Complexation of iron hexacyanides by cytochrome c. Evidence for electron exchange at the exposed heme edge

Electrostatic binding of at least two anionic iron hexacyanides to cationic horse heart cytochrome c was demonstrated by equilibrium dialysis measurements. No binding was detected following trifluoroacetylation of all of the 19 lysine residues. Replacement of the natural heme iron ligand methionine 80 by the alternative intrinsic ligand lysine 79 but not the extrinsic ligand imidazole resulted in the loss of one hexacyanide binding site. It is proposed that this site is located at the exposed heme edge and is functional in electron exchange.