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sidiansanke铜虫 (著名写手)
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请教过氧化氢酶和氢过氧化物酶
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| 过氧化氢酶（catalase peroxidase）和氢过氧化物酶（hydroperoxidase）有什么区别啊，谢谢大家！ |
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sidiansanke
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Catalase Vs Peroxidase
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catalase's function is to break down H2O2 into H2O and O2. H2O2 is a harmful product that must be broken down. Catalase is a member of the peroxidase family that specifically uses hydrogen peroxide as a substrate, hence it could be called "Hydrogen Peroxidase": however, so does Peroxidase (also called Myeloperoxidase). The major difference between Catalase and Peroxidase is that Catalase generates water and Oxygen, while Peroxidase generates water and an activated donor molecule. So "Hydrogen Peroxidase" could actually refer to more than one enzyme without identifying the more important aspect of the reaction: where the free radical goes. For this reason, the name "Hydrogen Peroxidase" is not accepted by the international scientific community. ----------------------------------------- http://en.wikipedia.org/wiki/Catalase Catalase is a common enzyme found in nearly all living organisms which are exposed to oxygen, where it functions to catalyze the decomposition of hydrogen peroxide to water and oxygen.[1] Catalase has one of the highest turnover numbers of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.[2] Catalase is a tetramer of four polypeptide chains, each over 500 amino acids long.[3] It contains four porphyrin heme (iron) groups that allow the enzyme to react with the hydrogen peroxide. The optimum pH for catalase is approximately 7,[4] while the optimum temperature varies by species.[5] Contents [hide] 1 History 2 Action of catalase 3 Molecular mechanism 4 Cellular role 5 Distribution among organisms 6 Applications 6.1 Catalase test 6.2 Gray hair 7 Pathology 8 See also 9 References 10 External links [edit] History Catalase was first noticed as a substance in 1811 when Louis Jacques Thénard, who discovered H2O2 (hydrogen peroxide), suggested that its breakdown is caused by a substance. In 1900, Oscar Loew was the first to give it the name catalase, and found its presence in many plants and animals.[6] In 1937 catalase from beef liver was crystallised by James B. Sumner[7] and the molecular weight worked out in 1938.[8] In 1969 the amino acid sequence of bovine catalase was worked out.[9] Then in 1981, the 3D structure of the protein was revealed.[10] [edit] Action of catalase The reaction of catalase in the decomposition of hydrogen peroxide is: 2 H2O2 → 2 H2O + O2 [edit] Molecular mechanism While the complete mechanism of catalase is not currently known, 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[11] Here Fe()-E represents the iron centre of the heme group attached to the enzyme. Fe(IV)-E(.+) ís a mesomeric form of Fe(V)-E, meaning that iron is not completely oxidized to +V but receives some "supporting electron" from the heme ligand. This heme has to be drawn then as radical cation (.+). As hydrogen peroxide enters the active site, it interacts with the amino acids Asn147 (asparagine at position 147) and His74, causing a proton (hydrogen ion) to transfer between the oxygen atoms. The free oxygen atom coordinates, freeing the newly-formed water molecule and Fe(IV)=O. Fe(IV)=O reacts with a second hydrogen peroxide molecule to reform Fe(III)-E and produce water and oxygen.[11] The reactivity of the iron center may be improved by the presence of the phenolate ligand of Tyr357 in the fifth iron ligand, which can assist in the oxidation of the Fe(III) to Fe(IV). The efficiency of the reaction may also be improved by the interactions of His74 and Asn147 with reaction intermediates.[11] In general, the rate of the reaction can be determined by the Michaelis-Menten equation.[12] Catalase can also oxidize different toxins, such as formaldehyde, formic acid, phenols, 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 inhibitor of catalase, strongly binding to the heme of catalase and stopping the enzyme's action. Three-dimensional 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. [edit] Cellular role Hydrogen peroxide is a harmful by-product of many normal metabolic processes: To prevent damage, 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.[13] The true biological significance of catalase is not always straightforward to assess: Mice genetically engineered to lack catalase are phenotypically normal, indicating that this enzyme is dispensable in animals under some conditions.[14] Some human beings have very low levels of catalase (acatalasia), yet show few ill effects. It is likely that the predominant scavengers of H2O2 in normal mammalian cells are peroxiredoxins rather than catalase. Human catalase works at an optimum temperature of 37°C,[15] which is approximately the temperature of the human body. In contrast, catalase isolated from the hyperthermophile archaea Pyrobaculum calidifontis has a temperature optimum of 90°C.[16] Catalase is usually located in a cellular organelle called the peroxisome.[17] Peroxisomes in plant cells are involved in photorespiration (the use of oxygen and production of carbon dioxide) and symbiotic nitrogen fixation (the breaking apart of diatomic nitrogen (N2) to reactive nitrogen atoms). Hydrogen peroxide is used as a potent antimicrobial agent when cells are infected with a pathogen. Pathogens that are catalase-positive, such as Mycobacterium tuberculosis, Legionella pneumophila, and Campylobacter jejuni, make catalase in order to deactivate the peroxide radicals, thus allowing them to survive unharmed within the host.[18] [edit] Distribution among organisms All known animals use catalase in every organ, with particularly high concentrations occurring in the liver. One unique use of catalase occurs in bombardier beetle. The beetle has two sets of chemicals ordinarily stored separately in its paired glands. The larger of the pair, the storage chamber or reservoir, contains hydroquinones and hydrogen peroxide, whereas the smaller of the pair, the reaction chamber, contains catalases and peroxidases. To activate the 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.[19] Catalase is also universal among plants, and many fungi are also high producers of the enzyme.[20] Very few aerobic microorganisms are known that do not use catalase. Streptococcus species are an example of aerobic bacteria that do not possess catalase. Catalase has also been observed in some anaerobic microorganisms, such as Methanosarcina barkeri.[21] ---------------------------------------------------------- Peroxidases (EC number 1.11.1.x) are a large family of enzymes. A majority of peroxidase protein sequences can be found in the PeroxiBase database. Peroxidases typically catalyze a reaction of the form: ROOR' + electron donor (2 e-) + 2H+ → ROH + R'OH For many of these enzymes the optimal substrate is hydrogen peroxide, but others are more active with organic hydroperoxides such as lipid peroxides. Peroxidases can contain a heme cofactor in their active sites, or redox-active cysteine or selenocysteine residues. The nature of the electron donor is very dependent on the structure of the enzyme. For example, horseradish peroxidase can use a variety of organic compounds as electron donors and acceptors. Horseradish peroxidase has an accessible active site, and many compounds can reach the site of the reaction. For an enzyme such as cytochrome c peroxidase, the compounds that donate electrons are very specific, because there is a very closed active site. While the exact mechanisms have yet to be elucidated, peroxidases are known to play a part in increasing a plant's defenses against pathogens.[1] Peroxidases are sometimes used as histological marker. Cytochrome c peroxidase is used as a soluble, easily purified model for cytochrome c oxidase. Glutathione peroxidase is a peroxidase found in humans, which contains selenocysteine. It uses glutathione as an electron donor and is active with both hydrogen peroxide and organic hydroperoxide substrates. Amyloid beta, when bound to heme, has been shown to have peroxidase activity.[2] A typical group of peroxidases are the haloperoxidases. This group is able to form reactive halogen species and, as a result, natural organohalogen substances. ------------------------------ [ Last edited by HarveyWang on 2009-4-10 at 02:39 ] |
4楼2009-04-10 02:35:15
