Substrate Activation and the Kinetics of Ferroxidase

The oxidation of Fe(II) and aromatic diamines as catalyzed by human ferroxidase (ceruloplasmin, EC 1.12.3.1) exhibits nonlinear kinetics in typical v against v/s plots. The possibility of two enzymes or more has been excluded by experiments indicating the chemical and kinetic homogeneity of crystalline ferroxidase. The kinetics of Fe(II) oxidation can be accounted for by a mechanism based on substrate activation resulting in a rate expression of the form: [see PDF for equation] Curves calculated from this expression fit the experimental points within allowed error at several temperatures. The rate constant, kxiii, for the proposed rate-determining step, was 7 to 10 times greater for the Fe(II)-activated reaction than for the rate constant, k13, for the nonactivated route. The loss of absorption at 610 nm in the presence of low concentrations of Fe(II) and the activation observed with other divalent metal ions, which are not substrates of ferroxidase, are also consistent with this mechanism.