THEORETICAL-STUDIES AND COMPARISON WITH EXPERIMENTS ON THE HORSERADISH-PEROXIDASE OXIDASE REACTION

We study numerically the changes in the kinetics and the thermodynamics for the periodically forced oxidation of reduced nicotinamide adenine dinucleotide (NADH) by molecular oxygen. This reaction, which is catalyzed by the horseradish peroxidase enzyme, is referred to as the PO reaction. The model used for the calculations is the Degn-Olsen-Perram (DOP) model, a simple four-variable model. We choose different forms of external periodic perturbations in the inflow of molecular oxygen to observe the effect of such forms on the dissipation of the system. On forcing a numerical limit cycle with a two-term Fourier series waveshape, we observe that the dissipation of the system is lowered relative to the autonomous system. Sinusoidal perturbations of a stable focus in the model system also show that the dissipation may be lowered relative to the autonomous state and that, as the perturbation amplitude increases, the dissipation decreases. The calculations indicate that NADH is an essential species in the PO reaction. By application of a perturbation to a stable focus, bistability between a steady state and an oscillatory state was observed. Numerical calculations on a chaotic state of the PO reaction show both periodic and chaotic responses, with the chaotic responses leading to lower dissipation than the periodic responses. Comparison of these calculations with the experiments described in the preceding paper leads to the conclusion that the DOP model is stiffer than the experiments; perturbation factors, such as the frequency of perturbation and the Fourier coefficient C2 for two-term series perturbations, lead to larger changes in the dissipation in the experiments than in the calculations. The experiments on a stable focus suggest that NADH is a nonessential species while the calculations suggest that NADH is an essential species.