cofactor

Definition: 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.

Cofactors can be inorganic cofactors (non derived from carbon) or organic cofactors (derived from carbon, associated with hydrogen, oxygen, nitrogen). Some sources also limit the use of the term "cofactor" to inorganic substances. An inactive enzyme, without the cofactor is called an apoenzyme, while the complete enzyme with cofactor is the holoenzyme.

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.

Some enzymes or enzyme complexes require several cofactors. A good example is the multienzyme complex pyruvate dehydrogenase. This enzyme complex at the junction of glycolysis and the citric acid cycle requires five organic cofactors and one metal ion : loosely bound thiamine diphosphate (ThDP), covalently bound lipoamide and flavin adenine dinucleotide (FAD), and the cosubstrates nicotinamide adenine dinucleotide (NAD+) and coenzyme A (CoA) and a metal ion (Mg2+). The succinate dehydrogenase complex also has several cofactors, including flavin, iron-sulfur centers and heme.

See also

 vitamins
 cofactors and vitamins metabolism