flavin adenine dinucleotide (FAD)

In biochemistry, flavin adenine dinucleotide (FAD) is a redox cofactor involved in several important reactions in metabolism.

FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecule consists of a riboflavin moiety (vitamin B2) bound to the phosphate group of an ADP molecule. The flavin group is bound to ribitol, a sugar alcohol, by a carbon-nitrogen bond, not a glycosidic bond. Thus, riboflavin is not technically a nucleotide; the name flavin adenine dinucleotide is a misnomer.

FAD accepts two electrons and two protons to become FADH₂. FADH2 can then be oxidized to the semireduced form (semiquinone) FADH by donating one electron and one proton.

A flavoprotein is a protein that contains a flavin moiety, this may be in the form of FAD or FMN. There are many flavoproteins besides components of the succinate dehydrogenase complex, including α-ketoglutarate dehydrogenase and a component of the pyruvate dehydrogenase complex.