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In biochemistry, a cofactor is a substance that needs to be present in addition to an enzyme for a certain reaction to take place.

Many enzymes require certain nonprotein "helpers" for their catalytic activities. These cofactors generally are either bound tightly to active sites, or may bind loosely with the substrate. Some cofactors are inorganic, such as the metal atoms zinc, iron, and copper in certain forms. However, if a cofactor is organic, it is considered a cofactor. Most vitamins and raw materials making up vitamins are cofactors. They are all necessary, in their certain cases, for catalysis to take place.

See also: Cofactor (mathematics)