Superoxide Dismutase (SOD)
Comprehensive Notes for CSIR-NET, GATE, SLET, JAM & Other Competitive Exams
Introduction and Discovery
Superoxide dismutase (SOD) is an important antioxidant metalloenzyme that protects cells against oxidative damage caused by the superoxide radical (O2•−). SOD catalyzes the dismutation of superoxide into molecular oxygen (O2) and hydrogen peroxide (H2O2).
- Superoxide dismutase is classified as EC 1.15.1.1.
- It is a metalloenzyme whose active site contains a redox-active metal ion.
- Depending on the SOD type, the metal may be Cu, Zn, Mn, Fe, or Ni.
- The enzyme was identified and characterized as superoxide dismutase by Joe M. McCord and Irwin Fridovich in 1969 during their work on erythrocuprein.
- SOD is one of the major enzymatic defenses against reactive oxygen species (ROS).
The hydrogen peroxide produced by SOD can subsequently be removed by enzymes such as catalase and various peroxidases.
Types and Isoforms of Superoxide Dismutase
| Type | Metal Cofactor | Major Location | Typical Structure | Distribution |
|---|---|---|---|---|
| Cu/Zn-SOD (SOD1) | Cu + Zn | Cytosol, nucleus and mitochondrial intermembrane space | Dimer | Many eukaryotes |
| Mn-SOD (SOD2) | Mn | Mitochondrial matrix in eukaryotic cells | Tetramer | Eukaryotic mitochondria and many bacteria |
| Extracellular SOD (SOD3) | Cu + Zn | Extracellular space and extracellular matrix | Tetramer | Mammals |
| Fe-SOD | Fe | Mainly bacterial cells and plant chloroplasts | Dimer or tetramer | Many prokaryotes and some plants |
| Ni-SOD | Ni | Some bacteria | Hexamer | Restricted distribution in prokaryotes |
Structure of SOD
- Cu/Zn-SOD: Characteristically contains a Greek-key β-barrel fold. The active site contains copper as the redox-active metal and zinc primarily contributes to structural stability.
- Mn-SOD and Fe-SOD: Have closely related structural folds dominated by α-helical elements.
- The metal at the active site undergoes reversible changes in oxidation state during catalysis.
- The active-site environment facilitates rapid reaction between superoxide molecules and the metal center.
Catalytic Mechanism of SOD
SOD catalyzes a two-step redox process commonly described as a ping-pong or double-displacement mechanism. The metal alternates between two oxidation states.
Step 1: Oxidation of Superoxide
Step 2: Reduction of Superoxide
Combining these two half-reactions gives the overall SOD reaction:
Biological Significance of SOD
- Provides an important first-line enzymatic defense against superoxide radicals.
- Protects cells from oxidative damage associated with mitochondrial respiration and other metabolic processes.
- In plants, SOD participates in protection against oxidative stress associated with drought, salinity, temperature stress, heavy metals, ozone and other environmental stresses.
- Converts superoxide into hydrogen peroxide, which can then be detoxified by catalase and peroxidases.
- Because ROS can also function as signaling molecules, SOD activity can influence cellular redox signaling.
SOD and Human Diseases
- Amyotrophic Lateral Sclerosis (ALS): Mutations in the SOD1 gene are associated with a significant proportion of familial ALS cases. Disease-associated SOD1 mutations are generally understood to cause toxicity through mechanisms involving a toxic gain of function.
- Down syndrome: The SOD1 gene is located on chromosome 21. Increased SOD1 dosage in trisomy 21 can alter cellular redox balance and contribute to oxidative stress.
- Altered SOD activity has also been investigated in several disorders involving oxidative stress, including cancer, diabetes, neurodegenerative diseases and cardiovascular disorders.
- SOD mimetics, including compounds such as Tempol and experimental salen-manganese compounds, have been investigated as potential therapeutic agents.
Inhibitors and SOD Assays
Important Inhibitors
- Cyanide: Strongly inhibits Cu/Zn-SOD, whereas Mn-SOD and Fe-SOD are relatively resistant under commonly used assay conditions.
- Azide: Can inhibit Cu/Zn-SOD and is used in biochemical studies involving SOD inhibition.
- Hydrogen peroxide: At sufficiently high concentrations, H2O2 can cause oxidative inactivation of some SODs, particularly Cu/Zn-SOD.
Common SOD Assays
- Xanthine–xanthine oxidase assay: Generates superoxide, which can then be detected using an appropriate indicator system.
- NBT assay: Superoxide reduces nitroblue tetrazolium (NBT) to a colored formazan product. SOD decreases this reduction by removing superoxide.
- Cytochrome c assay: Superoxide reduces cytochrome c, and SOD inhibits this reduction. This provides an indirect measurement of SOD activity.
Quick Comparison of Major SOD Types
| SOD | Metal | Major Eukaryotic Location | Typical Oligomeric State |
|---|---|---|---|
| SOD1 | Cu/Zn | Cytosol, nucleus, mitochondrial intermembrane space | Dimer |
| SOD2 | Mn | Mitochondrial matrix | Tetramer |
| SOD3 | Cu/Zn | Extracellular space and matrix | Tetramer |
| Fe-SOD | Fe | Plant chloroplasts | Dimer/tetramer |
| Ni-SOD | Ni | Not a major human/eukaryotic SOD | Hexamer |
Exam-Oriented Key Points
- SOD stands for Superoxide Dismutase.
- SOD is a major antioxidant metalloenzyme.
- Overall reaction: 2O2•− + 2H+ → O2 + H2O2
- SOD1 → Cu/Zn → mainly cytosolic.
- SOD2 → Mn → mitochondrial matrix.
- SOD3 → Cu/Zn → extracellular.
- Fe-SOD → Fe → found in many bacteria and plant chloroplasts.
- Ni-SOD → Ni → found in some bacteria.
- SOD1 mutations are strongly associated with familial ALS.
- Cu/Zn-SOD contains copper and zinc, with copper serving as the redox-active metal.
- Cyanide is a classic inhibitor of Cu/Zn-SOD.
- SOD converts superoxide into H2O2 and O2; it does not directly convert H2O2 into water.
Test Your Knowledge on Superoxide Dismutase (SOD)
1. Who identified and characterized superoxide dismutase as an enzyme in 1969?
B. McCord and Fridovich
C. Sanger and Pauling
D. Krebs and Calvin
Answer
B. McCord and Fridovich
2. Which isoform of SOD is primarily associated with the mitochondrial matrix in eukaryotic cells?
B. SOD2 (Mn-SOD)
C. SOD3 (extracellular SOD)
D. Fe-SOD
Answer
B. SOD2 (Mn-SOD)
3. Which reaction is catalyzed by superoxide dismutase?
B. 2O2•− + 2H+ → O2 + H2O2
C. H2O2 → H2O + ½O2
D. 2H2O2 → 2H2O + O2
Answer
B. 2O2•− + 2H+ → O2 + H2O2
4. Mutations in which SOD gene are associated with familial Amyotrophic Lateral Sclerosis (ALS)?
B. SOD1
C. SOD3
D. Fe-SOD
Answer
B. SOD1
5. Which compound is classically used to strongly inhibit Cu/Zn-SOD in biochemical studies?
B. Glucose
C. Urea
D. Iodoacetate
Answer
A. Cyanide
6. The catalytic reaction of SOD is generally considered to occur at a rate close to:
B. 106 M−1s−1
C. 109 M−1s−1
D. 1012 M−1s−1
Answer
C. Approximately 109 M−1s−1
7. Which metal ions are present in Cu/Zn-superoxide dismutase?
B. Cu and Zn
C. Cu and Fe
D. Cu and Mn
Answer
B. Cu and Zn
8. Which metal cofactors are characteristic of cytosolic SOD1 and mitochondrial SOD2, respectively?
B. Cu/Zn for SOD1, Mn for SOD2
C. Mn for SOD1, Cu/Zn for SOD2
D. Zn for both SOD1 and SOD2
Answer
B. Cu/Zn for SOD1, Mn for SOD2
9. Which SOD isoform is predominantly extracellular in mammals?
B. SOD2
C. SOD3
D. Fe-SOD
Answer
C. SOD3
10. Which enzyme commonly acts on the hydrogen peroxide produced by SOD?
B. Catalase
C. RNA polymerase
D. Hexokinase
Answer
B. Catalase
One-Minute Revision of SOD
SOD = Superoxide Dismutase
Substrate: Superoxide (O2•−)
Products: O2 + H2O2
SOD1: Cu/Zn, mainly cytosolic
SOD2: Mn, mitochondrial
SOD3: Cu/Zn, extracellular
Fe-SOD: Fe, common in bacteria and present in plant chloroplasts
Ni-SOD: Ni, found in some bacteria
SOD1 mutations: Associated with familial ALS
Classic Cu/Zn-SOD inhibitor: Cyanide
Major downstream H2O2-removing enzymes: Catalase and peroxidases