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A cofactor is a non-protein chemical compound that is bound tightly to an enzyme and is required for catalysis. They can be considered "helper molecules/ions" that assist in biochemical transformations. Certain substances such as water and various abundant ions may be bound tightly by enzymes, but are not considered to be cofactors since they are ubiquitous and rarely limiting.

An enzyme without a cofactor is referred to as an apoenzyme, and the completely active enzyme (in addition to the cofactor) is called a holoenzyme.

Apoenzyme + cofactor <=> Holoenzyme

Metal ions are common cofactors. In nutrition, the list of essential trace elements reflects their role as cofactors. This list includes manganese, iron, cobalt, nickel, copper, zinc, and molybdenum. Other organisms require additional metals, such as vanadium and tungsten. The study of these cofactors falls under the area of bioinorganic chemistry.

Some cofactors undergo chemical changes during the course of a reaction, undergoing reduction or oxidation. Nonetheless, as a catalyst, cofactors return to their original state in the course of the catalytic cycle. They are not consumed. In this respect, cofactors differ from substrates or coenzymes.

In many cases, the cofactor includes both an inorganic and organic components. One diverse set of examples are the heme proteins, which consists of a porphyrin ring coordinated to iron.

Cofactors vary in their location and the tightness of their binding to the host enzyme. When bound tightly to the enzyme, cofactors are called prosthetic groups. Loosely-bound cofactors typically associate in a similar fashion to enzyme substrates. These are better described as coenzymes, which are organic substances that directly participate as substrates in an enzyme reaction. Vitamins can serve as precursors to coenzymes (e.g. vitamins B1, B2, B6, B12, niacin, folic acid) or as coenzymes themselves (e.g. vitamin C).

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