The Peroxidase-Oxidase Reaction:
A Hierarchy of Re-oxidation.

Hierachy of conversion rates of Per³⁺ to coI according to BFSO. Shown here are the rates of reaction, R₂ (peroxidase cycle) and R₆ and R₁₁ (oxidase cycle). (Top.) R₆ (oxidase cycle) dominates. (Bottom.) Detail. R₆ leads, followed by R₂ and finally R₁₁. The hierachy of reactions can be understood by noting that in the absence of hydrogen peroxide, coIII accumulates because the reaction runs out of NAD radicals which are removed by the dimerization reaction, R₁₀. This breaks the oxidase cycle at two points: R₆ and R₁₁. At the same time, R_5a, which requires superoxide anion radical, continues to convert Per³⁺ to coIII. Eventually, hydrogen peroxide re-accumulates - principally via R₁ - and Per³⁺ is converted to coI via R₂ (peroxidase cycle. Reactions R_3a and R_3b follow with an accompanying production of NAD radicals which, in turn, allows for decomposition of coIII. At least for the parameter values considered here, the inclusion of R₁ (non-enzymic oxidation of NADH), which was re-introduced by Olson et al. (1995) is essential for perpetuation of the overall reaction (Bronnikova et al., 2001). In the present case, the rate constant, the dynamics correspond to a 3-cycle with k₆ = 1.50 x 10^-8 M^-1s^-1. Other constants as in the DCP experiments (Table I). Total enzyme is 0.9 mM.