Correlation of acid-induced conformational transition of ferricytochrome c with cyanide binding kinetics.

A relation between pH-induced conformational transitions of horse heart ferricytochrome c and the kinetics of external ligand coordination to heme iron was investigated by optical spectroscopy, circular dichroism and viscometry. The dependencies of both the association, k (a), and dissociation rate constants of cyanide binding on pH were determined from kinetic measurements. The association rate constant exhibits a bell-shaped form of dependence on pH in the region where this protein unfolds. The maximum of the dependence of k (a) on pH is found to be coincident with the pK values of conformational transitions of ferricytochrome c in solutions with both low and high ionic strengths. This observation is explained in terms of ferricytochrome c unfolding, which is characterized by two processes: the gradual opening of the heme crevice accompanied by the detachment of the axial Met80 and its replacement with a water molecule. The former process enhances the rate, whereas the latter results in the inhibition of the rate of cyanide binding.