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 and its biochemical role usually involves changing between these two states. FAD can be reduced to the FADH₂, whereby it accepts two hydrogen atoms:

FADH₂ is an energy-carrying molecule, and the reduced coenzyme can be used as a substrate for oxidative phosphorylation in the mitochondria. FADH₂ is reoxidized to FAD, which makes it possible to produce two moles of the universal energy carrier ATP. The primary sources of reduced FAD in eukaryotic metabolism are the citric acid cycle and the beta oxidation reaction pathways. In the citric acid cycle, FAD is a prosthetic group in the enzyme succinate dehydrogenase that oxidizes succinate to fumarate, whereas in beta oxidation it serves as a coenzyme in the reaction of acyl CoA dehydrogenase.

FAD is derived from riboflavin (vitamin B₂). Many oxidoreductases, called flavoenzymes or flavoproteins, require FAD as a prosthetic group which functions in electron transfers.

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