title: Horse heart ferricytochrome c: conformation and heme configuration of low ionic strength acidic forms.
abstract: Resonance Raman, absorption and circular dichroism spectroscopic studies of the stable forms of horse heart ferricytochrome c in the pH range 6-0.8 and at the lowest possible ionic strengths, in water, and at 30 degrees C are reported. The neutral pH form, state III, changes to the acidic pH form, state I, through a three-step process: state III in equilibrium with IIIa in equilibrium with state II in equilibrium with state I, with pKa's of 3.6 +/- 0.3, 2.7 +/- 0.2, and 1.2 +/- 0.2, depending on the monitoring probe, respectively. State IIIa ferricytochrome c is like state III (i.e., with the Met-80-sulfur-iron linkage and a closed heme crevice) but with a higher degree of folding and a slightly larger porphyrin core. State II ferricytochrome c is an unfolded form with an open heme crevice and no Met-80-sulfur-iron linkage. The heme iron is high-spin, and hexacoordinated with weak ligand-field groups, water, and nitrogen of the protonated/hydrogen-bonded imidazole of the His-18 residue at the axial positions. The state I form also lacks the Met-80-sulfur-iron linkage and has an open heme crevice like the state II form; however, it is less unfolded and has a high-spin pentacoordinated heme iron, with the nitrogen of the imidazole of His-18 as the axial ligate, which is out of the porphyrin plane by about 0.45 A.
authors.affiliation: Department of Chemistry, State University of New York, Albany 12222.
authors.forename: Y P
authors.initials: YP
authors.lastname: Myer
authors.forename: A F
authors.initials: AF
authors.lastname: Saturno
mesh_terms: D000818:Animals; D002942:Circular Dichroism; D003574:Cytochrome c Group / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D006736:Horses / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D009206:Myocardium / Q000737:chemistry*; D009994:Osmolar Concentration; D011487:Protein Conformation; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman; D014867:Water
chemical_list: D003574:Cytochrome c Group; D014867:Water; D006418:Heme