page_title: Flavin adenine dinucleotide
section_title: Chemical states
hierarchical_section_title: Flavin adenine dinucleotide / Chemical states
caption_attribution_description: English: Gain of H- and formation of N-
context_page_description: In biochemistry, flavin adenine dinucleotide is a redox-active coenzyme associated with various proteins, which is involved with several enzymatic reactions in metabolism. A flavoprotein is a protein that contains a flavin group, which may be in the form of FAD or flavin mononucleotide. Many flavoproteins are known: components of the succinate dehydrogenase complex, α-ketoglutarate dehydrogenase, and a component of the pyruvate dehydrogenase complex.
  FAD can exist in four redox states, which are the flavin-N-oxide, quinone, semiquinone, and hydroquinone. FAD is converted between these states by accepting or donating electrons. FAD, in its fully oxidized form, or quinone form, accepts two electrons and two protons to become FADH₂. The semiquinone can be formed by either reduction of FAD or oxidation of FADH₂ by accepting or donating one electron and one proton, respectively. Some proteins, however, generate and maintain a superoxidized form of the flavin cofactor, the flavin-N-oxide.
context_section_description: In biological systems, FAD acts as an acceptor of H⁺ and e⁻ in its fully oxidized form, an acceptor or donor in the FADH form, and a donor in the reduced FADH₂ form. The diagram below summarizes the potential changes that it can undergo.
  Along with what is seen above, other reactive forms of FAD can be formed and consumed. These reactions involve the transfer of electrons and the making/breaking of chemical bonds. Through reaction mechanisms, FAD is able to contribute to chemical activities within biological systems. The following pictures depict general forms of some of the actions that FAD can be involved in.
  Mechanisms 1 and 2 represent hydride gain, in which the molecule gains what amounts to be one hydride ion. Mechanisms 3 and 4 radical formation and hydride loss. Radical species contain unpaired electron atoms and are very chemically active. Hydride loss is the inverse process of the hydride gain seen before. The final two mechanisms show nucleophilic addition and a reaction using a carbon radical.
caption: Mechanism 1. Hydride transfer occurs by addition of H+ and 2 e−
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