The redox potential of horse heart cytochrome c

Abstract

Summary The absolute extinction of the 695-nm band of horse heart ferricytochrome c and its redox potentials as a function of urea concentration at pH 7.0 have been measured. The redox potential is found to be independent up to about the 5 M urea concentration, but drops at higher concentrations, reaching a value of about 175 mV in 8 M urea. Changes in the 695-nm band, on the other hand, are indicative of a weak transition centered at about 2.5 M urea concentration and a major asymmetric transition leading to its quenching at about 6.5 M urea. The insensitivity of redox potentials to the transition at 2.5 M, associated with the loosening of the heme crevice (Myer, Y. P. (1968) Biochemistry 7 , 765), clearly demonstrates the unimportance of the integrity of the heme crevice, whereas the drop in potential corresponding to the transition at 6.5 M urea, representing the disruption of the heme coordination and helix coil transition, points to the protein structures as determinants of the high redox potential of the protein. The correlation of the transition profiles further indicates that a process localized toward the high-urea-concentration limb, the disruption of heme coordination, determines the level of the redox potential of this protein.