Heat capacity changes in heme protein–ligand interactions

Isothermal titration calorimetry (ITC) has been used to determine thermodynamics of heme protein–ligand interactions with special emphasis of obtaining heat capacity changes (ΔCp-values) for the reactions. Cyanide and azide have a relative high affinity to metmyoglobin with ΔCp-values of −175 ± 41 J/K mol and −197 ± 42 J/K mol, respectively, while imidazole to metmyoglobin, and azide and imidazole to cytochrome c are low affinity systems with ΔCp-values of 67 ± 4 J/K mol, 17 ± 4 J/K mol, and 0 ± 8 J/K mol, respectively. The small ΔCp-values correlate well with there being minor changes is apolar solvent accessible surface areas (ASAapolar). Also, the determination of ΔCp-values allowed for the parameterization of the reaction entropy changes (ΔSr). Conformational entropy changes (ΔSconf) were large, and hence, found to be in accordance previous studies describing changes in the heme protein structures upon ligand binding.