Ligand accessibility to heme cytochrome b ₅ coordinating sphere and enzymatic activity enhancement upon tyrosine ionization

Recently, we observed that at extreme alkaline pH, cytochrome b ₅ (Cb ₅ ) acquires a peroxidase-like activity upon formation of a low spin hemichrome associated with a non-native state. A functional characterization of Cb ₅ , in a wide pH range, shows that oxygenase/peroxidase activities are stimulated in alkaline media, and a correlation between tyrosine ionization and the attained enzymatic activities was noticed, associated with an altered heme spin state, when compared to acidic pH values at which the heme group is released. In these conditions, a competitive assay between imidazole binding and Cb ₅ endogenous heme ligands revealed the appearance of a binding site for this exogenous ligand that promotes a heme group exposure to the solvent upon ligation. Our results shed light on the mechanism behind Cb ₅ oxygenase/peroxidase activity stimulation in alkaline media and reveal a role of tyrosinate anion enhancing Cb ₅ enzymatic activities on the distorted protein before maximum protein unfolding.