Cofactor (biochemistry)
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A cofactor is any substance that needs to be present in addition to an enzyme to catalyze a certain reaction. (However, more or less ubiquitous substances such as water do not qualify.) Some cofactors are inorganic, such as the metal atoms zinc, iron, and copper in certain forms. Others, such as most vitamins, are organic, and are known as coenzymes. Some cofactors undergo chemical changes during the course of a reaction (i.e. being reduced or oxidized). Nonetheless, as a catalyst, cofactors will be returned to their original state when the reaction in which they are needed has finished -- they are not consumed in the reaction or permanently converted to something else (that would be a substrate of the reaction). Cofactors vary in location and tightness of binding. When bound tightly to the enzyme, they are called prosthetic groups. Loosely bound cofactors typically bind in a similar fashion to enzyme substrates. When a cofactor is an organic substance that directly participates as a substrate in the reaction, it is called a coenzyme. Vitamins can serve as precursors to coenzymes (e.g. vitamins B1, B2, B6, B12, niacin, folic acid) or as cofactors themselves (e.g. vitamin C). Cofactors are inorganic ions and organic, non-protein molecules that help some enzymes function as catalysts. When inorganic, they are usually either copper, zinc or iron. Found on the active sites of enzymes, they attract electrons from bonds in a substrate to cause them to break.
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