Flavin adenine dinucleotide

Flavin adenine dinucleotide (FAD) is a redox-active coenzyme derived from riboflavin. Its flavin group reversibly accepts and donates electrons in enzymatic reactions.

Flavin mononucleotide: Flavin mononucleotide (FMN) is a riboflavin-derived coenzyme containing a phosphate but no adenine nucleotide. FMN shares FAD’s redox-active flavin group but lacks its adenine nucleotide.
Riboflavin: Riboflavin, or vitamin B₂, is a water-soluble vitamin used to synthesize FMN and FAD. FAD is made from this vitamin through phosphorylation and attachment of an adenine nucleotide.
Succinate dehydrogenase: Succinate dehydrogenase is a mitochondrial enzyme complex that oxidizes succinate to fumarate and transfers electrons to ubiquinone. Its tightly bound FAD accepts electrons during succinate oxidation.
Riboflavin kinase: Riboflavin kinase transfers a phosphate from ATP to riboflavin, producing FMN. This enzyme makes the immediate flavin precursor used to synthesize FAD.
Nicotinamide adenine dinucleotide: Nicotinamide adenine dinucleotide (NAD) is a soluble coenzyme that transfers hydride equivalents in redox reactions. NAD usually diffuses between enzymes, whereas FAD often remains tightly bound to one enzyme.
FADH₂: FADH₂ is the two-electron-reduced form of flavin adenine dinucleotide. This reduced form is produced when FAD accepts two electrons and two protons.
Adenosine monophosphate: Adenosine monophosphate is a nucleotide composed of adenine, ribose, and one phosphate group. FAD’s adenine-nucleotide portion is linked to the phosphate of FMN.
Acyl-CoA dehydrogenase: Acyl-CoA dehydrogenases catalyze the first oxidation step in mitochondrial fatty-acid breakdown. Their FAD accepts electrons as a carbon–carbon double bond forms in the fatty-acid substrate.
FAD synthetase: FAD synthetase is an enzyme that converts FMN and ATP into flavin adenine dinucleotide. Its reaction creates the adenine-containing coenzyme from FMN.
Ubiquinone: Ubiquinone is a lipid-soluble electron carrier that transfers electrons within membranes. Ubiquinone is a mobile membrane carrier, unlike enzyme-bound FAD.

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