Conformational Transitions of Ferricytochrome c in Strong Inorganic Acids

Abstract

Conformational transitions of horse heart ferricytochromec(ferricytc) have been investigated in the presence of strong inorganic acids and their salts by optical absorption spectroscopy, magnetic circular dichroism and circular dichroism. In the presence of acids (HClO4or H2SO4, pH 2) or their salts (1 M NaClO4or Na2SO4, pH 2, 25 °C), the three ligation states of ferricytcheme were identified. One is the high-spin state: His18-Fe-H2O (40-50%), and two are the low-spin states: His18-Fe-Met80 (30-25%) and His18-Fe-His (30-25%). Under these conditions low temperatures facilitate native heme coordination of ferricytc. Transition from low-spin to high-spin heme coordination of ferricytcis complete in 1 M HClO4or 3 M H2SO4. At the concentration of HClO4and H2SO4above 3 M, different behavior in spectral transitions of ferricytcnear the heme is observed. High-spin pentacoordinated ferricytcwith the heme ligand of His18-Fe is formed in 8 M H2SO4. This state is unstable at higher concentration of H2SO4and porphyrin ferricytcis formed. At HClO4concentration higher than 3 M, the new, until this time not observed heme coordination structure of ferricytcoriginates.