Hydrogen Peroxide, a Molecule with a Janus Face

In biology, H₂O₂ plays an important role as a component of oxidative stress and as a signaling molecule. This chapter describes the discovery and properties of H₂O₂, arguing that this molecule is not as reactive as often thought, and focusing on the Fenton reaction, as for decades this reaction illustrated the bad side of H₂O₂. The more recently discovered role of H₂O₂ as a redox mediator and messenger represents its good side. These two aspects define H₂O₂, a molecule with a Janus face. During the 1970s, the Fenton reaction was proposed as the causative agent of oxidative damage because it produced the reactive hydroxyl radical. The origin of the Fenton reaction was a case of serendipity. Thenard tested whether H₂O₂ reacted with many compounds—inorganic and organic. Scavenging of H₂O₂ by catalase and glutathione peroxidase is fast. Nature uses a trick to accelerate the reaction with the peroxidative cysteine in peroxiredoxin.