Catalase
Catalase is a common enzyme found in nearly all living organisms exposed to oxygen (such as bacteria, plants, and animals) which catalyzes the decomposition of hydrogen peroxide to water and oxygen.[5] It is a very important enzyme in protecting the cell from oxidative damage by reactive oxygen species (ROS). Catalase has one of the highest turnover numbers of all enzymes; one catalase molecule can convert millions of hydrogen peroxide molecules to water and oxygen each second.[6]
Catalase
Catalase
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[edit]
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[edit]
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 amino acid sequence of bovine catalase was determined in 1969,[16] and the three-dimensional structure in 1981.[17]
Distribution among organisms[edit]
The large majority of known organisms use catalase in every organ, with particularly high concentrations occurring in the liver in mammals.[33] Catalase is found primarily in peroxisomes and the cytosol of erythrocytes (and sometimes in mitochondria[34])
Almost all aerobic microorganisms use catalase. It is also present in some anaerobic microorganisms, such as Methanosarcina barkeri.[35] Catalase is also universal among plants and occurs in most fungi.[36]
One unique use of catalase occurs in the bombardier beetle. This beetle has two sets of liquids that are stored separately in two paired glands. The larger of the pair, the storage chamber or reservoir, contains hydroquinones and hydrogen peroxide, while the smaller, the reaction chamber, contains catalases and peroxidases. To activate the noxious spray, the beetle mixes the contents of the two compartments, causing oxygen to be liberated from hydrogen peroxide. The oxygen oxidizes the hydroquinones and also acts as the propellant.[37] The oxidation reaction is very exothermic (ΔH = −202.8 kJ/mol) and rapidly heats the mixture to the boiling point.[38]
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. Poikilothermic animals typically have catalases with optimum temperatures in the range of 15-25 °C, while mammalian or avian catalases might have optimum temperatures above 35 °C,[40][41] and catalases from plants vary depending on their growth habit.[40] In contrast, catalase isolated from the hyperthermophile archaeon Pyrobaculum calidifontis has a temperature optimum of 90 °C.[42]
Interactions[edit]
Catalase has been shown to interact with the ABL2[57] and Abl genes.[57] Infection with the murine leukemia virus causes catalase activity to decline in the lungs, heart and kidneys of mice. Conversely, dietary fish oil increased catalase activity in the heart, and kidneys of mice.[58]
Methods for determining catalase activity[edit]
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]