Preferred sites for electron exchange between cytochrome c and [Fe(edta)]2– and [Co(sep)]2+ complexes

Abstract

Rate constants (25 °C) for the reduction of eight different singly substituted 4-carboxy-2,6-dinitrophenyl (cdnp) horse heart cytochrome c derivatives, modified respectively at lysine 7,1 3,25,27,60,72,86, and 87, and of one 2,4,6-trinitrophenyl (tnp) derivative modified at lysine 13, by the 2– and 2+ complexes [Fe(edta)]^2–(edta = ethylenediaminetetra-acetate) and [Co(sep)]²⁺(sep = 1, 3,6,8,10,13,16,19-octa-azabicyclo[6.6.6]icosane) have been determined at pH 7.5 (Tris-HCI)[Tris = tris(hydroxymethyl)methylamine], I= 0.10 M (NaCI). The influence of the modified residues on second-order rate constants for these reactions has been used to define the regions on the protein surface involved. In both cases the solvent accessible edge of the heme prosthetic group on the ‘front’ surface of the molecule is relevant. The reaction with [Fe(edta)]^2– is most strongly influenced by the modification at lysine 72 followed by 27, the latter representing a shift in reactivity as compared to [Fe(CN)₆]^3–. With [Co(sep)]²⁺ the region around lysine 27 to the right of the heme edge is the most influential as in the case of [Co(phen)₃]³⁺(phen = 1, 1 0-phenanthroline). The preferred sites with [Fe(edta)]^2– and [Fe(CN)₆]^3– are the same as those identified using n.m.r.