Effects of 2,4-substituents of deuteroheme upon peroxidase functions. II. Reactions of peroxidases and metmyoglobin with azide, cyanide and fluoride

Artificial horseradish peroxidases and metmyoglobins were reconstituted from their apoproteins and the following unnatural hemes: mesoheme, deuteroheme, hematoheme, chlorocruoroheme, and diacetyldeuteroheme. The electron-withdrawing effects of the 2,4-substituents upon affinities of these hemoproteins for azide, cyanide, and fluoride were investigated. Peroxidase preparations used were acidic (A₁ + A₂) and neutral (B + C) enzymes.The electron withdrawal of the substituents increased the strength of ligaud binding. Plotting the logaritms of dissociation constants for the hemoprotein-ligand complexes against pK₃, a measure of relative basicities for metal-free porphyrins, the Hammett relationship was found to be applicable to this case except for a few. The affinities of deuterohemoproteins for ligands were definitely higher than those expected from the above relationship.The constant ρ, slope of the Hammett equation, varied with the ligand. For any particular ligand, the two peroxidase preparations gave the same ρ value, but it was different from the value for metmyoglobin.