(FAD; also riboflavin-5’-adenosine diphosphate), a nonprotein component (coenzyme) of most flavoproteins, which are present in all living cells; a derivative of riboflavin. Molecular wieght, 785.56.

FAD participates in the oxidation-reduction processes of organisms. It is soluble in water and organic solvents and is characterized by intensive fluorescence, with a maximum emission between 520 and 540 nanometers. In a number of cases, it exhibits strong covalent bonding with proteins (for example, in mono-amine oxidase); in some instances, it is capable of dissociating from an apoenzyme similar to nicotinamide adenine dinucleotide. Its catalytic activity is related to the capacity of nitrogen atoms in the FAD molecule’s isoalloxazine ring to affix two hydrogen atoms from the oxidized substrate. This process occurs in two stages, with the formation of the intermediate product semiquinone, which contains a nitrogen atom with an unshared electron. The oxidation-reduction potential of FAD depends on the nature of the apoenzyme; it changes from – 0.06 volt (at pH 7.0) for the free form of FAD to –0.185 volt for several flavoproteins. FAD biosynthesis is brought about by flavin mononucleotide (FMN) and adenosine triphosphate (ATP) with the participation of the enzyme adenylil transferase.

The acronym FAD is widely used in the biological literature.