FAD

FAD

Overview
In biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, flavin adenine dinucleotide (FAD) is a redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....

 involved in several important reactions in metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

. FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecule consists of a riboflavin
Riboflavin
Riboflavin, also known as vitamin B2 or additive E101, is an easily absorbed micronutrient with a key role in maintaining health in humans and animals. It is the central component of the cofactors FAD and FMN, and is therefore required by all flavoproteins. As such, vitamin B2 is required for a...

 moiety (vitamin B2) bound to the phosphate group of an ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....

 molecule. The flavin group is bound to ribitol
Ribitol
Ribitol or adonitol is a crystalline pentose alcohol formed by the reduction of ribose. It occurs naturally in the plant Adonis vernalis, as well as in the cell walls of Gram positive bacteria . It also contributes to the chemical structure of riboflavin.-External links:**...

, a sugar alcohol, by a carbon-nitrogen bond, not a glycosidic bond
Glycosidic bond
In chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate....

.
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Encyclopedia
In biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, flavin adenine dinucleotide (FAD) is a redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....

 involved in several important reactions in metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

. FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecule consists of a riboflavin
Riboflavin
Riboflavin, also known as vitamin B2 or additive E101, is an easily absorbed micronutrient with a key role in maintaining health in humans and animals. It is the central component of the cofactors FAD and FMN, and is therefore required by all flavoproteins. As such, vitamin B2 is required for a...

 moiety (vitamin B2) bound to the phosphate group of an ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....

 molecule. The flavin group is bound to ribitol
Ribitol
Ribitol or adonitol is a crystalline pentose alcohol formed by the reduction of ribose. It occurs naturally in the plant Adonis vernalis, as well as in the cell walls of Gram positive bacteria . It also contributes to the chemical structure of riboflavin.-External links:**...

, a sugar alcohol, by a carbon-nitrogen bond, not a glycosidic bond
Glycosidic bond
In chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate....

. Thus, riboflavin is not technically a nucleotide; the name flavin adenine dinucleotide is a misnomer.

FAD can be reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

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


FAD (fully oxidized form, or quinone
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...

 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
Oxidative phosphorylation
Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...

. FAD is a prosthetic group in the enzyme complex succinate dehydrogenase (complex II) that oxidizes succinate
Succinic acid
Succinic acid is a dicarboxylic acid. Succinate plays a biochemical role in the citric acid cycle. The name derives from Latin succinum, meaning amber, from which the acid may be obtained....

 to fumarate
Fumaric acid
Fumaric acid or trans-butenedioic acid is the chemical compound with the formula HO2CCH=CHCO2H. This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid. In fumaric acid the carboxylic acid groups are trans and in maleic acid they are cis...

 in the eighth step of the citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

. 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
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

; the energy in FADH2 is enough to produce 1.5 equivalents of ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 by oxidative phosphorylation. Another metabolic source of FADH2 is beta oxidation
Beta oxidation
Beta oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle....

, where FAD serves as a coenzyme to acyl CoA dehydrogenase
Acyl CoA dehydrogenase
Acyl-CoA dehydrogenases are a class of enzymes that function to catalyze the initial step in each cycle of fatty acid β -oxidation in the mitochondria of cells. Their action results in the introduction of a trans double-bond between C2 and C3 of the acyl-CoA thioester substrate...

.

Any oxidoreductase
Oxidoreductase
In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule to another...

 enzyme that uses FAD as an electron carrier is called a flavoprotein
Flavoprotein
Flavoproteins are proteins that contain a nucleic acid derivative of riboflavin: the flavin adenine dinucleotide or flavin mononucleotide ....

. There are many flavoproteins besides components of the succinate dehydrogenase complex, including α-ketoglutarate dehydrogenase and a component of the pyruvate dehydrogenase complex.

See also

  • Flavin mononucleotide
    Flavin mononucleotide
    Flavin mononucleotide , or riboflavin-5′-phosphate, is a biomolecule produced from riboflavin by the enzyme riboflavin kinase and functions as prosthetic group of various oxidoreductases including NADH dehydrogenase as well as cofactor in biological blue-light photo receptors...

  • NAD
    Nicotinamide adenine dinucleotide
    Nicotinamide adenine dinucleotide, abbreviated NAD, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base and the other nicotinamide.In metabolism, NAD is involved...