期刊
JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY
卷 13, 期 4, 页码 481-498出版社
SPRINGER
DOI: 10.1007/s00775-008-0372-9
关键词
cofactor; cytochrorne; electron paramagnetic resonance; electron transfer; heme
资金
- NIDDK NIH HHS [R01 DK044842, R01 DK044842-14, R01 DK031038-23, R01 DK031038, DK044842, DK031038, R56 DK031038] Funding Source: Medline
- NIGMS NIH HHS [GM062563, R01 GM062563-04A1, R01 GM062563] Funding Source: Medline
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, Delta phi = 57-90 degrees. 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 N-delta 1 atom of the heme-ligating histidine. The Delta phi 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.
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