JBC, Vol. 251, Issue 19, 6036-6042, Oct, 1976
Oxidation of p-cresol by horseradish peroxidase compound I
W. D. Hewson and H. B. Dunford
Rate constants for the reaction between horseradish peroxidase compound I
and p-cresol have been determined at several values of pH between 2.98 and
10.81. These rate constants were used to construct a log (rate) versus pH
profile from which it is readily seen that the most reactive form of the
enzyme is its most basic form within this pH range so that base catalysis
is occurring. At the maximum rate a second order rate constant of (5.1 +/-
0.3) x 10(-7) M-1 s-1 at 25 degrees is obtained. The activation energy of
the reaction at the maximum rate was determined from an Arrhenius plot to
be 5.0 +/- 0.5 kcal/mol. Evidence for an exception to the generally
accepted enzymatic cycle of horseradish peroxidase is presented. One-half
molar equivalent of p-cresol can convert compound I quantitatively to
compound II at high pH, whereas usually this step requires 1 molar
equivalent of reductant. The stoichiometry of this reaction is
pH-dependent.
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