The protein contains one 5-coordinate heme and three bis-His coordinated hemes in a 3D arrangement common to a newly characterized class of multiheme proteins. The ligand binding, electrochemical properties, and heme-heme interactions are investigated with Mössbauer and X- and Q-band (parallel/perpendicular mode) EPR spectroscopy. The results indicate that the 5-coordinate heme will not bind the common heme ligands, CN(-), F(-), CO, and NO in a wide pH range. Thus, cyt c(554) functions only in electron transfer. Analysis of a series of electrochemically poised and chemically reduced samples allows assignment of reduction potentials for heme 1 through 4 of +47, +47, -147, and -276 mV, respectively. The spectroscopic results indicate that the hemes are weakly exchange-coupled (J approximately -0.5 cm(-)(1)) in two separate pairs and in accordance with the structure: hemes 2/4 (high-spin/low-spin), hemes 1/3 (low-spin/low-spin). There is no evidence of exchange coupling between the pairs. A comparison of the reduction potentials between homologous hemes of cyt c(554) and other members of this new class of multiheme proteins is discussed. Heme 1 has a unique axial N(delta)-His coordination which contributes to a higher potential relative to the homologous hemes of hydroxylamine oxidoreductase (HAO) and the split-Soret cytochrome. Heme 2 is 300 mV more positive than heme 4 of HAO, which is attributed to hydroxide coordination to heme 4 of HAO.
authors.affiliation: Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
authors.forename: Anup K
authors.initials: AK
authors.lastname: Upadhyay
authors.forename: Doros T
authors.initials: DT
authors.lastname: Petasis
authors.forename: Dave M
authors.initials: DM
authors.lastname: Arciero
authors.forename: Alan B
authors.initials: AB
authors.lastname: Hooper
authors.forename: Michael P
authors.initials: MP
authors.lastname: Hendrich
mesh_terms: D003574:Cytochrome c Group / Q000737:chemistry* / Q000378:metabolism; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D008024:Ligands; D008958:Models, Molecular; D009606:Nitrosomonas / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D015204:Spectroscopy, Mossbauer / Q000379:methods; D017186:Titrimetry
chemical_list: D003574:Cytochrome c Group; D008024:Ligands; C083951:cytochrome c553; C017165:cytochrome c(3); C022302:cytochrome C-552