Flavin adenine dinucleotide

Flavin adenine dinucleotide is a redox coenzyme with a flavin ring system linked to an adenine-containing nucleotide unit. Reduction of the flavin gives FADH2.

In succinate dehydrogenase, an enzyme-bound flavin accepts reducing equivalents during conversion of succinate to fumarate. The flavin is reoxidized through the respiratory electron-transfer machinery, connecting a carbon-metabolism reaction with the mitochondrial respiratory chain.

FADH2 is the two-electron-reduced form of flavin adenine dinucleotide. The notation summarizes reduction of the flavin portion, while the surrounding enzyme controls how electrons and protons are transferred.

In succinate dehydrogenase, reduction and reoxidation occur with the flavin bound to the enzyme. Thus FADH2 should not always be pictured as a freely diffusing molecule that carries electrons between distant enzymes.