Ca2+ activation of wheat peroxidase: a possible physiological mechanism of control.

Peroxidation of substrates such as ascorbic acid, pyrogallol, or ferulic acid, as well as indole acetic acid oxidation catalyzed by wheat germ peroxidase (WGP)2 C2, were found to be activated by Ca2+. This activation is independent of the stabilizing effect of structural Ca2+ reported for peroxidases. Steady state kinetics of ferulic acid oxidation catalyzed by WGP C2 showed an increase in the rate of compound I formation and of compound II decomposition in the presence of the ion, evidenced as an increase in rate constants k1, from 8.9 x 10(5) to 4.5 x 10(5) M-1 cm-1, and k3, from 4.4 x 10(5) to 1.1 x 10(6) M-1 cm-1. The dissociation constant Kd, for the cyanide derivative of the enzyme showed a marked decrease from 220 to 34 microM in the presence of Ca2+, thus implying an effect of the ion in the H2O2 binding step. In the presence of Ca2+, a conformational change in the protein was revealed by tryptophan fluorescence, providing a basis for the activation mechanism. Other peroxidases such as horseradish peroxidase and WGP C3 were not activated by Ca2+. The results suggest the existence of a physiological mechanism of control of peroxidase isozymes activity mediated by Ca2+.