Superoxide dismutase

Superoxide dismutase (SOD, EC is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide (O2) radical into either ordinary molecular oxygen (O2) or hydrogen peroxide (H2O2). Superoxide is produced as a by-product of oxygen metabolism and, if not regulated, causes many types of cell damage.[2] Hydrogen peroxide is also damaging and is degraded by other enzymes such as catalase. Thus, SOD is an important antioxidant defense in nearly all living cells exposed to oxygen. One exception is Lactobacillus plantarum and related lactobacilli, which use a different mechanism to prevent damage from reactive (O2).

Superoxide dismutase
Superoxide dismutase 2 PDB 1VAR
Structure of a human Mn superoxide dismutase 2 tetramer.[1]
EC number1.15.1.1
CAS number9054-89-1
IntEnzIntEnz view
ExPASyNiceZyme view
MetaCycmetabolic pathway
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO

Chemical reaction

SODs catalyze the disproportionation of superoxide:

2 HO2 → O2 + H2O2

In this way, O2 is converted into two less damaging species.

The pathway by which SOD-catalyzed dismutation of superoxide may be written, for Cu,Zn SOD, with the following reactions :

  • Cu2+-SOD + O2 → Cu+-SOD + O2 (reduction of copper; oxidation of superoxide)
  • Cu+-SOD + O2 + 2H+ → Cu2+-SOD + H2O2 (oxidation of copper; reduction of superoxide)

The general form, applicable to all the different metal-coordinated forms of SOD, can be written as follows:

  • M(n+1)+-SOD + O2 → Mn+-SOD + O2
  • Mn+-SOD + O2 + 2H+ → M(n+1)+-SOD + H2O2.

where M = Cu (n=1) ; Mn (n=2) ; Fe (n=2) ; Ni (n=2).

In a series of such reactions, the oxidation state and the charge of the metal cation oscillates between n and n+1: +1 and +2 for Cu, or +2 and +3 for the other metals .



Irwin Fridovich and Joe McCord at Duke University discovered the enzymatic activity of superoxide dismutase in 1968.[3] SODs were previously known as a group of metalloproteins with unknown function; for example, CuZnSOD was known as erythrocuprein (or hemocuprein, or cytocuprein) or as the veterinary anti-inflammatory drug "Orgotein".[4] Likewise, Brewer (1967) identified a protein that later became known as superoxide dismutase as an indophenol oxidase by protein analysis of starch gels using the phenazine-tetrazolium technique.[5]

There are three major families of superoxide dismutase, depending on the protein fold and the metal cofactor: the Cu/Zn type (which binds both copper and zinc), Fe and Mn types (which bind either iron or manganese), and the Ni type (which binds nickel).

2SOD ribbon colorPencil WhBkgd
Ribbon diagram of bovine Cu-Zn SOD subunit[6]
Crystal Structure of Human Manganese SOD
Active site of Human Manganese SOD, manganese shown in purple[7]
94-SuperoxideDismutase-Mn Fe 2mers
Mn-SOD vs Fe-SOD dimers
  • Copper and zinc – most commonly used by eukaryotes, including humans. The cytosols of virtually all eukaryotic cells contain an SOD enzyme with copper and zinc (Cu-Zn-SOD). For example, Cu-Zn-SOD available commercially is normally purified from bovine red blood cells. The bovine Cu-Zn enzyme is a homodimer of molecular weight 32,500. It was the first SOD whose atomic-detail crystal structure was solved, in 1975.[8] It is an 8-stranded "Greek key" beta-barrel, with the active site held between the barrel and two surface loops. The two subunits are tightly joined back-to-back, mostly by hydrophobic and some electrostatic interactions. The ligands of the copper and zinc are six histidine and one aspartate side-chains; one histidine is bound between the two metals.[9]
  • Iron Superoxide Dismutase Active Site
    Active site for iron superoxide dismutase
    Iron or manganese – used by prokaryotes and protists, and in mitochondria and chloroplasts
    • Iron – Many bacteria contain a form of the enzyme with iron (Fe-SOD); some bacteria contain Fe-SOD, others Mn-SOD, and some (such as E. coli) contain both. Fe-SOD can also be found in the chloroplasts of plants. The 3D structures of the homologous Mn and Fe superoxide dismutases have the same arrangement of alpha-helices, and their active sites contain the same type and arrangement of amino acid side-chains. They are usually dimers, but occasionally tetramers.
    • Manganese – Nearly all mitochondria, and many bacteria, contain a form with manganese (Mn-SOD): For example, the Mn-SOD found in human mitochondria. The ligands of the manganese ions are 3 histidine side-chains, an aspartate side-chain and a water molecule or hydroxy ligand, depending on the Mn oxidation state (respectively II and III).[10]
  • Nickel – prokaryotic. This has a hexameric (6-copy) structure built from right-handed 4-helix bundles, each containing N-terminal hooks that chelate a Ni ion. The Ni-hook contains the motif His-Cys-X-X-Pro-Cys-Gly-X-Tyr; it provides most of the interactions critical for metal binding and catalysis and is, therefore, a likely diagnostic of NiSODs.[11][12]
Iron Superoxide Dismutase Active Site
Active site for iron superoxide dismutase
Copper/zinc superoxide dismutase
1sdy CuZnSOD dimer ribbon
Yeast Cu,Zn superoxide dismutase dimer[13]
Iron/manganese superoxide dismutases, alpha-hairpin domain
1n0j H mit MnSOD D1 rib
Structure of domain1 (color), human mitochondrial Mn superoxide dismutase[10]
Iron/manganese superoxide dismutases, C-terminal domain
1n0j H mit MnSOD D2 rib
Structure of domain2 (color), human mitochondrial Mn superoxide dismutase[10]
Nickel superoxide dismutase
94-SuperoxideDismutase-Ni 6mer
Structure of Streptomyces Ni superoxide dismutase hexamer[12]

In higher plants, SOD isozymes have been localized in different cell compartments. Mn-SOD is present in mitochondria and peroxisomes. Fe-SOD has been found mainly in chloroplasts but has also been detected in peroxisomes, and CuZn-SOD has been localized in cytosol, chloroplasts, peroxisomes, and apoplast.[14][15]


Three forms of superoxide dismutase are present in humans, in all other mammals, and most chordates. SOD1 is located in the cytoplasm, SOD2 in the mitochondria, and SOD3 is extracellular. The first is a dimer (consists of two units), whereas the others are tetramers (four subunits). SOD1 and SOD3 contain copper and zinc, whereas SOD2, the mitochondrial enzyme, has manganese in its reactive centre. The genes are located on chromosomes 21, 6, and 4, respectively (21q22.1, 6q25.3 and 4p15.3-p15.1).

SOD1, soluble
2c9v CuZn rib n site
Crystal structure of the human SOD1 enzyme (rainbow-color N-terminus = blue, C-terminus = red) complexed with copper (orange sphere) and zinc (grey sphere).[16]
Alt. symbolsALS, ALS1
Other data
EC number1.15.1.1
LocusChr. 21 q22.1
SOD2, mitochondrial
Active site of human mitochondrial Mn superoxide dismutase (SOD2).[1]
Alt. symbolsMn-SOD; IPO-B; MVCD6
Other data
EC number1.15.1.1
LocusChr. 6 q25
SOD3, extracellular
Crystallographic structure of the tetrameric human SOD3 enzyme (cartoon diagram) complexed with copper and zinc cations (orange and grey spheres respectively).[17]
Alt. symbolsEC-SOD; MGC20077
Other data
EC number1.15.1.1
LocusChr. 4 pter-q21


In higher plants, superoxide dismutase enzymes (SODs) act as antioxidants and protect cellular components from being oxidized by reactive oxygen species (ROS).[18] ROS can form as a result of drought, injury, herbicides and pesticides, ozone, plant metabolic activity, nutrient deficiencies, photoinhibition, temperature above and below ground, toxic metals, and UV or gamma rays.[19][20] To be specific, molecular O2 is reduced to O2 (a ROS called superoxide) when it absorbs an excited electron released from compounds of the electron transport chain. Superoxide is known to denature enzymes, oxidize lipids, and fragment DNA.[19] SODs catalyze the production of O2 and H2O2 from superoxide (O2), which results in less harmful reactants.

When acclimating to increased levels of oxidative stress, SOD concentrations typically increase with the degree of stress conditions. The compartmentalization of different forms of SOD throughout the plant makes them counteract stress very effectively. There are three well-known and -studied classes of SOD metallic coenzymes that exist in plants. First, Fe SODs consist of two species, one homodimer (containing 1-2 g Fe) and one tetramer (containing 2-4 g Fe). They are thought to be the most ancient SOD metalloenzymes and are found within both prokaryotes and eukaryotes. Fe SODs are most abundantly localized inside plant chloroplasts, where they are indigenous. Second, Mn SODs consist of a homodimer and homotetramer species each containing a single Mn(III) atom per subunit. They are found predominantly in mitochondrion and peroxisomes. Third, Cu-Zn SODs have electrical properties very different from those of the other two classes. These are concentrated in the chloroplast, cytosol, and in some cases the extracellular space. Note that Cu-Zn SODs provide less protection than Fe SODs when localized in the chloroplast.[18][19][20]


Human white blood cells use enzymes such as NADPH oxidase to generate superoxide and other reactive oxygen species to kill bacteria. During infection, some bacteria (e.g., Burkholderia pseudomallei) therefore produce superoxide dismutase to protect themselves from being killed.[21]


SOD out-competes damaging reactions of superoxide, thus protecting the cell from superoxide toxicity. The reaction of superoxide with non-radicals is spin-forbidden. In biological systems, this means that its main reactions are with itself (dismutation) or with another biological radical such as nitric oxide (NO) or with a transition-series metal. The superoxide anion radical (O2) spontaneously dismutes to O2 and hydrogen peroxide (H2O2) quite rapidly (~105 M−1s−1 at pH 7). SOD is necessary because superoxide reacts with sensitive and critical cellular targets. For example, it reacts with the NO radical, and makes toxic peroxynitrite.

Because the uncatalysed dismutation reaction for superoxide requires two superoxide molecules to react with each other, the dismutation rate is second-order with respect to initial superoxide concentration. Thus, the half-life of superoxide, although very short at high concentrations (e.g., 0.05 seconds at 0.1mM) is actually quite long at low concentrations (e.g., 14 hours at 0.1 nM). In contrast, the reaction of superoxide with SOD is first order with respect to superoxide concentration. Moreover, superoxide dismutase has the largest kcat/KM (an approximation of catalytic efficiency) of any known enzyme (~7 x 109 M−1s−1),[22] this reaction being limited only by the frequency of collision between itself and superoxide. That is, the reaction rate is "diffusion-limited".

The high efficiency of superoxide dismutase seems necessary: even at the subnanomolar concentrations achieved by the high concentrations of SOD within cells, superoxide inactivates the citric acid cycle enzyme aconitase, can poison energy metabolism, and releases potentially toxic iron. Aconitase is one of several iron-sulfur-containing (de)hydratases in metabolic pathways shown to be inactivated by superoxide.[23]

Stability and folding mechanism

SOD1 is an extremely stable protein. In the holo form (both copper and zinc bound) the melting point is > 90 °C. In the apo form (no copper or zinc bound) the melting point is ~ 60 °C.[24] By differential scanning calorimetry (DSC), holo SOD1 unfolds by a two-state mechanism: from dimer to two unfolded monomers.[24] In chemical denaturation experiments, holo SOD1 unfolds by a three-state mechanism with observation of a folded monomeric intermediate.[25]


Superoxide is one of the main reactive oxygen species in the cell. As a consequence, SOD serves a key antioxidant role. The physiological importance of SODs is illustrated by the severe pathologies evident in mice genetically engineered to lack these enzymes. Mice lacking SOD2 die several days after birth, amid massive oxidative stress.[26] Mice lacking SOD1 develop a wide range of pathologies, including hepatocellular carcinoma,[27] an acceleration of age-related muscle mass loss,[28] an earlier incidence of cataracts, and a reduced lifespan. Mice lacking SOD3 do not show any obvious defects and exhibit a normal lifespan, though they are more sensitive to hyperoxic injury.[29] Knockout mice of any SOD enzyme are more sensitive to the lethal effects of superoxide-generating compounds, such as paraquat and diquat (herbicides).

Drosophila lacking SOD1 have a dramatically shortened lifespan, whereas flies lacking SOD2 die before birth.

SOD knockdowns in the worm C. elegans do not cause major physiological disruptions. However, the lifespan of C. elegans can be extended by superoxide/catalase mimetics suggesting that oxidative stress is a major determinant of the rate of aging.[30]

Knockout or null mutations in SOD1 are highly detrimental to aerobic growth in the budding yeast Saccharomyces cerevisiae and result in a dramatic reduction in post-diauxic lifespan. In wild-type S. cerevisiae, DNA damage rates increased 3-fold with age, but more than 5-fold in mutants deleted for either the SOD1 or SOD2 genes.[31] Reactive oxygen species levels increase with age in these mutant strains and show a similar pattern to the pattern of DNA damage increase with age. Thus it appears that superoxide dismutase plays a substantial role in preserving genome integrity during aging in S. cerevisiae. SOD2 knockout or null mutations cause growth inhibition on respiratory carbon sources in addition to decreased post-diauxic lifespan.

In the fission yeast Schizosaccharomyces pombe, deficiency of mitochondrial superoxide dismutase SOD2 accelerates chronological aging.[32]

Several prokaryotic SOD null mutants have been generated, including E. coli. The loss of periplasmic CuZnSOD causes loss of virulence and might be an attractive target for new antibiotics.

Role in disease

Mutations in the first SOD enzyme (SOD1) can cause familial amyotrophic lateral sclerosis (ALS, a form of motor neuron disease).[33][34][35][36] The most common mutation in the U.S. is A4V, while the most intensely studied is G93A. The other two isoforms of SOD have not been linked to any human diseases, however, in mice inactivation of SOD2 causes perinatal lethality[26] and inactivation of SOD1 causes hepatocellular carcinoma.[27] Mutations in SOD1 can cause familial ALS (several pieces of evidence also show that wild-type SOD1, under conditions of cellular stress, is implicated in a significant fraction of sporadic ALS cases, which represent 90% of ALS patients.),[37] by a mechanism that is presently not understood, but not due to loss of enzymatic activity or a decrease in the conformational stability of the SOD1 protein. Overexpression of SOD1 has been linked to the neural disorders seen in Down syndrome.[38] In patients with thalassemia, SOD will increase as a form of compensation mechanism. However, in the chronic stage, SOD does not seem to be sufficient and tends to decrease due to the destruction of proteins from the massive reaction of oxidant-antioxidant.[39]

In mice, the extracellular superoxide dismutase (SOD3, ecSOD) contributes to the development of hypertension.[40][41] Diminished SOD3 activity has been linked to lung diseases such as Acute Respiratory Distress Syndrome (ARDS) or Chronic obstructive pulmonary disease (COPD).[42][43][44]

Superoxide dismutase is also not expressed in neural crest cells in the developing fetus. Hence, high levels of free radicals can cause damage to them and induce dysraphic anomalies (neural tube defects).

A cross-sectional study in humans suggests that serum SOD could be a marker of cardiovascular alterations in hypertensive and diabetic patients, since changes in its serum levels are correlated with alterations in vascular structure and function.[45]

Pharmacological activity

SOD has powerful antinflammatory activity. For example, SOD is a highly effective experimental treatment of chronic inflammation in colitis. Treatment with SOD decreases reactive oxygen species generation and oxidative stress and, thus, inhibits endothelial activation. Therefore, such antioxidants may be important new therapies for the treatment of inflammatory bowel disease.[46]

Likewise, SOD has multiple pharmacological activities. E.g., it ameliorates cis-platinum-induced nephrotoxicity in rodents.[47] As "Orgotein" or "ontosein", a pharmacologically-active purified bovine liver SOD, it is also effective in the treatment of urinary tract inflammatory disease in man.[48] For a time, bovine liver SOD even had regulatory approval in several European countries for such use. This was cut short by concerns about prion disease.

An SOD-mimetic agent, TEMPOL, is currently in clinical trials for radioprotection and to prevent radiation-induced dermatitis.[49] TEMPOL and similar SOD-mimetic nitroxides exhibit a multiplicity of actions in diseases involving oxidative stress.[50]

Cosmetic uses

SOD may reduce free radical damage to skin—for example, to reduce fibrosis following radiation for breast cancer. Studies of this kind must be regarded as tentative, however, as there were not adequate controls in the study including a lack of randomization, double-blinding, or placebo.[51] Superoxide dismutase is known to reverse fibrosis, possibly through de-differentiation of myofibroblasts back to fibroblasts.[52]

Commercial sources

SOD is commercially obtained from marine phytoplankton, bovine liver, horseradish, cantaloupe, and certain bacteria. For therapeutic purpose, SOD is usually injected locally. There is no evidence that ingestion of unprotected SOD or SOD-rich foods can have any physiological effects, as all ingested SOD is broken down into amino acids before being absorbed. However, ingestion of SOD bound to wheat proteins could improve its therapeutic activity, at least in theory.[53]

See also


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External links

Aerotolerant anaerobe

Aerotolerant anaerobes use fermentation to produce ATP. They do not utilize oxygen, but they can protect themselves from reactive oxygen molecules. In contrast, obligate anaerobes can be harmed by reactive oxygen molecules.

There are three categories of anaerobes. Obligate anaerobes are damaged by the presence of oxygen. Aerotolerant organisms cannot use oxygen for growth but are tolerate its presence. And facultative anaerobes can grow without oxygen but use oxygen if it is present.

Aerotolerant anaerobes have superoxide dismutase and peroxidase but don't have catalase.An example of an aerotolerant anaerobe is Streptococcus mutans. (Facultative anaerobe)


Antioxidants are compounds that inhibit oxidation. Oxidation is a chemical reaction that can produce free radicals, thereby leading to chain reactions that may damage the cells of organisms. Antioxidants such as thiols or ascorbic acid (vitamin C) terminate these chain reactions. To balance the oxidative state, plants and animals maintain complex systems of overlapping antioxidants, such as glutathione and enzymes (e.g., catalase and superoxide dismutase), produced internally, or the dietary antioxidants vitamin C, and vitamin E.

The term "antioxidant" is mostly used for two entirely different groups of substances: industrial chemicals that are added to products to prevent oxidation, and naturally occurring compounds that are present in foods and tissue. The former, industrial antioxidants, have diverse uses: acting as preservatives in food and cosmetics, and being oxidation-inhibitors in fuels.Antioxidant dietary supplements have not been shown to improve health in humans, or to be effective at preventing disease. Supplements of beta-carotene, vitamin A, and vitamin E have no positive effect on mortality rate or cancer risk. Additionally, supplementation with selenium or vitamin E do not reduce the risk of cardiovascular disease.

CCS (gene)

Copper chaperone for superoxide dismutase is a metalloprotein that is responsible for the delivery of Cu to superoxide dismutase (SOD1). CCS is a 54kDa protein that is present in mammals and most eukaryotes including yeast. The structure of CCS is composed of three distinct domains that are necessary for its function. Although CCS is important for many organisms, there are CCS independent pathways for SOD1, and many species lack CCS all together, such as C. elegans. In humans the protein is encoded by the CCS gene.

Down syndrome research

Research of Down syndrome-related genes is based on studying the genes located on chromosome 21. In general, this leads to an overexpression of the genes. Understanding the genes involved may help to target medical treatment to individuals with Down syndrome. It is estimated that chromosome 21 contains 200 to 250 genes. Recent research has identified a region of the chromosome that contains the main genes responsible for the pathogenesis of Down syndrome, located proximal to 21q22.3. The search for major genes involved in Down syndrome characteristics is normally in the region 21q21–21q22.3.


In cell biology, molecular biology and related fields, the word extracellular (or sometimes extracellular space) means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid (see extracellular matrix). The term is used in contrast to intracellular (inside the cell).

According to the Gene Ontology, the extracellular space is a cellular component defined as: "That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid. For multicellular organisms, the extracellular space refers to everything outside a cell, but still within the organism (excluding the extracellular matrix). Gene products from a multi-cellular organism that are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term".The composition of the extracellular space includes metabolites, ions, various proteins and non-protein substances (e.g. DNA, RNA, lipids, microbial products etc.) that might affect cellular function. For example, hormones, growth factors, cytokines and chemokines act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are various enzymes, including digestive enzymes (Trypsin, Pepsin), extracellular proteinases (Matrix metalloproteinases, ADAMTSs, Cathepsins) and antioxidant enzymes (extracellular superoxide dismutase). Often, proteins present in the extracellular space are stored outside the cells by attaching to various extracellular matrix components (Collagens, Proteoglycans, etc.). In addition, extracellular matrix proteolytic products are also present in the extracellular space, especially in tissues undergoing remodelling [2].


Glisodin is the registered trademark of a nutritional supplement based on two constituents:

Cantaloupe extract, which typically contains high quantities of the enzyme superoxide dismutase (SOD)

Gliadin, a wheat protein designed to protect SOD during the digestive process

Glutathione peroxidase

Glutathione peroxidase (GPx) (EC is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage. The biochemical function of glutathione peroxidase is to reduce lipid hydroperoxides to their corresponding alcohols and to reduce free hydrogen peroxide to water.

Irwin Fridovich

Irwin Fridovich is an American biochemist who, together with his graduate student Joe M. McCord, discovered the enzymatic activity of copper,zinc superoxide dismutase (SOD),—to protect organisms from the toxic effects of superoxide free radicals formed as a byproduct of normal oxygen metabolism. Subsequently, Fridovich's research group also discovered the manganese-containing and the iron-containing SODs from E coli and the mitochondrial MnSOD (SOD2), now known to be an essential mammalian protein. He spent the rest of his career studying the biochemical mechanisms of SOD and of biological superoxide toxicity, using bacteria as model systems. Fridovich is currently Professor Emeritus of Biochemistry at Duke University.

Joe M. McCord

Joe Milton McCord (born March 3, 1945) is an American biochemist. While serving as a graduate student, he and his supervisor Irwin Fridovich were the first to describe the enzymatic activity of superoxide dismutase. McCord joined the board of directors of the LifeVantage Corporation (makers of the dietary supplement Protandim) in 2006, serving as the company's chief science officer from 2011 to 2012, and retired from the company in June 2013.


Mangafodipir (sold under the brand name Teslascan as mangafodipir trisodium) is a contrast agent delivered intravenously to enhance contrast in magnetic resonance imaging (MRI) of the liver. It has two parts, paramagnetic manganese (II) ions and the chelating agent fodipir (dipyridoxyl diphosphate, DPDP). Normal liver tissue absorbs the manganese more than abnormal or cancerous tissue. The manganese shortens the longitudinal relaxation time (T1), making the normal tissue appear brighter in MRIs. This enhanced contrast allows lesions to be more easily identified.

Mangafodipir was withdrawn from the US market in 2003 and the European market in 2012.Reactive oxygen species (ROS) and reactive nitrogen species (RNS) participate in pathological tissue damage. Mitochondrial manganese superoxide dismutase (MnSOD) normally keeps ROS and RNS in check. During development of mangafodipir as an MRI contrast agent, it was discovered that it possessed MnSOD mimetic activity. Mangafodipir has been tested as a chemotherapy adjunct in cancer patients and as an adjunct to percutaneous coronary intervention in patients with myocardial infarctions, with promising results. Whereas MRI contrast depends on release of Mn2+, the MnSOD mimetic activity depends on Mn2+ that remains bound to DPDP. Calmangafodipir [Ca4Mn(DPDP)5] (brand name PledOx) is stabilized with respect to Mn2+ and has improved therapeutic activity. Calmangafodipir is being explored as a chemotherapy adjunct in cancer patients.


Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large number of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-binding protein domains although there may be up to 3000 human zinc metalloproteins.

Mitochondrial ROS

Mitochondrial ROS (mtROS or mROS) are reactive oxygen species (ROS) that are produced by mitochondria. Generation of mitochondrial ROS mainly takes place at the electron transport chain located on the inner mitochondrial membrane during the process of oxidative phosphorylation (OXPHOS). Leakage of electrons at complex I and complex III from electron transport chains leads to partial reduction of oxygen to form superoxide. Subsequently, superoxide is quickly dismutated to hydrogen peroxide by two dismutases including superoxide dismutase 2 (SOD2) in mitochondrial matrix and superoxide dismutase 1 (SOD1) in mitochondrial intermembrane space. Collectively, both superoxide and hydrogen peroxide generated in this process are considered as mitochondrial ROS.Once thought as merely the by-products of cellular metabolism, mitochondrial ROS are increasingly viewed as important signaling molecules. At low levels, mitochondrial ROS are considered to be important for metabolic adaptation as seen in hypoxia. Mitochondrial ROS, stimulated by danger signals such as lysophosphatidylcholine and Toll-like receptor 4 and Toll-like receptor 2 bacterial ligands lipopolysaccharide (LPS) and lipopeptides, are involved in regulating inflammatory response. Finally, high levels of mitochondrial ROS activate apoptosis/autophagy pathways capable of inducing cell death.

Nickel superoxide dismutase

Nickel superoxide dismutase (Ni-SOD) is a metalloenzyme that, like the other superoxide dismutases, protects cells from oxidative damage by catalyzing the disproportionation of the cytotoxic superoxide radical (O
) to hydrogen peroxide and molecular oxygen. Superoxide is a reactive oxygen species that is produced in large amounts during photosynthesis and aerobic cellular respiration. The equation for the disproportionation of superoxide is shown below:

Ni-SOD was first isolated in 1996 from Streptomyces bacteria and is primarily found in prokaryotic organisms. It has since been observed in cyanobacteria and a number of other aquatic microbes. Ni-SOD is homohexameric, meaning that it has six identical subunits. Each subunit has a single nickel containing active site. The disproportionation mechanism involves a reduction-oxidation cycle where a single electron transfer is catalyzed by the Ni2+/Ni3+ redox couple. Ni-SOD catalyzes close to the barrier of diffusion.

Petkau effect

The Petkau effect is an early counterexample to linear-effect assumptions usually made about radiation exposure. It was found by Dr. Abram Petkau at the Atomic Energy of Canada Whiteshell Nuclear Research Establishment, Manitoba and published in Health Physics March 1972. The Petkau effect was coined by Swiss nuclear hazards commentator Ralph Graeub in 1985 in this book Der Petkau-Effekt und unsere strahlende Zukunft (The Petkau effect and our Radiating Future).Petkau had been measuring, in the usual way, the radiation dose that would rupture a simulated artificial cell membrane. He found that 3500 rads delivered in ​2 1⁄4 hours (26 rad/min = 15.5 Sv/h) would do it. Then, almost by chance, Petkau repeated the experiment with much weaker radiation and found that 0.7 rad delivered in ​11 1⁄2 hours (1 millirad/min = 0.61 mSv/h) also ruptured the membrane. This was counter to the prevailing assumption of a linear relationship between total dose or dose rate and the consequences.The radiation was of ionizing nature, and produced negative oxygen ions (free radicals). Those ions were more damaging to the simulated membrane in lower concentrations than higher (a somewhat counter-intuitive result in itself) because in the latter, they more readily recombine with each other instead of interfering with the membrane. The ion concentration directly correlated with the radiation dose rate and the composition had non-monotonic consequences.


Superoxide dismutase [Cu-Zn] also known as superoxide dismutase 1 or SOD1 is an enzyme that in humans is encoded by the SOD1 gene, located on chromosome 21. SOD1 is one of three human superoxide dismutases. It is implicated in apoptosis and familial amyotrophic lateral sclerosis.


Superoxide dismutase 2, mitochondrial (SOD2), also known as manganese-dependent superoxide dismutase (MnSOD), is an enzyme which in humans is encoded by the SOD2 gene on chromosome 6. A related pseudogene has been identified on chromosome 1. Alternative splicing of this gene results in multiple transcript variants. This gene is a member of the iron/manganese superoxide dismutase family. It encodes a mitochondrial protein that forms a homotetramer and binds one manganese ion per subunit. This protein binds to the superoxide byproducts of oxidative phosphorylation and converts them to hydrogen peroxide and diatomic oxygen. Mutations in this gene have been associated with idiopathic cardiomyopathy (IDC), premature aging, sporadic motor neuron disease, and cancer.


Extracellular superoxide dismutase [Cu-Zn] is an enzyme that in humans is encoded by the SOD3 gene.

This gene encodes a member of the superoxide dismutase (SOD) protein family. SODs are antioxidant enzymes that catalyze the dismutation of two superoxide radicals into hydrogen peroxide and oxygen. The product of this gene is thought to protect the brain, lungs, and other tissues from oxidative stress. The protein is secreted into the extracellular space and forms a glycosylated homotetramer that is anchored to the extracellular matrix (ECM) and cell surfaces through an interaction with heparan sulfate proteoglycan and collagen. A fraction of the protein is cleaved near the C-terminus before secretion to generate circulating tetramers that do not interact with the ECM.

Superoxide dismutase mimetics

Superoxide dismutase (SOD) mimetics are synthetic compounds that mimic the native superoxide dismutase enzyme. SOD mimetics effectively convert the superoxide anion (O−2), a reactive oxygen species, into hydrogen peroxide, which is further converted into water by catalase. Reactive oxygen species are natural byproducts of cellular respiration and cause oxidative stress and cell damage, which has been linked to causing cancers, neurodegeneration, age-related declines in health, and inflammatory diseases. SOD mimetics are a prime interest in therapeutic treatment of oxidative stress because of their smaller size, longer half-life, and similarity in function to the native enzyme.The chemical structure of SOD mimetics generally consists of manganese, iron, or copper (and zinc) coordination complexes. Salen-manganese(III) complexes contain aromatic ring structures that increase the lipid solubility and cell permeability of the entire complex. Manganese (II) and iron (III) complexes are commonly used due to their high kinetic and thermodynamic stability, increasing the half-life of the mimetic. However, manganese-based SOD mimetics are found to be more therapeutically effective than their counterparts due to their low toxicity, higher catalytic activity, and increased stability in vivo.

Synthetic catalytic scavenger

A Synthetic catalytic scavenger is an artificial anti-oxidant that has been demonstrated to extend cellular life. It was successful in C. elegans and was effective in rat trials. Studies have shown that synthetic catalytic scavengers have superoxide dismutase and catalase activities which prevented injuries from reactive oxygen species, helping promote the livelihood of tissues.

Available protein structures:
PDBsumstructure summary
Available protein structures:
PDBsumstructure summary
Available protein structures:
PDBsumstructure summary
Available protein structures:
PDBsumstructure summary
Nonsteroidal anti-inflammatory drugs (NSAIDs) (primarily M01A and M02A, also N02BA)
Pyrazolones /
Acetic acid derivatives
and related substances
Propionic acid derivatives
N-Arylanthranilic acids
Other oxidoreductases (EC 1.15-1.21)
1.15: Acting on superoxide as acceptor
1.16: Oxidizing metal ions
1.17: Acting on CH or CH2 groups
1.18: Acting on iron-sulfur proteins as donors
1.19: Acting on reduced flavodoxin as donor
1.20: Acting on phosphorus or arsenic in donors
1.21: Acting on X-H and Y-H to form an X-Y bond

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