flavin

Flavin is a term used for a family of heterocyclic compounds based on the isoalloxazine ring, most commonly referring to riboflavin (vitamin B2) and its derivatives flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Riboflavin is a water-soluble vitamin essential in small amounts for humans; FMN and FAD function as tightly bound cofactors in many enzymes.

Chemistry and redox biology: Flavins can cycle between oxidized, semiquinone, and fully reduced states, enabling two-electron transfers and coupling with proton transfers. This redox versatility allows flavins to participate in a wide range of oxidative and reductive reactions. As prosthetic groups or co-substrates, flavins are central to numerous flavoproteins involved in metabolism and energy production.

Biological roles: Flavoproteins catalyze dehydrogenation, hydroxylation, and other oxidoreductive reactions. In mitochondria, FMN is a prosthetic group of complex I, and FAD is a prosthetic group of succinate dehydrogenase. Flavoproteins participate in fatty acid oxidation, amino acid metabolism, and the detoxification of reactive oxygen species. Some flavins are photoreactive and are used in fluorescence-based detection methods.

Dietary sources and biosynthesis: Humans cannot synthesize riboflavin and must obtain it from the diet, including dairy products, meat, eggs, and fortified foods. Many microorganisms and plants synthesize riboflavin de novo via a pathway starting from GTP and ribulose-5-phosphate. Riboflavin deficiency, though rare in developed countries, can cause ariboflavinosis, with symptoms such as sore throat, stomatitis, seborrheic dermatitis, and anemia.

History: The name flavin derives from the yellow color observed in many riboflavin-containing compounds.