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 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).

Living with oxygen is dangerous. We rely on oxygen to power our cells, but oxygen is a reactive molecule that can cause serious problems if not carefully controlled. One of the dangers of oxygen is that it is easily converted into other reactive compounds. Inside our cells, electrons are continually shuttled from site to site by carrier molecules, such as carriers derived from riboflavin and niacin. If oxygen runs into one of these carrier molecules, the electron may be accidentally transferred to it. This converts oxygen into dangerous compounds such as superoxide radicals and hydrogen peroxide, which can attack the delicate sulfur atoms and metal ions in proteins. To make things even worse, free iron ions in the cell occasionally convert hydrogen peroxide into hydroxyl radicals. These deadly molecules attack and mutate DNA. One theory, still controversial, is that this type of oxidative damage accumulates over the years of our life, causing us to age.

Antioxidants to the Rescue

Fortunately, cells make a variety of antioxidant enzymes to fight the dangerous side-effects of life with oxygen. Two important players are superoxide dismutase, which converts superoxide radicals into hydrogen peroxide, and catalase, which converts hydrogen peroxide into water and oxygen gas. The importance of these enzymes is demonstrated by their prevalence, ranging from about 0.1% of the protein in an Escherichia coli cell to upwards of a quarter of the protein in susceptible cell types. These many catalase molecules patrol the cell, counteracting the steady production of hydrogen peroxide and keeping it at a safe level.

Catalases are some of the most efficient enzymes found in cells. Each catalase molecule can decompose millions of hydrogen peroxide molecules every second. The cow catalase shown here (PDB entry 8cat) and our own catalases use an iron ion to assist in this speedy reaction. The enzyme is composed of four identical subunits, each with its own active site buried deep inside. The iron ion, shown in green, is gripped at the center of a disk-shaped heme group. Catalases, since they must fight against reactive molecules, are also unusually stable enzymes. Notice how the four chains interweave, locking the entire complex into the proper shape.

MODE OF ACTION:

Most catalases exist as tetramers of 60 or 75 kDa, each subunit containing an active site haem group buried deep within the structure, but which is accessible from the surface through hydrophobic channels. The very rigid, stable structure of catalases is resistant to unfolding, which makes them uniquely stable enzymes that are more resistant to pH, thermal denaturation and proteolysis than most other enzymes. Their stability and resistance to proteolysis is an evolutionary advantage, especially since they are produced during the stationary phase of cell growth when levels of proteases are high and there is a rapid rate of protein turnover.

Haem-containing catalases break down hydrogen peroxide by a two-stage mechanism in which hydrogen peroxide alternately oxidises and reduces the haem iron at the active site. In the first step, one hydrogen peroxide molecule oxidises the haem to an oxyferryl species. In the second step, a second hydrogen peroxide molecule is used as a reductant to regenerate the enzyme, producing water and oxygen. Some catalases contain NADPH as a cofactor, which functions to prevent the formation of an inactive compound. Catalases may have another role: the generation of ROS, possibly hydroperoxides, upon UVB irradiation. In this way, UVB light can be detoxified through the generation of hydrogen peroxide, which can then be degraded by the catalase. NADPH may play a role in providing the electrons needed to reduce molecular oxygen in the production of ROS.

Much of the hydrogen peroxide that is produced during oxidative cellular metabolism comes from the breakdown of one of the most damaging ROS, namely the superoxide anion radical (O2-). Superoxide is broken down by superoxide dismutases into hydrogen peroxide and oxygen. Superoxide is so damaging to cells that mutations in the superoxide dismutase enzyme can lead to ALS, which is characterised by the loss of motoneurons in the spinal cord and brain stem, possibly involving the activation of caspase-12 and the apoptosis cascade via oxidative stress.

Applications Of Catalase:

Catalase is also used in the textile industry, removing hydrogen peroxide from fabrics to make sure the material is peroxide-free. A minor use is in contact lens hygiene - some lens-cleaning systems disinfect the lenses by soaking them in a hydrogen peroxide solution, and catalase is used to decompose the peroxide before reinserting the lenses in the eye. Recently, catalase has begun to be used in the aesthetics industry in mask treatments combining the enzyme with hydrogen peroxide on the face to increase cellular oxygenation of cells in the upper layers of the epidermis.

3D structure of Catalase