Toward a multistep mechanism of cytochrome c reactivity. Answer to a comment

Koppenol's rejection (Biophys. Chem. 18 (1983) 203) of a model of polarity-dependent ferrocytochrome c oxidation (M. Fragata and F. Bellemare, Biophys. Chem. 15 (1982) 111) places emphasis on the role of the protein surface charges in reactivity but is at the same time too restrictive as it neglects largely the polarity (dielectric constant) of the aqueous and hydrophobic interfaces of the exposed heme edge and the inner cleft (heme crevice) of cytochrome c which appear to be the oxidation-reduction sites. It is suggested that a more general model should take into account (i) a recognition (or diffusion) step where the distance travelled by cytochrome c at the membrane surface and/or the Brownian displacements in the bulk solution are greatly influenced by ionic strength, and (ii) a redox step where low polarity effects prevail with concomitant weakening of ionic activity.