All Topics  
Catalase

 

 
   Email Print
   Bookmark   Link
 

 

Catalase
 
 
  Topics Catalase Topic Home

Catalase is a common enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
 found in nearly all living organisms which are exposed to oxygen, where it functions to catalyze the decomposition of hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
 to water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 and oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
. Catalase has one of the highest turnover number
Turnover number

Turnover number has two related meanings:In enzymology, turnover number is defined as the maximum number of molecules of Substrate that an enzyme can convert to product per catalytic site per unit of time and can be calculated as follows: kcat = Vmax/[E]T ....
s of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.

Catalase is a tetramer
Tetramer

A tetramer is a protein with four subunits . There are homo-tetramers such as glutathione S-transferase or single-strand binding protein, dimers of hetero-dimers such as haemoglobin , and hetero-tetramers, where each subunit is different....
 of four polypeptide chains, each over 500 amino acids long.






Discussion
Ask a question about 'Catalase'
Start a new discussion about 'Catalase'
Answer questions from other users
Full Discussion Forum



Recent Posts

Which fruit has the most catalase in it?








Encyclopedia


Catalase is a common enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
 found in nearly all living organisms which are exposed to oxygen, where it functions to catalyze the decomposition of hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
 to water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 and oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
. Catalase has one of the highest turnover number
Turnover number

Turnover number has two related meanings:In enzymology, turnover number is defined as the maximum number of molecules of Substrate that an enzyme can convert to product per catalytic site per unit of time and can be calculated as follows: kcat = Vmax/[E]T ....
s of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.

Catalase is a tetramer
Tetramer

A tetramer is a protein with four subunits . There are homo-tetramers such as glutathione S-transferase or single-strand binding protein, dimers of hetero-dimers such as haemoglobin , and hetero-tetramers, where each subunit is different....
 of four polypeptide chains, each over 500 amino acids long. It contains four porphyrin
Porphyrin

Porphyrins are a group of chemical compounds of which many occur in nature, such as in green leaves and red blood cells, and in bio-inspired synthetic catalysts and devices....
 heme
Heme

A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin....
 (iron) groups that allow the enzyme to react with the hydrogen peroxide. The optimum pH
PH

pH is a measure of the Acid or Base of a solution. It is defined as the cologarithm of the Activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations....
 for catalase is approximately 7, while the optimum temperature varies by species.

History


Catalase was first noticed as a substance in 1811 when Louis Jacques Thénard
Louis Jacques Thénard

Louis Jacques Th?nard , was a France chemist.His father, a poor peasant, managed to have him educated at the academy of Sens, and sent him at the age of sixteen to study pharmacy in Paris....
, who discovered H2O2 (hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
), suggested that its breakdown is caused by a substance. In 1900, Oscar Loew
Oscar Loew

Oscar Loew was a German people Agriculture chemist.Loew served as instructor at Tokyo Imperial University between 1893-1907, succeeding Oskar Kellner as professor of agricultural chemistry....
 was the first to give it the name catalase, and found its presence in many plants and animals. In 1937 catalase from beef liver was crystallised by James B. Sumner
James B. Sumner

James Batcheller Sumner was an American chemist. He shared the Nobel Prize in Chemistry in 1946 with John Howard Northrop and Wendell Meredith Stanley....
 and the molecular weight worked out in 1938.

In 1969 the amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 sequence of bovine catalase was worked out. Then in 1981, the 3D structure of the protein was revealed.

Action of catalase


The reaction of catalase in the decomposition of hydrogen peroxide is:

2 H2O2 ? 2 H2O + O2


Molecular mechanism


While the complete mechanism of catalase is not currently known, the reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 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


Here Fe-E represents the iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 centre of the heme
Heme

A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin....
 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
Active site

The active site of an enzyme contains the catalysis and binding sites. The structure and chemical properties of the active site allow the recognition and binding of the substrate ....
, it interacts with the amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
s Asn147 (asparagine
Asparagine

Asparagine is one of the 20 most common natural amino acids on Earth. It has carboxamide as the side chain's functional group. It is not an essential amino acid....
 at position 147) and His74
Histidine

Histidine is one of the 20 standard amino acids present in proteins. In the nutritional sense, in humans, histidine is considered an essential amino acid, but only in children....
, causing a proton
Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
 (hydrogen ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
) 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. The reactivity of the iron center may be improved by the presence of the phenolate ligand
Ligand

In chemistry, a ligand is either an atom, ion, or molecule that bonds to a central metal, generally involving formal donation of one or more of its electrons....
 of Tyr357
Tyrosine

Tyrosine or 4-hydroxyphenylalanine, is one of the 20 amino acids that are used by cell to protein biosynthesis proteins. This is a non-essential amino acid and it is found in casein....
 in the fifth iron ligand
Ligand

In chemistry, a ligand is either an atom, ion, or molecule that bonds to a central metal, generally involving formal donation of one or more of its electrons....
, 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. In general, the rate of the reaction can be determined by the Michaelis-Menten equation.

Catalase can also oxidize different toxins, such as formaldehyde
Formaldehyde

Formaldehyde is a chemical compound with the chemical formula H2CO. It is the simplest aldehyde. Formaldehyde exists in several forms aside from H2CO: the cyclic trimer trioxane and the polymer Polyoxymethylene....
, formic acid
Formic acid

Formic acid is the simplest carboxylic acid. Its formula is hydrogencarbonoxygenOH or CH2O2. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stingers....
, phenols
Phenols

In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl Functional group attached to an aromatic hydrocarbon group....
, and alcohols. In doing so, it uses hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
 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
Copper(II) sulfate

Copper sulfate is the chemical compound with the chemical formula CopperSulfurOxygen4. This salt exists as a series of compounds that differ in their degree of water of crystallization....
) will act as a noncompetitive inhibitor on catalase. Also, the poison cyanide
Cyanide

A cyanide is any chemical compound that contains the nitrile , which consists of a carbon atom chemical bond to a nitrogen atom. Inorganic cyanides are hydrogen cyanide salts in which cyanide is generally the anion CN-....
 is a competitive inhibitor of catalase, strongly binding to the heme
Heme

A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin....
 of catalase and stopping the enzyme's action.

Three-dimensional protein structure
Protein structure

Proteins are an important class of biological macromolecules present in all biological organisms, made up of such chemical element as carbon,hydrogen, nitrogen, oxygen, and sulphur....
s of the peroxidated catalase intermediates are available at the Protein Data Bank
Protein Data Bank

The Protein Data Bank is a repository for the 3-D structural data of large biological molecules, such as proteins and nucleic acids. . The data, typically obtained by X-ray crystallography or Protein NMR and submitted by biologists and biochemistry from around the world, are released into the public domain, and can be accessed at no charge...
. This enzyme is commonly used in laboratories as a tool for learning the effect of enzymes upon reaction rates.

Cellular role


Hydrogen peroxide is a harmful by-product of many normal metabolic
Metabolism

Metabolism is the set of chemical reactions that occur in living organisms in order to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments....
 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
Chemical decomposition

Chemical decomposition or analysis is the separation of a chemical compound into chemical element or smaller compounds. It is sometimes defined as the opposite of a chemical synthesis....
 of hydrogen peroxide into less reactive gas
Gas

In physics, a gas is a state of matter, consisting of a collection of particles without a definite shape or volume that are in more or less random motion....
eous oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
 and water molecules.

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. Some human beings have very low levels of catalase (acatalasia
Acatalasia

Acatalasia is an autosomal recessive peroxisomal disorder caused by a complete lack of catalase....
), 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, which is approximately the temperature of the human body. In contrast, catalase isolated from the hyperthermophile
Hyperthermophile

A hyperthermophile is an organism that thrives in extremely hot environments? from 60 degrees C upwards. An optimal temperature for the existence of Hyperthermophiles are above 80?C ....
 archaea
Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . Archaea, like bacteria, are prokaryotic....
 Pyrobaculum calidifontis has a temperature optimum of 90°C.

Catalase is usually located in a cellular organelle
Organelle

In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
 called the peroxisome
Peroxisome

Peroxisomes are organelles from the Microbody family and are present in almost any eukaryote cell. They participate in the metabolism of fatty acids and many other metabolites....
. Peroxisomes in plant cells are involved in photorespiration
Photorespiration

Photorespiration is the alternate pathway for production of glyceraldehyde 3-phosphate by RuBisCO, the main enzyme of the light-independent reactions of photosynthesis ....
 (the use of oxygen and production of carbon dioxide) and symbiotic nitrogen fixation
Nitrogen fixation

Nitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form in the Earth's atmosphere and converted into nitrogen compounds ....
 (the breaking apart of diatomic
Diatomic

Diatomic molecules are molecules made only of two atoms, of either the same or different chemical elements. The prefix di- means two in Greek....
 nitrogen
Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
 (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
Mycobacterium tuberculosis

Mycobacterium tuberculosis is a pathogenic bacterial species in the genus Mycobacterium and the causative agent of most cases of tuberculosis....
, Legionella pneumophila
Legionella pneumophila

Legionella pneumophila is a thin, Wiktionary:pleomorphism, flagellatedGram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogen in this group and is the causative agent of legionellosis or Legionnaires' disease....
, and Campylobacter jejuni
Campylobacter jejuni

Campylobacter jejuni is a species of curved, rod-shaped, non-spore forming, Gram-negative microaerophilic, bacteria commonly found in animal feces....
, make catalase in order to deactivate the peroxide radicals, thus allowing them to survive unharmed within the host
Host (biology)

In biology, a host is an organism that harbors a virus or parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter....
.

Distribution among organisms


All known animals use catalase in every organ
Organ (anatomy)

In biology, an organ is a biological tissue that performs a specific function or group of functions. Usually there is a main tissue and sporadic tissues....
, with particularly high concentrations occurring in the liver
Liver

The liver is a vital organ present in vertebrates and some other animals; it has a wide range of functions, a few of which are detoxification, protein synthesis, and production of biochemicals necessary for digestion....
. One unique use of catalase occurs in bombardier beetle
Bombardier beetle

Bombardier Beetles are ground beetles in the Tribe s Brachinini, Paussini, Ozaenini, or Metriini?more than 500 species altogether?that are most notable for the defense mechanism that gives them their name: When disturbed, the beetle ejects a noxious chemical spray in a rapid burst of pulses from special glands in its abdomen....
. 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 hydroquinone
Hydroquinone

Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound which is a type of Phenols, having the chemical formula C6H42....
s and hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
, whereas the smaller of the pair, the reaction chamber, contains catalases and peroxidase
Peroxidase

Peroxidases 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:...
s. 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.

Catalase is also universal among plants, and many fungi are also high producers of the enzyme.

Very few aerobic microorganisms are known that do not use catalase. Streptococcus
Streptococcus

Streptococcus is a genus of sphere Gram-positive bacterium belonging to the phylum Firmicutes and the lactic acid bacteria group. Cell division occurs along a single Coordinate axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek language st?ept?? streptos, meaning easily bent or twisted,...
 species are an example of aerobic bacteria that do not possess catalase. Catalase has also been observed in some anaerobic
Anaerobic

Anaerobic is a technical word which literally means without air , as opposed to aerobic .In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen....
 microorganisms, such as Methanosarcina barkeri.

Applications

Hydrogen Peroxide 2d
Catalase is used in the food industry for removing hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
 from milk
Milk

Milk is an opaque white liquid produced by the mammary glands of female mammals . It provides the primary source of nutrition for newborn mammals before they are able to digestion other types of food....
 prior to cheese
Cheese

Cheese is a food consisting of proteins and fat from milk, usually the milk of cattle, Water Buffalo, goats, or sheep's milk. It is produced by Coagulation of the milk protein casein....
 production. Another use is in food wrappers where it prevents food from oxidizing. Catalase is also used in the textile
Textile

A textile is a flexible material consisting of a network of natural or artificial fibres often referred to as thread or yarn. Yarn is produced by Spinning raw wool fibres, linen, cotton, or other material on a spinning wheel to produce long strands known as yarn....
 industry, removing hydrogen peroxide from fabrics to make sure the material is peroxide-free.

A minor use is in contact lens
Contact lens

A contact lens is a corrective lens, cosmetics, or therapeutic lens usually placed on the cornea of the eye. Modern soft contact lenses were invented by the Czech Republic chemists Otto Wichterle and Drahoslav L?m, who also invented the first gel used for their production....
 hygiene - a few lens-cleaning products disinfect
Disinfection

Disinfectants are antimicrobial agents that are applied to non-living objects to destroy microorganisms, the process of which is known as disinfection....
 the lens using a hydrogen peroxide solution; a solution containing catalase is then used to decompose the hydrogen peroxide before the lens is used again. Recently, catalase has also begun to be used in the aesthetics industry. Several mask treatments combine the enzyme with hydrogen peroxide on the face with the intent of increasing cellular oxygenation in the upper layers of the epidermis
Epidermis (skin)

The epidermis is the outermost layer of the skin, composed of terminally differentiated stratified squamous epithelium, acting as the body's major barrier against an inhospitable environment....
.

Catalase test


The catalase test is also one of the main three tests used by microbiologists to identify species of bacteria. The presence of catalase enzyme in the test isolate is detected using hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
. If the bacteria possess catalase (i.e. are catalase positive), when a small amount of bacterial isolate is added to hydrogen peroxide
Hydrogen peroxide

Hydrogen peroxide is a very pale blue liquid which appears colorless in a dilute solution, slightly more viscous than water. It is a weak acid....
, bubbles of oxygen are observed.

In microbiology
Microbiology

Microbiology is the study of microorganisms, which are unicellular or cell-cluster microscopic organisms. This includes eukaryote such as fungi and protists, and prokaryotes, which are bacteria and archaea....
, the catalase test is used to differentiate between bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
l species
Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring....
 in the lab. The test is done by placing a drop of hydrogen peroxide on a microscope slide
Microscope slide

A microscope slide was originally a 'slider' made of ivory or bone, containing specimens held between disks of transparent mica. These were popular in Victorian era England until the Royal Microscopical Society introduced the standardized microscope slide in the form of a thin sheet of glass used to hold objects for examination under a micro...
. Using an applicator stick, a scientist touches the colony and then smears a sample into the hydrogen peroxide drop.

  • If bubbles or froth forms, the organism is said to be catalase-positive. Staphylococci
    Staphylococcus

    Staphylococcus is a genus of Gram-positive Bacterium. Under the microscope they appear round , and form in grape-like clusters.The Staphylococcus genus include just thirty-three species....
     and micrococci
    Micrococcus

    Micrococcus is a genus of bacteria in the Micrococcaceae family . Micrococcus occurs in a wide range of environments, including water, dust, and soil....
     are catalase-positive.


  • If not, the organism is catalase-negative. Streptococci
    Streptococcus

    Streptococcus is a genus of sphere Gram-positive bacterium belonging to the phylum Firmicutes and the lactic acid bacteria group. Cell division occurs along a single Coordinate axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek language st?ept?? streptos, meaning easily bent or twisted,...
     and enterococci
    Enterococcus

    Enterococcus is a genus of lactic acid bacteria of the Phylum Firmicutes. Members of this genus were classified as Group D Streptococcus until 1984 when genomic DNA analysis indicated that a separate genus classification would be appropriate....
     are catalase-negative.


While the catalase test alone cannot identify a particular organism, combined 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.

Gray hair


According to recent scientific studies, low levels of catalase may play a role in the graying process of human hair. Hydrogen peroxide is naturally produced by the body and catalase breaks it down. If there is a dip in catalase levels, hydrogen peroxide cannot be broken down. This causes the hydrogen peroxide to bleach the hair from the inside out. Scientists believe this finding may someday lead to "anti" greying treatments for aging hair.

Pathology


The peroxisomal disorder
Peroxisomal disorder

Peroxisomal disorders are a class of conditions which lead to disorders of lipid metabolism....
 acatalasia
Acatalasia

Acatalasia is an autosomal recessive peroxisomal disorder caused by a complete lack of catalase....
 is due to a deficiency in the function of catalase.

See also


  • Enzyme kinetics
    Enzyme kinetics

    Enzyme kinetics is the study of the chemical reactions that are catalyst by enzymes, with a focus on their reaction rates. The study of an enzyme's chemical kinetics reveals the catalytic mechanism of this enzyme, its role in metabolism, how its activity is controlled, and how a drug or a poison might enzyme inhibitor the enzyme....
  • Peroxidases
  • Superoxide dismutase
    Superoxide dismutase

    The enzyme superoxide dismutase , catalyzes the dismutation of superoxide into oxygen and hydrogen peroxide. As such, it is an important antioxidant defense in nearly all cells exposed to oxygen....


External links