Anomalous Ligand Binding by a Class of High Spin c-Type Cytochromes

The kinetic and equilibrium properties of carbon monoxide binding by seven different examples of cytochrome c' have been investigated. It has been found that the ligand-binding mechanism is complex in all cases, with five equilibria controlling the process. Moreover, the stoichiometry of ligand binding has been found to be at least 2 carbon monoxide molecules per heme. A detailed mathematical model has been derived which describes the ligand-binding process. In agreement with a previous proposal, we find that the heme group is situated in a protein bounded with only hindered access of dissolved ligands to the heme-binding site. Once the ligand enters the cage it has two options: to bind to the heme to form a heme-CO complex with an attendant absorbance change, or to bind to some other site (unspecified) with no associated spectral change. Both the spectral and nonspectral forms can bind a 2nd CO molecule to form the fully liganded complex. In the final form 2 molecules of CO are bound per heme, only 1 eliciting a spectral change.