title: 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.
mesh_terms: D006735:Horseradish Peroxidase / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D010544:Peroxidases / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014443:Tyrosine / Q000378:metabolism
chemical_list: D014443:Tyrosine; D006735:Horseradish Peroxidase; D010544:Peroxidases