In the enzyme which catalyzes the breakdown of H₂O₂ $\to$H₂O + O_2, the prosthetic group is

The enzyme that catalyses the breakdown of hydrogen peroxide (H₂O₂) into water (H₂O) and oxygen (O₂) is called catalase. Catalase is a heme-containing enzyme found in many living organisms, including animals, plants and bacteria.