title: Redox properties of the diheme cytochrome c4 from Azotobacter vinelandii and characterisation of the two hemes by NMR, MCD and EPR spectroscopy.
abstract: From biphasic stopped-flow kinetic studies it has been established that the two heme centres of cytochrome c4 from Azotobacter vinelandii undergo redox change with [Co(terpy)2]3+/2+ (260 mV) at different rates. Rate constants for oxidation and reduction at pH 7.5 give reduction potentials for the two heme centres in agreement with previous values from spectrophotometric titrations (263 and 317 mV). From NMR studies on the fully reduced protein two sharp methyl methionine resonances are observed at -3.16 and -3.60 ppm, consistent with axial methionine coordination. On titration with [Fe(CN)6]3- the -3.16 ppm resonance is the first to disappear, and is assigned to the less positive reduction potential. Line-broadening effects are observed on partial oxidation, which are dominated by intermolecular processes in an intermediate time-range exchange process. The hemes of the oxidised protein are distinguishable by EPR g-values of 3.64 and 3.22. The former is of interest because it is at an unusually low field for histidine/methionine coordination, and has an asymmetric or ramp shape. The latter assigned to the low potential heme is similar to that of a cytochrome c551. The MCD spectra of the fully oxidised protein are typical of low-spin Fe(III) heme centres, with a negative peak at 710 nm characteristic of methionine coordination, and an NIR peak at 1900 nm characteristic of histidine/methionine (axial) coordination. Of the four histidines per molecule only two undergo diethyl pyrocarbonate (DEPC) modification.
authors.affiliation: Department of Chemistry, University of Newcastle upon Tyne, U.K.
authors.forename: P M
authors.initials: PM
authors.lastname: Gadsby
authors.forename: R T
authors.initials: RT
authors.lastname: Hartshorn
authors.forename: J J
authors.initials: JJ
authors.lastname: Moura
authors.forename: J D
authors.initials: JD
authors.lastname: Sinclair-Day
authors.forename: A G
authors.initials: AG
authors.lastname: Sykes
authors.forename: A J
authors.initials: AJ
authors.lastname: Thomson
mesh_terms: D001395:Azotobacter / Q000032:analysis*; D003574:Cytochrome c Group; D004047:Diethyl Pyrocarbonate; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme; D006639:Histidine; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008715:Methionine; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D013057:Spectrum Analysis
chemical_list: D003574:Cytochrome c Group; C034186:cytochrome C4; D006418:Heme; D006639:Histidine; D008715:Methionine; D004047:Diethyl Pyrocarbonate