FADH₂ in the context of "Flavoprotein"

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⭐ Core Definition: FADH₂

In biochemistry, flavin adenine dinucleotide (FAD) 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 (FMN). Many flavoproteins are known: components of the succinate dehydrogenase complex, α-ketoglutarate dehydrogenase, and a component of the pyruvate dehydrogenase complex.

FAD exists in two common oxidation states, the fully oxidized form (FAD) and the fully reduced, dihydrogenated form, FADH2. Other oxidation states also exist, including the N-oxide and semiquinone states. FAD, in its fully oxidized form, accepts two electrons and two protons to become FADH2. The semiquinone (FADH) 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(5)-oxide.

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FADH₂ in the context of Oxidative phosphorylation

Oxidative phosphorylation or electron transport-linked phosphorylation or terminal oxidation, is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing chemical energy in order to produce adenosine triphosphate (ATP). In eukaryotes, this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is so pervasive because it releases more energy than fermentation.

In aerobic respiration, the energy stored in the chemical bonds of glucose is released by the cell in glycolysis and subsequently the citric acid cycle, producing carbon dioxide and the energetic electron donors NADH and FADH₂. Oxidative phosphorylation uses these molecules and O2 to produce ATP, which is used throughout the cell whenever energy is needed. During oxidative phosphorylation, electrons are transferred from the electron donors to a series of electron acceptors in a series of redox reactions ending in oxygen, whose reaction releases half of the total energy.

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