title: Axial coordination of heme in ferric CcmE chaperone characterized by EPR spectroscopy.
abstract: In Escherichia coli cytochrome c maturation requires a set of eight proteins including the heme chaperone CcmE, which binds heme transiently, yet covalently. Several variants of CcmE were purified and analyzed by continuous-wave electron paramagnetic resonance, electron nuclear double resonance, and hyperfine sublevel correlation spectroscopy to investigate the heme axial coordination. Results reveal the presence of a number of coordination environments, two high-spin heme centers with different rhombicities, and at least one low-spin heme center. The low-spin species was shown to be an artifact induced by the presence of available histidines in the vicinity of the iron. Both of the high-spin forms are five-coordinated, and comparison of the spectra of the wild-type CcmE with those of the mutant CcmE(Y134H) proves that the higher-rhombicity form is coordinated by Tyr134. The low-rhombicity (axial) form does not have a histidine residue or a water molecule as an axial ligand. However, we identified exchangeable protons coupled to the iron ion. We propose that the axial form can be coordinated by a carboxyl group of an acidic residue in the flexible domain of the protein. The two species would represent two different conformations of the flexible alpha-helix domain surrounding the heme. This conformational flexibility confers CcmE special dynamic properties that are certainly important for its function.
authors.affiliation: Laboratorium für Physikalische Chemie, ETH Zurich, 8093 Zurich, Switzerland. garciarubio@esr.phys.chem.ethz.ch
authors.forename: Inés
authors.initials: I
authors.lastname: García-Rubio
authors.forename: Martin
authors.initials: M
authors.lastname: Braun
authors.forename: Igor
authors.initials: I
authors.lastname: Gromov
authors.forename: Linda
authors.initials: L
authors.lastname: Thöny-Meyer
authors.forename: Arthur
authors.initials: A
authors.lastname: Schweiger
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000235:genetics; D004578:Electron Spin Resonance Spectroscopy; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics; D006418:Heme / Q000737:chemistry*; D006420:Hemeproteins / Q000737:chemistry* / Q000235:genetics; D006639:Histidine / Q000737:chemistry; D018832:Molecular Chaperones / Q000737:chemistry* / Q000235:genetics; D009154:Mutation
chemical_list: D001425:Bacterial Outer Membrane Proteins; C115384:CcmE protein, E coli; D029968:Escherichia coli Proteins; D006420:Hemeproteins; D018832:Molecular Chaperones; D006418:Heme; D006639:Histidine