Cofactor (biochemistry)

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A cofactor is non-protein chemical compound required by an enzyme to catalyze a certain reaction. They can be considered "helper molecules/ions" that assist in biochemical transformations. Certain substances such as water and various abundant ions 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 fact, the list of essential transition metal ions reflects their role as cofactors. This list includes manganese, iron, cobalt, nickel, copper, zinc, and molybdenum. Other organisms require addition metals, such as vanadium and tungsten. The study of these cofactors falls under the area of bioinorganic chemistry.

Organic cofactors include most vitamins, which are also known as coenzymes. 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. The machinery for methanogenesis is a particularly rich source of coenzymes.

In many cases, the cofactor includes both an inorganic and organic components. One diverse examples are the heme proteins, which consist of both a tetrapyrrole and 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 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).