The Reaction of Cytochrome c with Imidazole

Ferricytochrome c binds imidazole and 1-methylimidazole in reactions with 1:1 stoichiometry. The pH and temperature dependence of imidazole binding indicate that the reaction is analogous to the binding of the enzyme by ligands such as cyanide and azide. At pH 7.4 and 18° the observed thermodynamic parameters for imidazole binding are: ΔF = -1.9 kcal/mole, ΔH = +1.1 kcal/mole, and ΔS = +10.2 eu. The spectral changes induced by imidazole binding are similar to those that occur when cytochrome c is dimerized, heated to 50°, or denatured with urea. This result is interpreted as showing that in all the cases mentioned, the iron binding sulfur of methionine 80 is displaced by nitrogenous ligands. At low concentrations, imidazole does not affect the kinetics of the reduction of ferricytochrome c by ascorbate, but higher concentrations of the ligand cause a reduction in the rate, while guanidine hydrochloride increases the rate. The reduction of ferricytochrome c imidazole is very fast; the spectrum of the reduced complex is identical with that of the reduced free enzyme, except for an increase of 2% in the absorbancy at the peak of the Soret region. This difference disappears with first-order kinetics, with k = 1.08 × 10^-1sec^-1at 21° and an energy of activation of 21.8 kcal/ mole.