A catalase is a catalyst. As a type of enzyme present in the cells of plants, animals and aerobic bacteria, catalases are very large, complex organic molecules made by the cell to perform very specific functions and to speed up the rate of chemical reactions that would otherwise be too slow to sustain life. Catalases are important to life as they accelerate the rate of the conversion of hydrogen peroxide, a powerful and potentially harmful oxidizing agent, to water and oxygen.

Possessing the general characteristics of an enzyme, catalases can be isolated but still remain active outside of the living cell. Catalases are very efficient catalysts that can greatly accelerate chemical reactions at concentrations so low that it is almost not detectable by chemical tests for proteins. As an enzyme, catalases can also speed up an indefinite amount of chemical reactions without being affected by the reaction itself. However, since most isolated enzymes are relatively unstable, they will gradually lose activity under the conditions used for their study.

As all enzymes are proteins, catalases also consist of chains of amino acids linked together by peptide bonds. The sequence of amino acids within the polypeptide chains determines the characteristic three-dimensional structure of the enzyme. It is also believed that the conformation is responsible for the activity of the enzyme. This conformation is stabilized by interactions of amino acids in different parts of the peptide chains with each other and with the surrounding medium. However, these interactions may be disrupted by high temperature, acid or alkaline conditions, or changes in the polarity of the medium. The result of such changes leads to an unfolding of the peptide chains (denaturation) and the loss of enzymatic activity, solubility and other properties of the enzyme.

Finally, one of the most characteristic properties of the enzyme catalase is that it is specific. Like the majority of enzymes, catalases act only on hydrogen peroxide. Their relationship is comparable to that of a lock and key. In other words, the substrate fits into active site of the enzyme like a key that fits into a lock. Sometimes, even a small structural change, or a change in the part of the molecule remote from that altered by the enzymatic reaction, would affect the ability of a compound to serve as a substrate.

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