Horseradish peroxidase. XLII. Oxidations of L-tyrosine and 3,5-diiodo-L-tyrosine by compound II

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Abstract
The oxidations of both L-tyrosine and 3,5-diiodo-L-tyrosine by compound II of horseradish peroxidase were studied over the pH range .apprx. 3-10 at 25.degree. C and at a constant ionic strength of 0.11. The rate vs. pH profile for the tyrosine-compound II reaction illustrates the influences of at least 2 acid group ionizations. An enzyme dissociation (pKa .apprx. 6.2) has a small effect on the reaction rate; whereas, a second pKa of 9.2, which may be attributed to either the enzyme or substrate, has a greater influence on the rate. The oxidation of tyrosine by compound II is fastest at pH 7.6. In the case of the diiodotyrosine-compound II reaction, 3 acid dissociations are necessary to describe the plot of log (kapp) vs. pH. These include 2 enzyme pKa values of 3.6 and 8.6 and 1 substrate pKa of 6.6. The rate optimum for the reaction occurs at pH 5.2 and deprotonation of the phenolic group of diiodotyrosine results in a dramatic decrease in kapp. Diiodotyrosine is required in only a 0.5 M equivalent for the conversion of horseradish peroxidase compound I to compound II. The diiodotyrosine pKa values were estimated as 6.4 and 9.4 for the phenolic and amino groups, respectively.