Everything about Fadh totally explained
Flavin (from Latin
flavus, "yellow") is the common name for a group of
organic compounds based on
pteridine, formed by the
tricyclic heteronuclear organic ring isoalloxazine. The biochemical source is the 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.
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:
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
redH
2 ⇌ FlH
•
where Fl
ox is the oxidized flavin, Fl
redH
2 the reduced flavin (upon addition of two hydrogen atoms) and FlH
• the semiquinone form (addition of one hydrogen atom).
In the form of FADH
2, it's 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
aminoacids,
carboxylic acids and
amines.
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 FADH
2 are
reduced forms of FAD. FADH
2 is produced as a prosthetic group in
succinate dehydrogenase, an enzyme involved in the
citric acid cycle. In
oxidative phosphorylation, two molecules of FADH
2 typically yield 1.5
ATP each, or three ATP combined.
FMN
Flavin mononucleotide is a
prosthetic group found in, among other proteins,
NADH dehydrogenase,
E.coli nitroreductase and
old yellow enzyme.
Use as a dye
Flavin is the most important coloring matter obtained from
quercitron (the bark of
Quercus velutina, also called dyer's oak).
Flavin is the commercial name for quercitron. Two preparations are "yellow flavin" and "red flavin," which contains only
quercetin.
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