Horseradish peroxidase. XXXII. pH dependence of the oxidation of L-(−)-tyrosine by compound I

Abstract

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.

Identification and structural basis of the reaction catalyzed by CYP121, an essential cytochrome P450 in Mycobacterium tuberculosis

“…5 (28,29). CpdI can promote the formation of two tyrosyl radicals, as demonstrated with the horseradish peroxidase, another hemecontaining protein forming CpdI as the reactive species (30,31). Moreover, one of the major reactions observed between two tyrosyl radicals is the formation of a tyrosyl cross link between the carbon atoms positioned ortho with respect to the hydroxyl (32).…”

The Fate of the Oxidizing Tyrosyl Radical in the Presence of Glutathione and Ascorbate

“…4). Tyrosine is oxidized by horseradish peroxidase compound I at a rate of 5.0 ϫ 10 4 M Ϫ1 s Ϫ1 (19) and compound II at a rate of 1.1 ϫ 10 3 M Ϫ1 s Ϫ1 (20). The observed tyrosyl radical ESR spectrum (Fig.…”

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

“…A second ionization with a pK, of 9.16 corresponding to the amino group of tyrosine, results in a large decrease in k,,,,,. The opposite effect was noted for the oxidation of tyrosine by compound I where the rate increased with deprotonation of the ainino moiety (29). Since it has been found that dityrosine (linked at the ortho positions) is the favoured reaction product for the hydrogen peroxide oxidation of tyrosine ( 4 0 ) , it seems unlikely that electron (or hyd;ogen atom) transfer would occur at the aliphatic side chain.…”