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Flavins (from Latinflavus, "yellow") refers generally to the class oforganic compounds containing the tricyclicheterocycleisoalloxazine or its isomer alloxazine, and derivatives thereof. The biochemical source of flavin is the yellow B vitaminriboflavin. The flavinmoiety is often attached with anadenosine diphosphate to formflavin adenine dinucleotide (FAD), and, in other circumstances, is found asflavin mononucleotide (orFMN), aphosphorylated form ofriboflavin. It is in one or the other of these forms that flavin is present as aprosthetic group inflavoproteins. Despite the similar names, flavins (with "i") are chemically and biologically distinct from theflavanoids (with "a"), and theflavonols (with "o").
The flavin group is capable of undergoingoxidation-reduction reactions, and can accept either oneelectron in a two-step process or two electrons at once. Reduction is made with the addition ofhydrogen atoms to specificnitrogen atoms on theisoalloxazine ring system:
Inaqueous 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 fastequilibrium with the semiquinone (radical) form, shifted against the formation of the radical:[2]
where Flox is the oxidized flavin, FlredH2 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 theelectron transfer chain.
Both free and protein-bound flavins arephotoreducible, that is, able to be reduced bylight, in a mechanism mediated by several organic compounds, such as someamino acids,carboxylic acids andamines.[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]
Flavin adenine dinucleotide is a group bound to many enzymes includingferredoxin-NADP+ reductase,monoamine oxidase,D-amino acid oxidase,glucose oxidase,xanthine oxidase, andacyl CoA dehydrogenase.[citation needed]
FADH and FADH2 arereduced forms of FAD. FADH2 is produced as a prosthetic group insuccinate dehydrogenase, an enzyme involved in thecitric acid cycle. Inoxidative phosphorylation, two molecules of FADH2 typically yield 1.5ATP each, or three ATP combined.[citation needed]
Flavin mononucleotide is aprosthetic group found in, among other proteins,NADH dehydrogenase,E.coli nitroreductase andold yellow enzyme.[citation needed]