Flavin is a tricyclic heteronuclear organic ring based on pteridine whose biochemical source is the vitamin 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. 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. Since FMN fluoresces more than FAD, these flavins can be readily distinguished experimentally through a simple fluorescence-based assay. In this assay, the 530 nm fluorescence (upon a 360 nm excitation) of a sample solution is measured before and after treatment with Naja naja venom. Under these conditions the fluorescence of FMN solutions does not change, but the phosphodiesterases present in the venom cleave FAD into FMN and adenine, yielding a 10-fold increase of fluorescence of FAD solutions .
FADH and FADH2
FADH and FADH2 are (respectively) one-electron and two-electron reduced forms of FAD. The redox potential (E0)of the FAD/FADH2 pair is approximately 0 mV. It is used instead of NAD+ (E0=-320 mV) as an electron acceptor in oxidation reactions with a reduction potential between approximately -300 mV and 0 mV. This occurs in some steps of the citric acid cycle and the [[beta-oxidation|&beta-oxidation] of fatty acids. Since the oxidized form (FAD) of the electron-carrier is a reactant in metabolism, it must be regenerated: this is accomplished by transferring the electrons carried by FADH2 to a final electron acceptor. In aerobic respiration, it donates its electrons to O2 (through the mitochondrial electron transport chain), releasing enough energy to synthesize approximately 1.5 ATP per FADH2.
- Voet, D.; Voet, J.G. (2004). Biochemistry (3rd ed.). John Wiley & Sons. ISBN 0-471-39223-5