Editing Flavin group

{{Short description|Group of chemical compounds}}
{{Lead rewrite|date=September 2022|reason=Per [[MOS:INTRO]], "avoid difficult-to-understand terminology" in the lead.}}[[File:Flavin group.svg|alt=|thumb|There are 18 key atoms in [[isoalloxazine]] that make up its characteristic three-ring structure. The R-group varies and differentiates various flavins.]]
[[Image:Riboflavin.svg|thumb|Riboflavin]]

'''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 [[functional group|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 [[flavanoid]]s (with "a"), and the [[flavonol]]s (with "o").

The flavin group is capable of undergoing [[oxidation-reduction reaction]]s, 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|isoalloxazine ring system]]:
[[Image:FAD FADH2 equlibrium.png|thumb|400px|center|[[Chemical equilibrium|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 [[Chemical equilibrium|equilibrium]] with the semiquinone ([[Radical (chemistry)|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<sub>ox</sub> + Fl<sub>red</sub>H<sub>2</sub> ⇌ FlH<sup>•</sup>

where Fl<sub>ox</sub> is the oxidized flavin, Fl<sub>red</sub>H<sub>2</sub> the reduced flavin (upon addition of two hydrogen atoms) and FlH<sup>•</sup> the semiquinone form (addition of one hydrogen atom).

In the form of FADH<sub>2</sub>, it is one of the cofactors that can transfer electrons to the [[electron transfer chain]].

==Photoreduction==
Both free and protein-bound flavins are [[Photoreduction|photoreducible]], that is, able to be reduced by [[light]], in a mechanism mediated by several organic compounds, such as some [[amino acid]]s, [[carboxylic acid]]s and [[amine]]s.<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>

==FAD==
[[image:Flavin adenine dinucleotide.png|right|thumb|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/FADH<sub>2</sub>==
FADH and FADH<sub>2</sub> are [[redox|reduced]] forms of FAD. FADH<sub>2</sub> is produced as a prosthetic group in [[succinate dehydrogenase]], an enzyme involved in the [[citric acid cycle]]. In [[oxidative phosphorylation]], two molecules of FADH<sub>2</sub> typically yield 1.5 [[adenosine triphosphate|ATP]] each, or three ATP combined.

==FMN==
[[image:Flavin mononucleotide.png|right|thumb|FMN]]
[[Flavin mononucleotide]] is a [[prosthetic group]] found in, among other proteins, [[NADH dehydrogenase]], [[E. coli nitroreductase|''E.coli'' nitroreductase]] and [[old yellow enzyme]].

== 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| ]]