Metalloprotein electron transfer reactions: analysis of reactivity of horse heart cytochrome c with inorganic complexes.

Abstract

The reactions of horse heart cytochrome c with Fe(EDTA)2-, Co (1,10-phenanthroline)33+, Ru(NH3)62+, and Fe(CN)63- were analyzed within the formalism of the Marcus theory of outer-sphere electron transfer, including compensation for electrostatic interactions. Calculated protein self-exchange rate constants based on crossreactions varied over 3 orders of magnitude, decreasing according to Fe(CN)63- > Co(phen)33+ > Ru(NH3)62+ > Fe(EDTA)2-. The mechanism of electron transfer probably involved attack by the small molecule reagents near the most nearly exposed region of the heme; this attack was affected by electrostatic interactions with the positively charged protein, by hydrophobic interactions that permitted reagent penetration of the protein surface, and by the availability of .pi. symmetry ligand (or extended metal) orbitals that overlapped with the .pi. redox orbitals of the heme group.