The Interaction of Porphyrins and Metalloporphyrins with Apocytochrome b5*

The preparation of undenatured apocytochrome bs and the apoprotein of an iodinated and acetylated derivative of cytochrome bs made possible the examination of the characteristics of hemei binding in this heme protein (2). A specific imidazole group on the apocytochrome was implicated in this reaction by stoichiometric titration of this single group on the iodinated and acetylated apoprotein with diazotized sulfanilic acid. The reactions of the apocytochrome b5 with group-specific reagents also indicate that whereas sulfhydryl and tyrosyl groups are not involved in heme binding, the acetylation of one or more LYor e-amino residues completely blocks this interaction. In the present report, the characterization of this heme binding has been extended by employing heme analogues in which the substituents on the pyrrole rings, as well as the metal in the porphyrin complex, were varied. These studies emphasize the role of the metal in heme binding in cytochrome bS and suggest a direct relationship between specific protein conformation and coenzyme binding.