When mitochondria are overloaded with more energy than they can handle, they begin making increasing amounts of the free radical superoxide. Superoxide carries out important signaling roles. Among them, it directs excess energy into fat synthesis. But it can also wreak havoc on the cell by forming oxidants that can damage vulnerable proteins, lipids, and other important molecules. Thus, a manganese-dependent enzyme called superoxide dismutase converts it into hydrogen peroxide. Hydrogen peroxide can also damage important molecules, but increasing evidence suggests it also regulates the activity of hundreds of proteins by controlling several “redox switches,” including glutathione.