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Enzymes are thought to have an area with a very particular shape. When a molecule of the right chemical for that enzyme comes along it will fit exactly into the shape. The area of particular shape is called the active site of the enzyme, as that is where the reaction takes place. The molecule that the enzyme works on is called the substrate. After the reaction has taken place and the products of the reaction leave the active site leaving it ready for another molecule of the chemical.
The active site of an enzyme has such a particular shape that only one kind of molecule will fit it, rather like a particular key fitting a lock. This is why enzymes are specific in their action.