Flavin is a tricyclic heteronuclear organic ring whose biochemical source is the vitamin riboflavin. The flavin moiety is often conjugated with an adenosine diphosphate to form flavin adenine dinucleotide (e.g. FAD), and in other circumstances, is found as flavin mononucleotide (or FMN), a phosphorylated form of riboflavin. The flavin group is capable of undergoing oxidation-reduction reactions, and can accept either one electron in a two step process or can accept two electrons at once. In the form of FADH2, it is one of the cofactors that can transfer electrons to the electron transfer chain. FMN is a prosthetic group found in NADH dehydrogenase and FAD is a cofactor in the enzymes D-amino acid oxidase, glucose oxidase, and xanthine oxidase.
NADH and FADH2 then transfer protons and electrons to the electron transport chain to produce additional ATPs from oxidative phosphorylation.
During the process of aerobic respiration, coupled oxidation-reduction reactions and electron carriers are often part of an electron transport chain, a series of electron carriers that eventually transfers electrons from NADH and FADH2 to oxygen.
The diffusible electron carriers NADH and FADH2 carry hydrogen atoms (protons and electrons) from substrates in exergonic catabolic pathways such as glycolysis and the citric acid cycle to other electron carriers that are embedded in membranes.
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