title: Cytochrome c'' isolated from Methylophilus methylotrophus. An example of bis-histidine-co-ordinated Fe3+ haem, with near-perpendicular orientation of the ligands.
abstract: Cytochrome c'' (Methylophilus methylotrophus) is a soluble protein, Mr 15,000, possessing one haem which is high-spin in the reduced state but switches to a low-spin form on oxidation. Low-temperature electron-paramagnetic-resonance spectroscopy of the oxidized state shows a low-spin signal at gz = 3.65 with a folded line-shape typical of a haem of low rhombicity, and the near-infrared magnetic-circular-dichroism (m.c.d.) spectra reveal an unusually intense (delta epsilon = 400 M-1.cm-1 at 5 T, 4.2 K) charge-transfer band at 1560 nm, establishing that the oxidized haem is co-ordinated by two His residues in a near-perpendicular orientation. This conformation is well established for transmembrane b cytochromes, but this appears to be the first example in a water-soluble cytochrome. The low-temperature m.c.d. spectra of the reduced form of the protein confirms that the haem contains a high-spin Fe2+ ligated by one His residue. The redox-linked spin-state change releases a His group. Since this residue is likely to bind a proton at pH values less than 6.5, this cytochrome may provide a useful model of a molecular mechanism of a redox-linked proton uptake and release process.
authors.affiliation: School of Chemical Sciences, University of East Anglia, Norwich, U.K.
authors.forename: M J
authors.initials: MJ
authors.lastname: Berry
authors.forename: S J
authors.initials: SJ
authors.lastname: George
authors.forename: A J
authors.initials: AJ
authors.lastname: Thomson
authors.forename: H
authors.initials: H
authors.lastname: Santos
authors.forename: D L
authors.initials: DL
authors.lastname: Turner
mesh_terms: D001419:Bacteria / Q000032:analysis*; D002942:Circular Dichroism; D003574:Cytochrome c Group / Q000302:isolation & purification*; D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds; D006418:Heme; D006639:Histidine; D010084:Oxidation-Reduction; D011487:Protein Conformation; D011522:Protons
chemical_list: D003574:Cytochrome c Group; D005290:Ferric Compounds; D011522:Protons; D006418:Heme; D006639:Histidine