Comparison of function of the distal base between myoglobin and peroxidase

The heme-linked protonation of a ferrous horseradish peroxidase is assigned to a distal amino acid residue. The conclusion is drawn from analyses of reactions that involve the protonation. Unfortunately it is difficult to apply it to the myoglobin case because no reaction has been found which is coupled with the protonation of the distal histidine. It is therefore of special interest to note that the pK value of 5.7 has been assigned to the distal histidine of metmyoglobin from binding kinetics with ligands20). It is well known that the reaction with hydrogen peroxide is quite different for the two types of hemoproteins. The distal base may be associated with the stabilization of the primary compound with hydrogen peroxide and also another amino acid residue may serve as a nucleophile for the stabilization of the pi-cation radical of porphyrin in the case if peroxidases. Numerous papers have dealt with heme substitution, heme linked protonation and reactions of hemoproteins related to the present subject. In this short paper, however, the discussion has mostly centered around the data obtained recently in our laboratory.