Studies on the Peroxidase Effect of Cytochrome c. V. Spectrophotometric Titration of a Reversible Conformational Change of Ferricytochrome c in Acid Aqueous Solution.

Abstract

A reversible conformational change of native, monomeric beef heart cytochrome c (ferric form) takes place in acid aqueous solution. The conformational change was studied by means of ultraviolet difference spectra: the main peaks were observed at 290.5 m[mu] (- [image] max (cm-1 X m-1) = 3.6 and at 230.5 m[mu] (- [image] max (cm-1 X mM-1) = 24.0). Equilibrium data were obtained for the spectral transition at 290.5 m[mu]. Thus, the pH-dependence revealed an S-shaped titration curve with pK'' = 2.80 ([mu] = 0.1; 23.2[degree]C) for the single protonated group (probably carboxyl) involved in the spectral transition; the process was endothermic ([DELTA] H[image] = 9.1 kcal mole-1). The titration of 1 abnormal carboxyl group in ferricytochrome c agrees well with previous acid-base titrations. The spectral transition is paralleled by an activation of ferricytochrome c as a peroxidase, as well as by a change in its reactivity towards ionic ligands and its optical rotatory dispersion spectrum, indicating a common mechanism for these effects.