Homo sapiens (human)
Free radicals can cause significant damage to biologically important molecules and to cell structures that contain them. Free radicals have unpaired electrons in their outermost shell. They are chemically instable and seek to react with other molecules in order to obtain a chemically balanced electron shell. Most free radicals of biological importance contain oxygen, called reactive oxygen. Although free radicals are mostly thought of as damaging the body, there are some uses for them. Excess amounts of free radicals are however undesirable. Antioxidants like vitamin C can help by binding free radicals. In addition to a variety of small molecules like vitamin C the body has two proteins to deal with free radicals. One is superoxide dismutase and the other catalase. Superoxide is diatomic oxygen with an excess electron. Superoxide dismutase converts the superoxide into regular oxygen and hydrogen peroxide. Hydrogen peroxide can be further processed by catalase to water and molecular oxygen.
Superoxide dismutase, the structure here, is a dimeric molecule. Defects of superoxide dismutase have been linked to fatal diseases like amyotrophic lateral sclerosis, better known as Lou Gehrig's disease. It leads to paralysis and to death within 5 years. To understand the basis of the disease causing effects the structure of the enzyme were determined. Its enzymatic reaction is associated with two metal irons, one copper and one zinc each per monomer. These two metal ions have interesting coordination. Except for one aspartic acid sidechain that interacts with the zinc ion, all the remaining metal-protein contacts come from nitrogens of histidine sidechains. When the location of disease-causing mutations was determined, none of them were associated with the active site of the enzyme. It seems that mutations mainly contribute to destabilize the folding of the protein and to have negative effect on the formation of dimers. It thus seems that defective superoxide dismutase is not entirely broken, but less efficient than the intact protein is.
Protein Data Bank (PDB)
author: Arno Paehler