Fadh

FADH is a biochemistry term that appears in the literature as a reduced form of flavin adenine dinucleotide (FAD). Because flavin redox chemistry involves multiple states, the exact meaning of FADH is context-dependent. In many sources, FADH refers to the fully reduced cofactor FADH2, which carries two electrons and two protons. In other contexts, FADH may denote the semiquinone radical FADH• formed during single-electron transfers.

Flavin adenine dinucleotide (FAD) is a redox-active cofactor derived from riboflavin. It participates in oxidation–reduction reactions by cycling between oxidized FAD, semiquinone FADH•, and reduced FADH2. Enzymes can accept either one electron (to form FADH•) or two electrons (to form FADH2), depending on the reaction.

Biological role: In mitochondria and other organisms, FADH2 produced by flavoprotein reactions donates electrons to the electron transport chain, typically via ubiquinone, contributing to ATP synthesis. Each FADH2 generally yields about 1.5 ATP during oxidative phosphorylation, less than the approx­imate 2.5 ATP per NADH.

Occurrences: FAD is a prosthetic group in many oxidoreductases, including succinate dehydrogenase (Complex II), fatty acyl-CoA dehydrogenases, and monoamine oxidases. The FAD/FADH2 couple participates in diverse metabolic pathways, including energy production and biosynthetic processes.

See also: flavin adenine dinucleotide, FADH2, semiquinone, flavoproteins.