Flavin group

Flavins (from Latin flavus, "yellow") refers generally to the class of organic compounds containing the tricyclic heterocycle isoalloxazine or its isomer alloxazine, and derivatives thereof. The biochemical source of flavin is the yellow B 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. Despite the similar names, flavins (with "i") are chemically and biologically distinct from the flavanoids (with "a"), and the flavonols (with "o").

There are 18 key atoms in isoalloxazine that make up its characteristic three-ring structure. The R-group varies and differentiates various flavins.

Riboflavin

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. The oxidized and reduced forms are in fast equilibrium with the semiquinone (radical) form, shifted against the formation of the radical:

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. This property of flavins is exploited by various light-sensitive proteins. For example, the LOV domain, found in many species of plant, fungi and bacteria, undergoes a reversible, light-dependent structural change which involves the formation of a bond between a cysteine residue in its peptide sequence and a bound FMN.

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)

Further reading

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