Catalase K-12 Experiments
Catalase
Catalase (human erythrocyte catalase: PDB 1DGF, EC 1.11.1.6) is a common enzyme found in living organisms. Its functions include catalyzing the decomposition of hydrogen peroxide to water and oxygen. Catalase has one of the highest turnover rates for all enzymes; one molecule of catalase can convert 83,000 molecules of hydrogen peroxide to water and oxygen per second. Catalase is a tetramer of 4 polypeptide chains which are at least 500 amino acids in length. Within this tetramer there are 4 porphyrin heme (iron) groups which are what allows it to react with the hydrogen peroxide. Its optimum pH is at a neutral level. The reaction of catalase in the decomposition of hydrogen peroxide is:
The catalase test is done by placing a drop of hydrogen peroxide on a slide. Using an applicator stick, touch the colony and then smear into the hydrogen peroxide. If bubbles or froth form, the organism is catalase positive and if no bubbles or froth appear, then the organism is catalase negative. The catalase test alone cannot identify a particular organism, but with other tests, it can aid diagnosis. The presence of catalase in bacterial cells depends on both the growth condition and the medium used to grow the cells.
Role in organismsHydrogen peroxide is formed as a waste product of metabolism in many living organisms. It is toxic and must be quickly converted into other, less dangerous, chemicals. To manage this problem, the enzyme catalase is frequently used to rapidly catalyse the decomposition of hydrogen peroxide into harmless oxygen gas and water. Role in pathogenesisHydrogen peroxide is used as a potent antimicrobial agent when cells are infected with a pathogen. Pathogens that are catalase positive make catalase in order to deactivate the peroxide radicals, thus allowing them to survive unharmed in the host cell. Human applicationsCatalase 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. Molecular mechanismThe complete mechanism of catalase is not yet known; however, the reaction occurs in two stages:
(Where Fe-E represents the iron centre of the heme group attached to the enzyme.) As hydrogen peroxide enters the active site it is forced to interact with the amino acids His74 and Asn174. This causes a proton (hydrogen ion) to transfer from the first oxygen to the second, polarizing and stretching the O-O bond, which breaks heterolytically. The free oxygen atom coordinates with the iron centre of the active site, displacing the newly formed water molecule and forming Fe(IV)=O. In the second stage, the Fe(IV)=O reacts with another hydrogen peroxide to reform Fe(III)-E plus water and oxygen molecules. Catalase can also oxidize different toxins, such as formaldehyde, formic acid, and alcohols. In doing so, it uses hydrogen peroxide according to the following reaction: H2O2 + H2R → 2H2O + R Again, the exact mechanism of this reaction is not known. Any heavy metal ion (such as Copper cations in Copper(II) sulfate) will act as a noncompetitive inhibitor on catalase. Also, the poison cyanide is a competitive inhibitors of catalase, strongly binding to the haem of catalase and stopping the enzyme's action. 3D protein structures of the peroxidated catalase intermediates are available at the Protein Data Bank. This enzyme is commonly used in laboratories as a tool for learning the effect of enzymes upon reaction rates. See alsoExternal links
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