Cyanide binding to the cytochrome c ferric heme octapeptide. A model for anion binding to the active site of high spin ferric heme proteins.

Equilibrium constants for the binding of cyanide to the ferric heme c octapeptide in 20% ethylene glycol, 50% buffer were measured spectrophotometrically. The equilibrium constant for cyanide binding at 20 degrees C and pH 7.4 is 3.47 X 10(7), which is approximately 15-fold lower than that observed for cyanide binding to methemoglobin or metmyoglobin. Equilibrium constants at several temperatures exhibited an apparent van't Hoff relationship, yielding thermodynamic values of delta H degrees = -79,000 J/mol (-18,900 cal/mol) and delta S degrees = J/degrees K mol (-30.1 e.u.). Comparison of the ratio of equilibrium constants for cyanide ligation to methemoglobin the heme octapeptide with the ratio of equilibrium constants for azide ligation to methemoglobin and the heme octapeptide suggests that cyanide binding to the methemoproteins is much smaller than expected by comparison to azide binding. The differences in the ratios, the thermodynamic values, and the preferred binding geometries suggest that CN- ligation, like CO ligation, is sterically hindered. A comparison of these ratios to similar ratios for CO, O2, and NO binding suggests that the Fe-CN bond angle is less subject to distortion than the Fe-CO bond and/or additional binding interactions contribute to N3- but not to CN-binding to the protein.