Nitric oxide binding to ferrous native horse heart cytochrome c and to its carboxymethylated derivative: A spectroscopic and thermodynamic Study

Nitric oxide binding to ferrous native horse heart cytochrome c and to its carboxymethylated derivative has been investigated quantitatively by EPR and absorbance spectroscopy. The X-band EPR spectra and the absorption spectra in the Soret region of the nitrosylated derivative of ferrous native and carboxymethylated cytochrome c display the same basic characteristics reported for the beef heart cytochrome a₃ in cytochrome c oxidase, and horseradish and baker's yeast cytochrome c peroxidase, as well as the high affinity form of oxygen carrying proteins. Values of the dissociation equilibrium constant for nitrosylation of ferrous native and carboxymethylated cytochrome c are 8.2 × 10^-6 M and ≤ 5 × 10^-8 M, respectively, at pH 7.0 and 10°C. The results here reported represent clearcut evidence for the nitric oxide-induced cleavage of the FeMet80 bond in ferrous native cytochrome c, and allow estimation of the free energy associated to the heme-iron sixth coordination bond ( > 10 kJ mol^-1, at 10°C).