Anion Binding to Cytochromec2: Implications on Protein–Ion Interactions in Class I Cytochromesc

The binding of several inorganic and carboxylate anions to cytochromec2fromRhodopseudomonas palustrishas been investigated by monitoring the salt-induced changes in the redox potential of the heme, using an interpretative model based on the extended Debye–Hückel equation. Most anions were found to interact specifically with the protein at one or multiple sites. Binding constants to the oxidized protein in the range 101–102m−1were determined from the anion concentration dependence of the chemical shift of the isotropically shifted heme methyl resonances. For several anions the stoichiometry and strength of the binding to cytochromec2were found comparable with those determined for mitochondrial cytochromesc, in spite of the limited sequence similarity (less than 40%) and the lower positive charge of the bacterial protein. These analogies were interpreted as indicative of the existence of common binding sites which are proposed to be located in the conserved lysine-rich domain around the solvent-exposed heme edge, which is also the surface area likely involved in the interaction with redox partners. The changes inE° due to partial neutralization of the positive charge of cytochromec2due to specific anion binding were found comparable with those for the mitochondrial species.