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An enzyme is an organic catalyst. A catalyst is a substance which brings about or greatly speeds up a chemical reaction and it is not used up or changed during the reaction. Enzymes speed up metabolic reactions in living cells without changing their composition in the process. Enzymes are protein molecules that are manufactured by an organism’s own cells.


Enzymes are classified based on the nature of chemical changes brought about on a substrate. Example;

  • Hydrolases: These bring about the decomposition of a substrate by the addition of water molecules to it. This process is called hydrolysis. However, they can also bring about a reverse reaction in which complex substances are synthesized from simple substances by the removal of water (condensation). Most digestive enzymes are hydrolysis enzymes.
  • Oxido – reductase: These bring about oxidation of a substrate by the addition of oxygen or removal of hydrogen. This reaction is usually accompanied by the reduction of another substance. Most respiratory enzymes such as dehydrogenases, oxidases and catalases belong to this group.
  • Transferases: These catalyze reversible reactions
  • Isomerases: These usually change the spatial configuration of a molecule in a living cell. They work on isomers.
  • Lyases: These catalyze the breaking up of bonds by elimination or addition reactions.
  • Ligases: These catalyze the joining of two molecules with accompanying hydrolysis of a high energy bond.
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  • They are proteins in nature.
  • They are soluble
  • Some enzymes require the presence of non-proteinous molecules called co-enzymes to activate them.
  • Enzymes are specific in action e.g. amylases will only act on starch.
  • Enzymes are organic catalysts i.e. they speed up chemical reactions in the body but are unchanged at the end of the reaction.

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