Which enzyme breaks down hydrogen peroxide in the body?

The enzyme that breaks down hydrogen peroxide in the body is catalase.

Catalase protects cells by converting hydrogen peroxide into water and oxygen. Hydrogen peroxide is produced during normal metabolism and can damage proteins, membranes, and DNA when it accumulates. Catalase is therefore one part of the body’s antioxidant defense system, working alongside enzymes such as superoxide dismutase and peroxidases.

Human catalase is a tetramer, meaning it is built from four protein subunits. Each subunit contains an iron-bearing heme group that helps the enzyme react with hydrogen peroxide. Catalase is especially concentrated in the liver and is commonly found in peroxisomes, cellular compartments involved in several oxidative reactions.

The enzyme has an unusually high reaction rate: a single catalase molecule can process millions of hydrogen peroxide molecules per second. Its activity is also useful beyond human physiology. Microbiologists perform a catalase test by adding hydrogen peroxide to bacteria; bubbling indicates that oxygen is being released. In a striking natural example, bombardier beetles use catalase and related enzymes to help produce their hot defensive spray. Catalase was first noticed in 1818 and received its name in 1900.