Copper, Zinc Superoxide Dismutase

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Copper, zinc superoxide dismutase is an oxidoreductase enzyme responsible for the very rapid two-step dismutation of the toxic superoxide radical to molecular oxygen and hydrogen peroxide through alternate reduction and oxidation of the active-site copper^[1].

Introduction

Cu/Zn Superoxide Dismutase

Oxygen is vital to sustain life; our cells cannot live without it. Oxygen is the final acceptor of electrons in the electron transport chain, allowing us to produce much more energy from food. But oxygen is also a dangerous compound.^[2]. Reactive forms of oxygen, such as superoxide, leak from the respiratory chain and wreak havoc on the cell. Superoxide is a free radical, a molecule that readily accept electrons, which make them highly reactive. They can strip electrons from proteins, lipids, or nucleic acids, thereby destroying their functions and resulting in cell dysfunction or death. Free-radical damage has been implicated in Amyotrophic lateral sclerosis, Arteriosclerosis, Arthritis, Cancer, and Ageing^[3] .

Copper, zinc superoxide dismutase is an important antioxidant defense in nearly all cells exposed to oxygen. This enzyme detoxifies superoxide via a special reaction known as dismutation. Copper, zinc superoxide dismutase belongs to the superfamily of oxidoreductases, specifically those acting on superoxide as acceptor

Superoxide Dismutase Family

In mammals there are three known isomers of superoxide dismutase (SOD). Copper and Zinc containing SOD1 is located in the cytoplasm. Manganese containing SOD2 is located in the mitochondria while a second Copper and Zinc containing SOD3 is located in the extracellular space. SOD3 and SOD2 are tetramers whereas SOD1 is a dimer. These enzymes perform the dismutation reaction by a similar mechanism.

General Structure

Cu/Zn Superoxide dismutase`s protein structure was solved by X-Ray Difraction at 2.00 Å resolution. It is a homotetramer, containing four polypeptide chains (B,O,G,Y), 152 residues in length. Each chain contains one alpha-helix and 12-14 beta-sheets. It has a structural weight of 62913.70 with the ligand identifier of C2 H4 O. Cu/Zn Superoxide dismutase contains six disulfide bonds with each subunits active site containing a copper and zinc ion ^[4].

Oxidative Capabilities

Free radicals such as superoxide are a type of reactive oxygen species that can strip electrons from proteins, lipids, or nucleic acids, thereby destroying their functions and resulting in cell dysfunction or death. They are molecules with a highly reactive unpaiared electron in an outer orbital,which can initiate chain reactions by removal of an electron from another molecule to complete its own orbital ^[5]. Apparently, if SOD is defective, superoxide is not degraded and can destroy cells. Motor neurons appear to be particularly sensitive to superoxide attack.

The Reaction of Dismutase

spinqualitylabelsall models Cu/Zn Superoxide Dismutase Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Dismutation is a term that refers to a special type of reaction, where two equal but opposite reactions occur on two separate molecules ^[6]. Superoxide is produced both accidentally and also as a reactive oxygen species required for a number of enzyme-catalyzed reactions. Copper-Zinc superoxide dismutase catalyze the reaction between superoxide and water to yield oxygen and hydrogen peroxide. Most enzymes that produce and require superoxide are in the peroxisomes,together together with superoxide dismutase, catalase and peroxidases^[7]. SOD takes two molecules of superoxide, strips the extra electron off of one, and places it on the other. So, one ends up with an electron less, forming normal oxygen, and the other ends up with an extra electron. The one with the extra electron then rapidly picks up two hydrogen ions to form hydrogen peroxide.

Of course, hydrogen peroxide is also a dangerous compound, so the cell must use the enzyme catalase to detoxify it. Hydrogen peroxide is then scavenged by catalase, a ubiquitous heme protein that catalyzes the dismutationof hydrogen peroxide into water and molecular oxygen

2H2O2 → 2H2O + O2

Cu/Zn Superoxide dismutase polypeptide chain coloured by B-factors

Importance of Vitamins

Superoxide dismutase and catalase are remarkably efficiently, performing their reactions at or near the diffusion-limited rate. Glutathione peroxide also plays a role in scavenging H2O2. Other cellular defences against oxidative damage include the antioxidant vitamins, vitamins E and C. Because it is lipophilic, vitamin E is especially useful in protecting membranes from lipid peroxidation.

One of the long-term benefits of exercise may be to increase the amount of superoxide dismutase in the cell. The elevated aerobic metabolism during exercise causes more ROS to be generated. In response, the cell synthesizes more protective enzymes. The net effect is one of protection, because the increase in superoxide dismutase more effectively protects the cell during periods of rest ^[8].

See Also

Free-Radical Theory of Aging (FRTA)

References

1. ↑ Tainer JA, Getzoff ED, Richardson JS, Richardson DC. Structure and mechanism of copper, zinc superoxide dismutase. Nature. 1983 Nov 17-23;306(5940):284-7. PMID:6316150
2. ↑ http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb94_1.html
3. ↑ Seeley, Stephens, Tate., Anatomy & Physiology, 8th edition, McGraw Hill, 2008
4. ↑ http://www.rcsb.org/pdb/explore/explore.do?structureId=2SOD
5. ↑ Metabolic Pathways and their Control pg.574
6. ↑ http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb94_1.html
7. ↑ Free Radicals and Antioxidant Nutrients ch.6 pg.484
8. ↑ Oxidative Phosphorylation Ch.18, pg. 518