Flavin group

{{Short description|Group of chemical compounds}} [[Image:Riboflavin.svg|thumb|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:alt=|center|framelessFlavins 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: [[Image:FAD FADH2 equlibrium.png|thumb|400px|center|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.<ref name="Michaelis">{{cite journal | vauthors = Michaelis L, Schubert MP, Smythe CV | title = Potentiometric study of the flavins. | journal = Journal of Biological Chemistry | date = 1936 | volume = 116 | issue = 2 | pages = 587–607 | doi = 10.1016/S0021-9258(18)74634-6 | url = http://www.jbc.org/cgi/reprint/116/2/587 | doi-access = free | access-date = 2008-04-25 | archive-date = 2009-08-08 | archive-url = https://web.archive.org/web/20090808044001/http://www.jbc.org/cgi/reprint/116/2/587 | url-status = dead }}</ref> The oxidized and reduced forms are in fast equilibrium with the semiquinone (radical) form, shifted against the formation of the radical:<ref name="Massey1">{{cite journal | vauthors = Massey V, Stankovich M, Hemmerich P | title = Light-mediated reduction of flavoproteins with flavins as catalysts | journal = Biochemistry | volume = 17 | issue = 1 | pages = 1–8 | date = January 1978 | pmid = 618535 | doi = 10.1021/bi00594a001 }}</ref>

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.<ref name="Michaelis">{{cite journal | vauthors = Michaelis L, Schubert MP, Smythe CV | title = Potentiometric study of the flavins. | journal = Journal of Biological Chemistry | date = 1936 | volume = 116 | issue = 2 | pages = 587–607 | doi = 10.1016/S0021-9258(18)74634-6 | url = http://www.jbc.org/cgi/reprint/116/2/587 | doi-access = free | access-date = 2008-04-25 | archive-date = 2009-08-08 | archive-url = https://web.archive.org/web/20090808044001/http://www.jbc.org/cgi/reprint/116/2/587 | url-status = dead }}</ref> The oxidized and reduced forms are in fast equilibrium with the semiquinone (radical) form, shifted against the formation of the radical:<ref name="Massey1">{{cite journal | vauthors = Massey V, Stankovich M, Hemmerich P | title = Light-mediated reduction of flavoproteins with flavins as catalysts | journal = Biochemistry | volume = 17 | issue = 1 | pages = 1–8 | date = January 1978 | pmid = 618535 | doi = 10.1021/bi00594a001 }}</ref>

::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&nbsp;— 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.<ref name="Massey1" /> 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.<ref>{{cite journal | vauthors = Alexandre MT, Domratcheva T, Bonetti C, van Wilderen LJ, van Grondelle R, Groot ML, Hellingwerf KJ, Kennis JT | title = Primary reactions of the LOV2 domain of phototropin studied with ultrafast mid-infrared spectroscopy and quantum chemistry | journal = Biophysical Journal | volume = 97 | issue = 1 | pages = 227–37 | date = July 2009 | pmid = 19580760 | pmc = 2711383 | doi = 10.1016/j.bpj.2009.01.066 | bibcode = 2009BpJ....97..227A }}</ref>

== 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 === right|thumb|FAD{{Main|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.{{cn|date=August 2025}}

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.{{cn|date=August 2025}}

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

=== FMN === {{Main|Flavin mononucleotide}} right|thumb|FMN Flavin mononucleotide is a prosthetic group found in, among other proteins, NADH dehydrogenase, ''E.coli'' nitroreductase and old yellow enzyme.{{cn|date=August 2025}}

== See also == *Pteridine *Pterin *Deazaflavin (5-deazaflavin)

== References == {{Reflist}}

== Further reading == {{refbegin}} * {{cite book | vauthors = Voet D, Voet JG | date = 2004 | title = Biochemistry | edition = 3rd | publisher = John Wiley & Sons | isbn = 0-471-39223-5 | url-access = registration | url = https://archive.org/details/biochemistry00voet_1 }} {{refend}}

{{DEFAULTSORT:Flavin Group}} Category:Cellular respiration Category:Flavins