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

The temperature dependence of the magnetic susceptibility of the ferric hemeoctapeptide from cytochrome c was measured in the presence and absence of fluoride ion to study the fluoride binding equilibrium of the hemeoctapeptide. The shift in the 1H NMR signal of sodium 4,4-dimethyl-4-silapentanesulfonate caused by the hemeoctapeptide was measured from -11 to 80 degrees C in a D2O/ethylene glycol solution. Magnetic susceptibilities obtained from the shifts were used to calculate the binding constant at each temperature. The equilibrium constant is 0.95 M-1 at 25 degrees C. Thermodynamic values determined from a plot of ln K versus 1/T are delta H0 = 19,700 J/mol (4,700 cal/mol) and delta S0 = 66.1 J/K mol (15.8 entropy units). The equilibrium and thermodynamic values are compared with those for fluoride binding to hemeproteins and with values for azide and cyanide binding to the hemeoctapeptide and hemoproteins. The differential binding data are used to assess the proposed bonded and nonbonded interactions between the distal histidine and the protein on the axial anionic ligand affinity. The results suggest that bonded interactions between the distal histidine and the ligand may contribute considerable stabilization to the hemoprotein-ligand complex.