Hydrogen peroxide manganese catalase reaction

Manganese is used by nature to catalyze a number of important biological reactions that include the dismutation of superoxide radicals, the decomposition of hydrogen peroxide, and the oxidation of water to dioxygen. The dinuclear manganese centers that occur in Lactobacillus plantar-aum catalase and Thermus thermophilus catalase have attracted considerable attention and many model compounds have now been synthesized that attempt to mimic aspects of these biological systems.The catalases have at least four accessible oxidation states (Mn Mn , Mn°Mn , Mn" Mn", and Mn Mn ) it is believed that the Mn"Mn"/Mn"Mn" redox couple is effective in catalyzing the disproportionation of water. 

Manganese is required by mitochondrial superoxide dismutase. This enzyme catalyzes the same reaction catalyzed by the cytosolic form of the enzyme, namely the conversion of superoxide to hydrogen peroxide. Superoxide is a molecule of oxygen containing one additional electron. It is produced as a byproduct of various reactions in which molecular oxygen (O ) is involved. These reactions may include those of cytochrome c oxidase and various flavoproteins- The HOOH formed by superoxide is decomposed by catalase ... 

Catalase is a more effective catalyst on this reaction than manganese(I V) oxide. This can be shown by adding a piece of liver or liquidised celery (which both contain catalase) to a solution of hydrogen peroxide and observing that oxygen gas is evolved instantly and dramatically. 

Enzymes are long-chain proteins that have a complicated structure and normally have only one particular site at which a reaction can take place. X-Ray diffraction results have shown that this region, called the active site, has a definite shape for each enzyme, and therefore only molecules with a similar complementary shape will fit into it - rather like keys in a lock. For example, only hydrogen peroxide molecules will fit the active site of catalase. Those molecules that do fit the active site are called substrate molecules. Thus, enzymes are much more specific catalysts than inorganic substances such as manganese(IV) oxide. 

There are several different types of catalase. Most animal cells have a form that has four haem molecules embedded in its core. In contrast, some microbes have a different sort of catalase, which contains manganese instead of haem at its core. Despite their different structures, both enzymes are equally fast, and are correctly called catalase, in the sense that they work in the same way — they both catalyse the reaction of two molecules of hydrogen peroxide with each other to form oxygen and water ... 

Experiment 19.1 Decomposition qfH202 by various catalysts. When a solution of iron(III) chloride is added to the hydrogen peroxide solution, a noticeable development of oxygen can be observed after a while. When manganese dioxide is added instead, the reaction proceeds much faster than in the first case and also produces fog. (Therefore, the expression genie in a bottle is used for a variation of this experiment.) When the enzyme catalase is added, a vigorous reaction producing foam occurs. 

This process is thermodynamically favorable. It has a Mt of-98.2 kJ-mol and a ACT of-119.2 kJ mor and a AS of 70.5 TmoP -K . The rate of decomposition is dependent on the temperature and concentration of the peroxide, as well as the pH and the presence of impurities and stabilizers. Hydrogen peroxide is incompatible with many substances that catalyse its decomposition, including most of the transition metals and their compounds. Common catalysts include manganese dioxide, silver, and platinum. The same reaction is catalysed by the enzyme catalase, found in the liver, whose main function in the body is the removal of toxic byproducts of metabolism and the reduction of oxidative stress. The decomposition occurs more rapidly in alkali, so acid is often added as a stabilizer. 

---