flavin adenine dinucleotide

FAD is a prosthetic group composed of riboflavin (vitamin B2) linked to adenine dinucleotide through a ribitol phosphate chain. In cellular metabolism, FAD functions as an electron acceptor, becoming reduced to FADH2 when it accepts two electrons and two protons. This occurs primarily in the citric acid cycle (succinate → fumarate reaction via succinate dehydrogenase) and during β-oxidation of fatty acids (acyl-CoA dehydrogenase reactions). FADH2 then donates electrons to Complex II (succinate dehydrogenase) of the electron transport chain, ultimately producing approximately 1.5 ATP molecules per FADH2 (compared to 2.5 ATP from NADH). FAD is also essential for amino acid metabolism, particularly in reactions catalyzed by monoamine oxidase and D-amino acid oxidase. Riboflavin deficiency leads to decreased FAD synthesis, resulting in ariboflavinosis with symptoms including cheilosis, glossitis, seborrheic dermatitis, and corneal vascularization.