title: Identification of the ligand-exchange process in the alkaline transition of horse heart cytochrome c.
abstract: Magnetic-circular-dichroism (m.c.d.) spectra over the wavelength range 300-2000 nm at room temperature and at 4.2K of horse heart cytochrome c are reported at a series of pH values between 7.8 and 11.0, encompassing the alkaline transition. The effect of glassing agents on the e.p.r. spectrum at various pH values is also reported. Comparison of these results with spectra obtained for the n-butylamine adduct of soybean leghaemoglobin support the hypothesis that lysine is the sixth ligand in the alkaline form of horse heart cytochrome c. The m.c.d. and e.p.r. spectra of horse heart cytochrome c in the presence of 1-methylimidazole have also been examined. These studies strongly suggest that histidine-18, the proximal ligand of the haem, is the ionizing group that triggers the alkaline transition. Low-temperature m.c.d. and e.p.r. spectra are also reported for Pseudomonas aeruginosa cytochrome c551. It is shown that no ligand exchange takes place at the haem in this species over the pH range 6.0-11.3.
authors.affiliation: School of Chemical Sciences, University of East Anglia, Norwich, U.K.
authors.forename: P M
authors.initials: PM
authors.lastname: Gadsby
authors.forename: J
authors.initials: J
authors.lastname: Peterson
authors.forename: N
authors.initials: N
authors.lastname: Foote
authors.forename: C
authors.initials: C
authors.lastname: Greenwood
authors.forename: A J
authors.initials: AJ
authors.lastname: Thomson
mesh_terms: D000818:Animals; D001426:Bacterial Proteins; D002942:Circular Dichroism; D003574:Cytochrome c Group; D004578:Electron Spin Resonance Spectroscopy; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles; D008024:Ligands; D009206:Myocardium / Q000201:enzymology; D011550:Pseudomonas aeruginosa / Q000032:analysis; D013696:Temperature
chemical_list: D001426:Bacterial Proteins; D003574:Cytochrome c Group; D007093:Imidazoles; D008024:Ligands; C017167:cytochrome C(551); C018100:1-methylimidazole