Catalase

By Jennifer McDowall

To view structure of catalase

The ability of organisms to use molecular oxygen was a major evolutionary breakthrough that enabled the production of significantly more energy from the breakdown of foods, amongst many other advantages. However, these advantages came at a cost: toxic by-products known as reactive oxygen species (ROS) are produced, which if left unchecked would seriously effect an organism’s viability. These ROS include hydrogen peroxide, superoxide anion radicals, singlet oxygen, hydroxyl radicals and nitric oxide. ROS serve as normal signalling molecules, but unchecked they can damage a wide variety of molecules within cells, leading to oxidative stress. In order to limit the crippling effects of oxidative stress, a cell can respond by committing suicide, whereby the ROS produced by a cell’s mitochondria can act as a trigger for apoptotic cell death through the activation of caspases. This is effective in the short-term, but high levels of oxidative stress can lead to serious tissue damage through excessive cell death and oxidative damage. Just how harmful these ROS can be is evidenced by the diseases they are involved in when their levels become too high, which include inflammatory joint disease (destruction of cartilage), insulin-dependent diabetes mellitus (destruction of pancreatic beta cells), asthma, cardiovascular disease, and many neurodegenerative diseases (destruction of nerve cells) including Alzheimer’s and amyotrophic lateral sclerosis (ALS). To help protect against the destructive effects of ROS, aerobic organisms produce protective antioxidant enzymes such as catalase (EC 1.11.1.6), superoxide dismutase (EC 1.15.1.1), and glutathione peroxidase (EC 1.11.1.9). It was the evolution of these enzymes that made oxidative cellular metabolism possible.

Catalases, Protective Enzymes

Catalases are produced by aerobic organisms ranging from bacteria to man. Catalases (EC 1.11.1.6) are haem-containing proteins that catalyse the conversion of hydrogen peroxide (H₂O₂) to water and molecular oxygen, thereby protecting cells from the toxic effects of hydrogen peroxide:

2H₂O₂ à 2H₂O + O₂

Some haem-containing catalases are bifunctional, acting as a catalase and a peroxidase (EC 1.11.1.7). In these bifunctional catalase-peroxidases, a variety of organic substances can be used as a hydrogen donor, for example alcohol, which can be oxidised in the liver. These bifunctional catalases are closely related to plant peroxidases. There are also non-haem manganese-containing catalases, which occur in bacteria. This review concentrates on the mono-functional, haem-containing catalases (EC 1.11.1.6).

Next: How Catalase Works