Flavin group

Riboflavin, a flavin vitamin

Flavins (from Latin flavus, "yellow") refers generally to the class of organic compounds generally derived from isoalloxazine by varying the R group shown:

Flavins have extensive reduction-oxidation chemistry, and are named for the characteristic yellow color they exhibit in certain oxidation states. Their importance derives from biology: flavins are ubiquitous cofactors for biochemical redox reactions, particularly the electron transport chain.

Despite the lexicographic similarity, flavins are chemically and biologically distinct from the flavanoids and flavonols.

Redox properties

Isoalloxazine is a chemically-aromatic compound with multiple rings and quinone-like oxygenation. It, and the flavins in general, are thus capable of undergoing single-electron oxidation-reduction reactions. 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]

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).

Photoreduction

Both free and protein-bound flavins are photoreducible — that is, able to be reduced by light. The process is mediated by several organic compounds, such as some amino acids, carboxylic acids and amines. ^[2] 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. ^[3]

In biology

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); in other circumstances, it 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.

FAD, FADH, and FADH₂

FAD

Main article: Flavin adenine dinucleotide

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.^{[ citation needed ]}

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.^{[ citation needed ]}

FADH₂ is one of the cofactors that can transfer electrons to the electron transfer chain.

FMN

Main article: Flavin mononucleotide

FMN

Flavin mononucleotide is a prosthetic group found in, among other proteins, NADH dehydrogenase, E.coli nitroreductase and old yellow enzyme.^{[ citation needed ]}

See also

• Pteridine
• Pterin
• Deazaflavin (5-deazaflavin)

References

1. Michaelis L, Schubert MP, Smythe CV (1936). "Potentiometric study of the flavins". Journal of Biological Chemistry. 116 (2): 587–607. doi: 10.1016/S0021-9258(18)74634-6 . Archived from the original on 2009-08-08. Retrieved 2008-04-25.
2. Massey V, Stankovich M, Hemmerich P (January 1978). "Light-mediated reduction of flavoproteins with flavins as catalysts". Biochemistry. 17 (1): 1–8. doi:10.1021/bi00594a001. PMID   618535.
3. Alexandre MT, Domratcheva T, Bonetti C, van Wilderen LJ, van Grondelle R, Groot ML, Hellingwerf KJ, Kennis JT (July 2009). "Primary reactions of the LOV2 domain of phototropin studied with ultrafast mid-infrared spectroscopy and quantum chemistry". Biophysical Journal. 97 (1): 227–37. Bibcode:2009BpJ....97..227A. doi:10.1016/j.bpj.2009.01.066. PMC   2711383 . PMID   19580760.

Further reading

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