Characterization of Non-Native Heme Coordination Structures Emerging upon Guanidine Hydrochloric Acid-Induced Unfolding of Pseudomonas aeruginosa Ferricytochrome c551

Abstract

Abstract The non-native heme coordination structures emerging upon guanidine hydrochloric acid (GdnHCl)-induced unfolding of Pseudomonas aeruginosa ferricytochrome c551 were characterized by absorption, circular dichroism, paramagnetic NMR, and resonance Raman spectroscopy. Hexacoordinated high-spin ferriheme bearing axial His and H2O ligands, and pentacoordinated high-spin ferriheme with an axial His ligand were identified in the presence of GdnHCl concentrations ≥ 1.5 M (M = mol dm−3). These non-native species were rapidly interconverted to each other through cleavage/formation of the Fe–H2O coordination bond, and were also in dynamic equilibrium, at a rate of < 2 × 104 s−1, with the native species. Thus, the present study demonstrated the presence of dynamic equilibrium through cleavage/formation of a Fe–H2O coordination bond in an unfolding intermediate. These results provide a deeper insight into structure transitions of the heme active site upon folding and unfolding of cytochrome c.