Flavin group

The tricyclic ring isoalloxazine, from which flavins are derived

Riboflavin

Flavin (from Latin flavus, "yellow") is the common name for a group of organic compounds based on pteridine, formed by the tricyclic heterocycle isoalloxazine. The 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 two electrons at once. Reduction is made with the addition of hydrogen atoms to specific nitrogen atoms on the isoalloxazine ring system:

Equilibrium between the oxidized (left) and totally reduced (right) forms of flavin.

In aqueous solution, flavins are yellow-coloured when oxidized, taking a red colour in the semi-reduced anionic state or blue in the neutral (semiquinone) state, and colourless when totally reduced.^[1] The oxidized and reduced forms are in fast equilibrium with the semiquinone (radical) form, shifted against the formation of the radical:^[2]

Fl_ox + Fl_redH₂ ⇌ FlH^•

where Fl_ox is the oxidized flavin, Fl_redH₂ the reduced flavin (upon addition of two hydrogen atoms) and FlH^• the semiquinone form (addition of one hydrogen atom).

In the form of FADH₂, it is one of the cofactors that can transfer electrons to the electron transfer chain.

Photoreduction

Both free and protein-bound flavins are photoreducible, that is, able to be reduced by light, in a mechanism mediated by several organic compounds, such as some amino acids, carboxylic acids and amines.^[2]

FAD

FAD

Flavin adenine dinucleotide is a 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 1.5 ATP each, or three ATP combined.

FMN

FMN

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

See also

• Pteridine
• Pterin
• Deazaflavin (5-deazaflavin)

References

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