Catalase

Catalase is a common

Catalase is a tetramer of four polypeptide chains, each over 500 amino acids long.^[7] It contains four iron-containing heme groups that allow the enzyme to react with hydrogen peroxide. The optimum pH for human catalase is approximately 7,^[8] and has a fairly broad maximum: the rate of reaction does not change appreciably between pH 6.8 and 7.5.^[9] The pH optimum for other catalases varies between 4 and 11 depending on the species.^[10] The optimum temperature also varies by species.^[11]

Structure

Human catalase forms a tetramer composed of four subunits, each of which can be conceptually divided into four domains.^[12] The extensive core of each subunit is generated by an eight-stranded antiparallel β-barrel (β1-8), with nearest neighbor connectivity capped by β-barrel loops on one side and α9 loops on the other.^[12] A helical domain at one face of the β-barrel is composed of four C-terminal helices (α16, α17, α18, and α19) and four helices derived from residues between β4 and β5 (α4, α5, α6, and α7).^[12] Alternative splicing may result in different protein variants.

History

Catalase was first noticed in 1818 by Louis Jacques Thénard, who discovered hydrogen peroxide (H₂O₂). Thénard suggested its breakdown was caused by an unknown substance. In 1900, Oscar Loew was the first to give it the name catalase, and found it in many plants and animals.^[13] In 1937 catalase from beef liver was crystallized by James B. Sumner and Alexander Dounce^[14] and the molecular weight was measured in 1938.^[15]

The

While the complete mechanism of catalase is not currently known,[18] the reaction is believed to occur in two stages:

H₂O₂ + Fe(III)-E → H₂O + O=Fe(IV)-E(.+)

H₂O₂ + O=Fe(IV)-E(.+) → H₂O + Fe(III)-E + O₂^[18]

Here Fe()-E represents the iron center of the heme group attached to the enzyme. Fe(IV)-E(.+) is a mesomeric form of Fe(V)-E, meaning the iron is not completely oxidized to +V, but receives some stabilising electron density from the heme ligand, which is then shown as a radical cation (.+).

As hydrogen peroxide enters the

Cellular role

Hydrogen peroxide is a harmful byproduct of many normal metabolic processes; to prevent damage to cells and tissues, it must be quickly converted into other, less dangerous substances. To this end, catalase is frequently used by cells to rapidly catalyze the decomposition of hydrogen peroxide into less-reactive gaseous oxygen and water molecules.^[24]

Mice genetically engineered to lack catalase are initially phenotypically normal.

Like alcohol dehydrogenase, catalase converts ethanol to acetaldehyde, but it is unlikely that this reaction is physiologically significant.^[32]

Distribution among organisms

The large majority of known organisms use catalase in every

Long-lived queens of the termite Reticulitermes speratus have significantly lower oxidative damage to their DNA than non-reproductive individuals (workers and soldiers).^[39] Queens have more than two times higher catalase activity and seven times higher expression levels of the catalase gene RsCAT1 than workers.^[39] It appears that the efficient antioxidant capability of termite queens can partly explain how they attain longer life.

Catalase enzymes from various species have vastly differing optimum temperatures.

Catalase is used in the food industry for removing

The catalase test is one of the three main tests used by microbiologists to identify species of bacteria. If the bacteria possess catalase (i.e., are catalase-positive), bubbles of oxygen are observed when a small amount of bacterial isolate is added to hydrogen peroxide. The catalase test is done by placing a drop of hydrogen peroxide on a microscope slide. An applicator stick is touched to the colony, and the tip is then smeared onto the hydrogen peroxide drop.

• If the mixture produces bubbles or froth, the organism is said to be 'catalase-positive'.
  Cryptococcus, and Rhodococcus equi
  .
• If not, the organism is 'catalase-negative'. Streptococcus^[49] and Enterococcus spp. are catalase-negative.

While the catalase test alone cannot identify a particular organism, it can aid identification when combined with other tests such as antibiotic resistance. The presence of catalase in bacterial cells depends on both the growth condition and the medium used to grow the cells.

Acatalasia is a condition caused by homozygous mutations in CAT, resulting in a lack of catalase. Symptoms are mild and include oral ulcers. A heterozygous CAT mutation results in lower, but still present catalase.^[53]

Gray hair

Low levels of catalase may play a role in the graying process of human hair. Hydrogen peroxide is naturally produced by the body and broken down by catalase. Hydrogen peroxide can accumulate in hair follicles and if catalase levels decline, this buildup can cause oxidative stress and graying.^[54] These low levels of catalase are associated with old age. Hydrogen peroxide interferes with the production of melanin, the pigment that gives hair its color.^[55][56]

Interactions

Catalase has been shown to

In 1870, Schoenn discovered a formation of yellow color from the interaction of hydrogen peroxide with molybdate;[59] then, from the middle of the 20th century, this reaction began to be used for colorimetric determination of unreacted hydrogen peroxide in the catalase activity assay.^[60] The reaction became widely used after publications by Korolyuk et al. (1988)^[61] and Goth (1991).^[62] The first paper describes serum catalase assay with no buffer in the reaction medium; the latter describes the procedure based on phosphate buffer as a reaction medium. Since phosphate ion reacts with ammonium molybdate,^[62] the use of MOPS buffer as a reaction medium is more appropriate.^[63]

Direct UV measurement of the decrease in the concentration of hydrogen peroxide is also widely used after the publications by Beers & Sizer^[64] and Aebi.^[65]

See also

• Enzyme kinetics
• Glutathione peroxidase
• Peroxidase
• Superoxide dismutase

References