Mechanism and Catalytic Site Atlas

Introduction

The mechanism proceeds via a two step process. In the first step, the enzyme is oxidised by hydrogen peroxide to an intermediate, Compound I, which in the second stage is reduced back to the original resting state releasing water and molecular oxygen. The total reaction is 2 moles of hydrogen peroxide gives 2 moles of water and one mole of oxygen. H2O2 enters the active site and binds to the heme Fe, histidine 128, and asparagine 201. The oxygen of H2O2 that is bound to the Fe donates a proton to the NE of histidine 128. Histidine 128 then in turn donates the proton to the other H2O2 oxygen bound to asparagine 201. This leaves water bound to asparagine which can then leave and the oxidised heme iron. A second molecule of hydrogen peroxide enters the active site and there is now an enzyme-compound 1-H2O2 complex, which evolves spontaneously to a compound II-like species in which formally a hydrogen atom has transferred (proton transfer to the oxoferryl oxygen and porphyrin radical of Compound I). In concert with this step a hydrogen atom is donated to His392 forming a positively charged imidazolium ring. This then acts as a general acid and donates it to the iron-bound hydroxide group, which leaves as water. Restoring the enzyme back to it's original state and releasing molecular oxygen and water.