Sf. Delauder et al., THERMODYNAMICS OF HYDROGEN-CYANIDE AND HYDROGEN-FLUORIDE BINDING TO CYTOCHROME-C PEROXIDASE AND ITS ASN-82-]ASP MUTANT, Biochemical journal, 302, 1994, pp. 437-442
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 cons
tants determined from the titration calorimetry results were in agreem
ent with those determined from difference-spectroscopy measurements. F
or cyanide binding to CCP at 297.9 K, the binding constant decreased f
rom 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 1
0(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 f
or cyanide and fluoride to D82CCP were respectively a factor of two le
ss and at least an order of magnitude less than the corresponding bind
ing constants of CCP. Decreased ligand-binding strength in the D82CCP
mutant is thus entirely due to entropic effects.