Coenzymes are small organic molecules that act as a Cofactor to an enzyme. The other type of cofactors are metals. The role of cofactors varies with the type of cofactor and enzyme. An enzyme with a cofactor is catalytically active and is known as a Holoenzyme.
Coenymes usually originate from vitamins. They can be tightly or loosely bound to the enzyme. Coenzymes that are tightly bound are known as prosthetic groups, whilst loosely bound coenzymes are called cosubstrates. This is becuase they actually bind and release 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 .
- ↑ Berg JM, Tymoczko, Stryer L. (2011), Biochemistry,7th Edition, New York: WH Freeman (p229, p456-8)