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Coenzymes are small organic molecules that have the ability to act as a Cofactor to an enzyme. Other well know cofactors are metals. The role of cofactors varies with the type of cofactor and enzyme. An enzyme with a cofactor is catalytically active, and therefore is known as a Holoenzyme

Coenymes usually originate from vitamins. They can bind tightly or loosely to an enzyme. Coenzymes that are tightly bound are known as prosthetic groups, whilst loosely bound coenzymes are called cosubstrates. This is because they actually bind and are released from the enzyme, much like a substrate molecule.

Coenzymes act as electron/proton carriers in reactions, so they are involved in Oxidation and Reduction reactions. NAD+ can accept 2 electrons whereas FAD can be reduced by taking up 2 protons to form FADH2 [1].


  1. Berg JM, Tymoczko, Stryer L. (2011), Biochemistry,7th Edition, New York: WH Freeman (p229, p456-8)

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