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- Title
Bis-histidine-coordinated hemes in four-helix bundles: how the geometry of the bundle controls the axial imidazole plane orientations in transmembrane cytochromes of mitochondrial Complexes II and III and related proteins.
- Authors
Berry, Edward A.; Walker, F. Ann
- Abstract
Early investigation of the electron paramagnetic resonance spectra of bis-histidine-coordinated membrane-bound ferriheme proteins led to the description of a spectral signal that had only one resolved feature. These became known as “highly anisotropic low-spin” or “large g max” ferriheme centers. Extensive work with small-molecule model heme complexes showed that this spectroscopic signature occurs in bis-imidazole ferrihemes in which the planes of the imidazole ligands are nearly perpendicular, Δ φ = 57–90°. In the last decade protein crystallographic studies have revealed the atomic structures of a number of examples of bis-histidine heme proteins. A frequent characteristic of these large g max ferrihemes in membrane-bound proteins is the occurrence of the heme within a four-helix bundle with a left-handed twist. The histidine ligands occur at the same level on two diametrically opposed helices of the bundle. These ligands have the same side-chain conformation and ligate heme iron on the bundle axis, resulting in a quasi-twofold symmetric structure. The two non-ligand-bearing helices also obey this symmetry, and have a conserved small residue, usually glycine, where the edge of the heme ring makes contact with the helix backbones. In many cases this small residue is preceded by a threonine or serine residue whose side-chain hydroxyl oxygen acts as a hydrogen-bond acceptor from the Nδ1 atom of the heme-ligating histidine. The Δ φ angle is thus determined by the common histidine side-chain conformation and the crossing angle of the ligand-bearing helices, in some cases constrained by hydrogen bonds to the serine/threonine residues on the non-ligand-bearing helices.
- Subjects
ELECTRON paramagnetic resonance spectroscopy; IMIDAZOLES; CYTOCHROMES; HEME; SERINE; SPECTRUM analysis
- Publication
Journal of Biological Inorganic Chemistry (JBIC), 2008, Vol 13, Issue 4, p481
- ISSN
0949-8257
- Publication type
Article
- DOI
10.1007/s00775-008-0372-9