Analysis of acid-base properties of peroxidase and myoglobin.

Two heme-linked groups of horseradish peroxidases, characterized by pKa = 5.8 (isoenzyme A) and 7.3 (isoenzyme C), are believed to be distal amino acid residues and, judging by their acid-base properties in various derivatives, influence the functions of these isoenzymes. In contrast, in myoglobin ionization of the distal histidine does not influence its reactivity. Accordingly, we conclude that the interaction of the distal base with a 6th ligand is weak in myoglobin but very strong in peroxidases.