Flavin (from Latin flavus, "yellow") is a tricyclic heteronuclear organic ring based on pteridine, whose biochemical source is the vitamin riboflavin. The flavin moiety is often attached with an adenosine diphosphate to form flavin adenine dinucleotide (FAD), and, in other circumstances, is found as flavin mononucleotide (or FMN), a phosphorylated form of riboflavin. It is in one or the other of these forms that flavin is present as a prosthetic group in flavoproteins.

The flavin group is capable of undergoing oxidation-reduction reactions, and can accept either one electron in a two-step process or can accept two electrons at once. In the form of FADH₂, it is one of the cofactors that can transfer electrons to the electron transfer chain.

FAD

Flavin adenine dinucleotide is a prosthetic group bound to many enzymes including ferredoxin-NADP+ reductase, monoamine oxidase, D-amino acid oxidase, glucose oxidase, xanthine oxidase, and acyl CoA dehydrogenase.  

FADH / FADH₂

FADH and FADH₂ are reduced forms of FAD. FADH₂ is produced as a prosthetic group in succinate dehydrogenase, an enzyme involved in the citric acid cycle. In oxidative phosphorylation, two molecules of FADH₂ typically yield 2.5 ATP.

FMN

Flavin mononucleotide is a prosthetic group found in, among other proteins, NADH dehydrogenase, E.coli nitroreductase and old yellow enzyme.  

Use as a dye

Flavin is the most important coloring matter obtained from quercitron (the bark of Quercus velutina, also called dyer's oak). Flavin is the commercial name for quercitron. Two preparations are "yellow flavin" and "red flavin," which contains only quercetin. [1]

See also

• Pteridine
• Pterin

References

• Voet, D.; Voet, J.G. (2004). Biochemistry (3rd ed.). John Wiley & Sons. ISBN 0-471-39223-5