The rate of oxidation of L-(-)-tyrosine by horseradish peroxidase compound 1 has been studied as a function of pH at 25 degrees C and ionic strength 0.11. Over the pH range of 3.20--11.23 major effects of three ionizations were observed. The pKa values of the phenolic (pKa = 10.10) and amino (pKa = 9.21) dissociations of tyrosine and a single enzyme ionization (pKa = 5.42) were determined from nonlinear least squares analysis of the log rate versus pH profile. It was noted that the less acidic form of the enzyme was most reactive; hence, the reaction is described as base catalyzed. The rate of tyrosine oxidation falls rapidly with the deprotonation of the phenolic group.
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Ralston et al. (1978) studied this question.