Horse heart ferricytochromec: Conformation and heme configuration of low ionic strength acidic forms

Resonance Raman, absorption and circular dichroism spectroscopic studies of the stable forms of horse heart ferricytochromec in thepH range 6–0.8 and at the lowest possible ionic strengths, in water, and at 30°C are reported. The neutralpH form, state III, changes to the acidicpH form, state I, through a three-step process: state III ? state IIIa ? state II ? 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 ferricytochromec 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 ferricytochromec 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 Å.