The modulation of cytochrome c electron self‐exchange by site‐specific chemical modification and anion binding

Abstract

The site‐specific chemical modification of horse heart cytochrome c at Lys‐13 and ‐72 using 4‐chloro‐3,5‐dinitrobenzoic acid (CDNB) increases the electron self‐exchange rate of the protein. In the presence of 0.24 M cacodylate (pH^∗ 7.0) the electron self‐exchange rate constants, k _ex, measured by a ₁H NMR saturation transfer method at 300 K, are 600, 6 × 10³ and 6 × 10⁴ M⁻¹·s⁻¹ for native, CDNP‐K13 and CDNP‐K72 cytochromes c respectively. Repulsive electrostatic interactions, which inhibit cytochrome c electron selfexchange, are differentially affected by modification. Measurements of ¹H NMR line broadening observed with partially oxidised samples of native cytochrome c show that ATP and the redox inert multivalent anion Co(CN)₆ ³⁻ catalyse electron self‐exchange. At saturation a limiting value of ~ 1.4 × 10⁵ M⁻¹·s⁻¹ is observed for both anions.