Stepwise modification of the electrostatic charge of cytochrome c. Effects on protein conformation and oxidation-reduction properties.

Horse heart cytochrome c was progressively maleylated, and fractions containing increasing numbers of modified lysines were obtained. The 695 nm band was present in derivatives containing up to 14 maleylated residues. Circular dichroic spectra showed minor changes beginning with 8 substituted lysines; in derivatives with 14 or more maleylated lysines, circular dichroism indicated total disruption of the native conformation. The ionic strength dependence of the measured oxidation reduction potentials and second order rate constants of reduction with ascorbate varied as expected from application of Debye-Huckel theory to the differently charged derivatives. The thermodynamic oxidation-reduction potentials decreased with the increase in the number of negatively charged groups, in a manner similar to that observed for simple iron complexes.