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Class of enzymes Superoxide dismutase (SOD, EC 18.104.22.168) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide (O2−) radical into ordinary molecular oxygen (O2) and hydrogen peroxide (H2O2). Superoxide is produced as a by-product of oxygen metabolism and, if not regulated, causes many types of cell damage.
Thus, SOD is an important antioxidant defense in nearly all living cells exposed to oxygen. One exception is and related lactobacilli, which use a different mechanism to prevent damage from reactive O2−. SODs catalyze the disproportionation of superoxide: 2 HO2 → O2 + H2O2 In this way, O2− is converted into two less damaging species.
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.
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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. 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).
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.
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. Active site for iron superoxide dismutaseIron 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 ) contain both.
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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.
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. In higher plants, SOD isozymes have been localized in different cell compartments.
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. 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.
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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). In higher plants, superoxide dismutase enzymes (SODs) act as antioxidants and protect cellular components from being oxidized by reactive oxygen species (ROS).
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. SODs catalyze the production of O2 and H2O2 from superoxide (O2−), which results in less harmful reactants.
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).
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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.
Note that Cu-Zn SODs provide less protection than Fe SODs when localized in the chloroplast. 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., ) therefore produce superoxide dismutase to protect themselves from being killed.
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.
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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).
Moreover, superoxide dismutase has the largest kcat/KM (an approximation of catalytic efficiency) of any known enzyme (~7 x 109 M−1s−1), 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.
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. By differential scanning calorimetry (DSC), holo SOD1 unfolds by a two-state mechanism: from dimer to two unfolded monomers.
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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.
Mice lacking SOD3 do not show any obvious defects and exhibit a normal lifespan, though they are more sensitive to hyperoxic injury. Knockout mice of any SOD enzyme are more sensitive to the lethal effects of superoxide-generating compounds, such as paraquat and diquat (herbicides). lacking SOD1 have a dramatically shortened lifespan, whereas flies lacking SOD2 die before birth.