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Superoxide dismutase (SOD) is an enzyme that catalyzes the conversion of superoxides into oxygen and hydrogen peroxide by disproportionation. It is widely found in various animals, plants and microorganisms and is an important antioxidant. Superoxide dismutase protects cells exposed to oxygen, which can remove superoxide anion radicals in the body and effectively resist the damage of oxygen free radicals to organisms.
Classification and Distribution
SOD is a kind of protease that scavenges free radicals, plays an important role in the survival of aerobic organisms, and is the key to the defense of oxygen toxicity. So far, scientists have isolated SOD from organisms such as bacteria, fungi, protozoa, algae, insects, fish, plants and mammals. These SODs can be classified into at least three types: Cu/Zn-SOD, Mn-SOD, and Fe-SOD, depending on the metal auxiliary group.
Table 1. Classification and distribution of SOD.
| Classification | Color | Molecular weight | Molecular conformation | Subunit number | Distribution |
| Cu/Zn-SOD | Blue-green | 32000 | β-fold | 2 | Eukaryotic cell |
| Mn-SOD | Pink | 80000 | α-helix | 4 | Eukaryotic and prokaryotic cell |
| Fe-SOD | Yellow | 40000 | α-helix | 2 | Prokaryotic cell |
Structure
The SOD can be divided into two groups in structure: Cu/Zn-SOD is the first group, and Mn-SOD and Fe-SOD are the second group. The naturally occurring SOD has different active center ions, but the catalytically active sites have a high degree of structural identity and evolutionary conservation, that is, the active center ions are a tetragonal pyramid or a tetrahedron composed of 3 or 4 histidine (His), imidazolyl and 1 H2O molecule. Cu/Zn-SOD, as the first group of SOD, is a breakthrough for people to study SOD structure, and it is also the most widely understood SOD. Comparing the amino acid sequences of Cu/Zn-SOD from different sources, it was found that their homology was high. Some amino acids are also very conservative and do not change in all sequences, suggesting that these amino acids are related to the active center. Each molecule of Cu/Zn-SOD is dimerized by two subunits through hydrophobic interaction and hydrogen bonding. A disulfide bridge composed of thiol groups of cysteine C55 and C144 inside the peptide chain plays an important role in subunit association.
According to the three-dimensional structure of Cu/Zn-SOD, the active site of SOD is a "hydrophobic pocket" centered on Cu. Cu and Zn are at the bottom of the hydrophobic pocket. Cu(II) coordinates with the N atom on the imidazole ring of the four histidine residues to form a planar square structure, which also incorporates a H2O molecule in the axial position. Zn(II) coordinates with three histidines and one aspartic acid to form a distorted tetrahedral structure. Cu(II) and Zn(II) form an "imidazole bridge" structure by co-joining a molecule of histidine.
Both Mn-SOD and Fe-SOD belong to the second group of SOD. Mn-SOD is a tetramer composed of 203 amino acid residues. Mn(III) is in a triangular bipyramidal coordination environment, in which one axial coordinate is water molecule, the other axial direction is occupied by the coordination group His-28, and the other three ligands His-83, His- 170 and Asp-166 are located on the equatorial plane. The structure of Fe-SOD is relatively simple and similar to Mn-SOD, and the active center is composed of 3 His, 1 Asp and 1 H2O.
Catalytic Mechanism
The substrate of superoxide dismutase is the superoxide anion radical (O2▪), which has both a negative charge and an unpaired electron. Under different conditions, O2▪ can be used as a reducing agent to become O2, and as an oxidant to become H2O2. H2O2 generates H2O and O2 under the action of catalase (CAT). It can be seen that the toxic O2 transfers to non-toxic H2O and O2 under the joint action of SOD and CAT.
Figure 1. Reactions catalyzed by superoxide dismutase (SOD), catalase, and glutathione peroxidases. (Cerqueira M.D. et al. 2011)
SOD-catalyzed conversion of superoxide to oxygen and hydrogen peroxide by disproportionation is accomplished in two main steps:
M3+ + O2▪ + H+ → M2+(H+) + O2
M2+(H+) + H+ + O2▪ → M3+ + H2O2
M represents the metal cofactor, M3+ represents the highest valence of the metal cofactor, and M2+ represents the valence of the metal cofactor after oxidation. This step-in-step mechanism has the following advantages in terms of reaction kinetics: First, a molecular reaction can overcome the electrostatic repulsion between the simultaneous reaction of two molecules, and the positively charged active metal specifically binds to the negatively charged superoxide anion (O2▪). Second, the electrostatic attraction of the metal ions at the active site is absorbed and preserved by a proton. In this mechanism, the products of the disproportionation reaction are neutral and do not constrain each other. Third, the energy released by the first step reaction can be supplied to the second step to reduce the superoxide anion (O2▪), and then the H2O2 is reduced to H2O by the hydrogen peroxide reductase.
Application
Since human skin is directly in contact with oxygen, it can cause skin aging and damage. The effect of SOD on protecting the skin and preventing oxidation is outstanding. A lot of cosmetics and health care products have added SOD, which has certain effects on skin care, anti-aging and prevention of age spots.
The formation and elimination of free radicals in normal healthy organisms is in a dynamic equilibrium, but with the increase of age, the content of SOD in the body shows a downward trend. Correspondingly, the growth of free radicals will destroy the balance of the body and cause various kinds of disease. Therefore, maintaining an appropriate amount of SOD in the body is an effective way to maintain health and delay aging. According to this principle, SOD, one of the free radical scavengers, has been used in clinical treatment research and has achieved certain effects.
Like other oxidants, SOD can be used as an antioxidant for canned foods, juices, beer, etc., to prevent food deterioration and spoilage caused by peroxidase, and as a good preservative for fruits and vegetables. In order to expand the application range of SOD in food, it is necessary to enhance the stability of SOD. It is necessary to chemically modify SOD and select substances with no toxic side effects as modifiers, such as heparin compounds, chondroitin sulfate, and fatty acids. Among them, SOD modified by glass acid and lauric acid has been put into commercial production. Studies have shown that the preparation of SOD liposomes not only enhances stability, but also promotes the absorption of human skin.
The overexpression of SOD in transgenic plants can increase the resistance of plants to stress, and the overexpression of Mn-SOD gene can increase the tolerance of transgenic plants to oxygen stress. By genetic engineering, increasing the expression of SOD in plants can greatly enhance the stress resistance of plants.
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