- Flavin adenine dinucleotide (FAD) is a redox-active coenzyme derived from vitamin B₂ (riboflavin) and plays a crucial role in a wide variety of cellular metabolic processes. It is one of the two principal coenzymes formed from riboflavin, the other being flavin mononucleotide (FMN).
- FAD is essential in biological oxidation-reduction (redox) reactions, serving as an electron carrier in many enzymatic systems, particularly in pathways involved in energy production, such as the citric acid cycle and electron transport chain.
- Structurally, FAD consists of a flavin mononucleotide (FMN) moiety linked via a phosphate bridge to an adenosine monophosphate (AMP) molecule. This gives FAD a larger and more complex structure compared to FMN, with a molecular formula of C₂₇H₃₃N₉O₁₅P₂ and a molecular weight of approximately 785.55 g/mol. The flavin portion of FAD contains an isoalloxazine ring system, which is responsible for its distinctive yellow color and redox activity. This ring system is the site of reversible electron transfer, making FAD a key cofactor in redox reactions.
- FAD is synthesized intracellularly from riboflavin in a two-step process. First, riboflavin is phosphorylated by riboflavin kinase to produce FMN. Then, FAD synthetase (also called FMN adenylyltransferase) catalyzes the addition of an AMP group from ATP to FMN, forming FAD. This activation of riboflavin into FAD enables its incorporation into flavoproteins, which are enzymes that rely on flavin cofactors for their catalytic activity.
- Biologically, FAD functions primarily as a redox coenzyme, undergoing reversible two-electron redox reactions. It can exist in three redox states: the oxidized form (FAD), the semiquinone radical (FADH•), and the fully reduced form (FADH₂). In this role, FAD accepts electrons and protons during enzymatic reactions, becoming reduced to FADH₂. It then donates these electrons to downstream components in metabolic pathways, such as the electron transport chain, where it contributes to ATP generation via oxidative phosphorylation.
- One of the most well-known roles of FAD is in the citric acid cycle, where it acts as a coenzyme for the enzyme succinate dehydrogenase. In this reaction, FAD accepts electrons during the oxidation of succinate to fumarate, forming FADH₂. Unlike NADH, which donates electrons to Complex I of the mitochondrial electron transport chain, FADH₂ transfers electrons directly to Complex II, bypassing the first proton pump and thus yielding slightly less ATP per molecule.
- FAD is also involved in other important enzymatic reactions, including those catalyzed by acyl-CoA dehydrogenases (in fatty acid β-oxidation), monoamine oxidases (in neurotransmitter metabolism), glutathione reductase, and various oxidases and hydroxylases. In many flavoproteins, FAD is tightly but non-covalently bound; however, in some enzymes, it is covalently attached via a histidine or cysteine residue, which can affect its redox potential and functional dynamics.
- From a physiological and clinical perspective, FAD is essential for normal metabolic function. Since it is synthesized from riboflavin, inadequate dietary intake of riboflavin can impair FAD-dependent enzymatic activity, leading to symptoms like fatigue, cracked lips, sore throat, skin disorders, and anemia. Inborn errors of flavin metabolism or mutations in flavoproteins can also result in metabolic disorders, such as glutaric acidemia or multiple acyl-CoA dehydrogenase deficiency.
- In research and biotechnology, FAD’s redox properties and ability to absorb and emit light have been leveraged in biosensors, fluorescent probes, and enzyme assays. It is also being explored in synthetic biology for engineering redox-active proteins and metabolic pathways. The dual nucleotide structure of FAD—containing both flavin and adenine groups—makes it especially versatile in protein interactions and electron transfer processes.
