A
cofactor is a non-protein
chemical compound that is bound (either tightly or loosely) to a
protein and is required for the protein's biological activity. These proteins are commonly enzymes and cofactors can be considered "helper molecules/ions" that assist in biochemical transformations. With enzymes, cofactors are also often further classified depending on how tightly they bind to the protein, with loosely-bound cofactors termed
coenzymes and tightly-bound cofactors termed
prosthetic groups. Some sources also limit the use of the term "cofactor" to inorganic substances.
[1][2] An inactive enzyme, without the cofactor is called an
apoenzyme, while the complete enzyme with cofactor is the
holoenzyme.
[3]Some enzymes or enzyme complexes require several cofactors. A good example is the multienzyme complex pyruvate dehydrogenase.[4] This enzyme complex at the junction of glycolysis and the citric acid cycle requires five organic cofactors and one metal ion&_160; loosely bound thiamine pyrophosphate (TPP), covalently bound lipoamide and flavin adenine dinucleotide (FAD), and the cosubstrates nicotinamide adenine dinucleotide (NAD+) and coenzyme A (CoA) and a metal ion (Mg2+).
Organic cofactors are often vitamins or are made from vitamins. Many contain the nucleotide adenosine monophosphate (AMP) as part of their structures, such as ATP, coenzyme A, FAD and NAD+. This common structure may reflect a common evolutionary origin as part of ribozymes in an ancient RNA world. It has been suggested that the AMP part of the molecule can be considered a kind a "handle" by which the enzyme can "grasp" the coenzyme to switch it between different catalytic centers.[5]
Cofactors can be divided into two broad groups organic cofactors, such as flavin or heme, and inorganic cofactors such as the metal ions Mg2+, Cu+, Mn2+ or iron-sulfur clusters.
