THERMODYNAMICS OF HYDROGEN-CYANIDE AND HYDROGEN-FLUORIDE BINDING TO CYTOCHROME-C PEROXIDASE AND ITS ASN-82-]ASP MUTANT

The thermodynamics of binding of fluoride and cyanide to cytochrome c peroxidase (CCP) and its Asn-82-->Asp mutant (D82CCP) in phosphate and acetate buffer at an ionic strength of 0.15 mol.kg(-1) from pH 5.0 to 7.1 were investigated by titration calorimetry at 289 and 297 K. The binding reactions are en thalpically driven. The fluoride-binding constants determined from the titration calorimetry results were in agreement with those determined from difference-spectroscopy measurements. For cyanide binding to CCP at 297.9 K, the binding constant decreased from 8.95(+/-0.83) x 10(5) M(-1) at pH 7.0 to 4.04(+/-0.23) x 10(5) M(-1) at pH 5.0, and the binding enthalpy increased from -57.2 +/- 1.4 kJ.mol(-1) at pH 7.0 to -48.6 +/- 1.8 kJ.mol(-1) at pH 5.0. For fluoride binding to CCP, the binding constant increased from 8.41(+/-0.54) x 10(3) M(-1) at pH 7.0 to 3.11(+/-0.09) x 10(5) M(-1) at pH 5.0 and the binding enthalpy increased from -71.9 +/- 1.1 kJ.mol(-1) at pH 7.0 to -67.0 +/- 1.9 kJ.mol(-1) at pH 5.0. The binding enthalpies for D82CCP were about the same as those for CCP. However, the binding constants for cyanide and fluoride to D82CCP were respectively a factor of two less and at least an order of magnitude less than the corresponding binding constants of CCP. Decreased ligand-binding strength in the D82CCP mutant is thus entirely due to entropic effects.