A coenzyme important in various biochemical reactions. It comprises a phosphorylated vitamin B2 (riboflavin) molecule linked to the nucleotide adenine monophosphate (AMP). FAD is usually tightly bound to the enzyme forming a flavoprotein. It functions as a hydrogen acceptor in dehydrogenation reactions, being reduced to FADH2. This in turn is oxidized to FAD by the electron transport chain, thereby generating ATP (two molecules of ATP per molecule of FADH2).