FAD

In biochemistry, flavin adenine dinucleotide (FAD) is a redox cofactor involved in several important reactions in metabolism. FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecule consists of a riboflavin moiety (vitamin B2) bound to the phosphate group of an ADP molecule. The flavin group is bound to ribitol, a sugar alcohol, by a carbon-nitrogen bond, not a glycosidic bond. Thus, riboflavin is not technically a nucleotide; the name flavin adenine dinucleotide is a misnomer.

FAD can be reduced to FADH2, whereby it accepts two hydrogen atoms (a net gain of two electrons):


 * [[Image:FAD FADH2 equlibrium.png|422px]]

FAD (fully oxidized form, or quinone form) accepts two electrons and two protons to become FADH2 (hydroquinone form). FADH2 can then be oxidized to the semireduced form (semiquinone) FADH by donating one electron and one proton. The semiquinone is then oxidized once more by losing an electron and a proton and is returned to the initial quinone form (FAD).

FAD is an aromatic ring system, whereas FADH2 is not. This means that FADH2 is significantly higher in energy, without the stabilization that aromatic structure provides. FADH2 is an energy-carrying molecule, because, if it is oxidized, it will regain aromaticity and release all the energy represented by this stabilization.

The primary biochemical role of FADH2 in eukaryotes is to carry high-energy electrons used for oxidative phosphorylation. FAD is a prosthetic group in the enzyme complex succinate dehydrogenase (complex II) that oxidizes succinate to fumarate in the eighth step of the citric acid cycle. The high-energy electrons from this oxidation are stored momentarily by reducing FAD to FADH2. FADH2 then reverts to FAD, sending its two high-energy electrons through the electron transport chain; the energy in FADH2 is enough to produce 1.5 equivalents of ATP by oxidative phosphorylation. Another metabolic source of FADH2 is beta oxidation, where FAD serves as a coenzyme to acyl CoA dehydrogenase.

Any oxidoreductase enzyme that uses FAD as an electron carrier is called a flavoprotein. There are many flavoproteins besides components of the succinate dehydrogenase complex, including α-ketoglutarate dehydrogenase and a component of the pyruvate dehydrogenase complex.