ANION-BINDING TO CYTOCHROME-C(2) - IMPLICATIONS ON PROTEIN-ION INTERACTIONS IN CLASS-I CYTOCHROMES-C

The binding of several inorganic and carboxylate anions to cytochrome C-2 from Rhodopseudomonas palustris has been investigated by monitorin g the salt-induced changes in the redox potential of the heme, using a n interpretative model based on the extended Debye-Huckel equation. Mo st anions were found to interact specifically with the protein at one or multiple sites. Binding constants to the oxidized protein in the ra nge 10(1)-10(2) M(-1) were determined from the anion concentration dep endence of the chemical shift of the isotropically shifted heme methyl resonances. For several anions the stoichiometry and strength of the binding to cytochrome c(2) were found comparable with those determined for mitochondrial cytochromes c, in spite of the limited sequence sim ilarity (less than 40%) and the lower positive charge of the bacterial protein. These analogies were interpreted as indicative of the existe nce of common binding sites which are proposed to be located in the co nserved lysine-rich domain around the solvent-exposed heme edge, which is also the surface area likely involved in the interaction with redo x partners. The changes in E degrees due to partial neutralization of the positive charge of cytochrome c(2) due to specific anion binding w ere found comparable with those for the mitochondrial species. (C) 199 7 Academic Press.