Two major Enzymes that remove hydrogen peroxide

There are two major enzymes that remove hydrogen peroxide (a source of free radicals): catalase and glutathione peroxidase. A paper published in J. Biol. Chem, 269, 1020-1025 (1994) examines their roles in cultured fibroblasts. The authors wrote:H2​O2​concentration dependence of the reaction rate was interpreted as that the process involves two kinetically different reactions (referred to as reactions 1 and 2).

Reaction 1 was characterized by a relatively lowKm​=40μM, and reaction 2 by linear dependence of the rate up to500μMH2​O2​…. Catalase shows a very highKm​value forH2​O2​, and its reaction apparently follows the first-order kinetics, but GSH peroxidase shows a relatively lowKm​, value, obeying the Michaelis-Menten kinetics.

On the basis of their kinetics, it is believed that in intact cells GSH peroxidase is more effective at relatively lowH2​O2​concentrations, while catalase is more effective at high concentrations. Based on this information, explain why the curve behaves as it does. Comment on its shape in different regions.

Fic. 4. The rate of extracellularH2​O2​removal by fibroblasts an a function ofH2​O2​concentration. The data$$(\Theta )$$were obtained at cell densities ranging from$$0.15$$to$$0.35\mathrm{mg}$$as protein. The solid lines show the fitted curve on the basis of Eauations 3-5. The fitted parameter values are given in Table III.