Flavin

Overview
Flavin 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, amongst other proteins, NADH dehydrogenase, E.coli nitroreductase and old yellow enzyme.