The chemical reactivity of fully maleylated cytochrome c.

The chemical reactivity of fully maleylated horse heart cytochrome c with oxidants, reductants, and iron ligands was studied in the presence and absence of MgCl2. In the absence of salt, the partly unfolded protein reacts rapidly with cyanide in the ferric state (k = 235 M-1s-1); the reaction is exothermic (delta H = -9.4 kcal/mol) and entropically unfavored (delta S = -7.0 e.u.). In 0.01 M MgCl2, the refoleded protein recovers the behavior towards cyanide of native cytochrome c. The oxidation-reduction potential of the refolded species is 190 mV. In the course of reduction of the unfolded form with dithionite, a reduced intermediate species (t 1/2 = 5.9 s) is observed; this species binds carbon monoxide rapidly, but the ligand dissociates thereafter. This shows that full maleylation does not disrupt the conformational stability of reduced cytochrome c.