<P> FAD plays a major role as an enzyme cofactor along with flavin mononucleotide, another molecule originating from riboflavin . Bacteria, fungi and plants can produce riboflavin, but other eukaryotes, such as humans, have lost the ability to make it . Therefore, humans must obtain riboflavin, also known as vitamin B2, from dietary sources . Riboflavin is generally absorbed in the small intestine and then transported to cells via carrier proteins . Riboflavin kinase (EC 2.7. 1.26) adds a phosphate group to riboflavin to produce flavin mononucleotide, and then FAD synthetase attaches an adenine nucleotide; both steps require ATP . Bacteria generally have one bi-functional enzyme, but archaea and eukaryotes usually employ two distinct enzymes . Current research indicates that distinct isoforms exist in the cytosol and mitochondria . It seems that FAD is synthesized in both locations and potentially transported where needed . </P> <P> Flavoproteins utilize the unique and versatile structure of flavin moieties to catalyze difficult redox reactions . Since flavins have multiple redox states they can participate in processes that involve the transfer of either one or two electrons, hydrogen atoms, or hydronium ions . The N5 and C4a of the fully oxidized flavin ring are also susceptible to nucleophilic attack . This wide variety of ionization and modification of the flavin moiety can be attributed to the isoalloxazine ring system and the ability of flavoproteins to drastically perturb the kinetic parameters of flavins upon binding, including flavin adenine dinucleotide (FAD). </P> <P> The number of flavin - dependent protein encoded genes in the genome (the flavoproteome) is species dependent and can range from 0.1% - 3.5%, with humans having 90 flavoprotein encoded genes . FAD is the more complex and abundant form of flavin and is reported to bind to 75% of the total flavoproteome and 84% of human encoded flavoproteins . Cellular concentrations of free or non-covalently bound flavins in a variety of cultured mammalian cell lines were reported for FAD (2.2 - 17.0 amol / cell) and FMN (0.46 - 3.4 amol / cell). </P> <P> FAD has a more positive reduction potential than NAD+ and is a very strong oxidizing agent . The cell utilizes this in many energetically difficult oxidation reactions such as dehydrogenation of a C-C bond to an alkene . FAD - dependent proteins function in a large variety of metabolic pathways including electron transport, DNA repair, nucleotide biosynthesis, beta - oxidation of fatty acids, amino acid catabolism, as well as synthesis of other cofactors such as CoA, CoQ and heme groups . One well - known reaction is part of the citric acid cycle (also known as the TCA or Krebs cycle); succinate dehydrogenase (complex II in the electron transport chain) requires covalently bound FAD to catalyze the oxidation of succinate to fumarate by coupling it with the reduction of ubiquinone to ubiquinol . The high - energy electrons from this oxidation are stored momentarily by reducing FAD to FADH . FADH then reverts to FAD, sending its two high - energy electrons through the electron transport chain; the energy in FADH is enough to produce 1.5 equivalents of ATP by oxidative phosphorylation . There are also redox flavoproteins that non-covalently bind to FAD like Acetyl - CoA - dehydrogenases which are involved in beta - oxidation of fatty acids and catabolism of amino acids like leucine (isovaleryl - CoA dehydrogenase), isoleucine, (short / branched - chain acyl - CoA dehydrogenase), valine (isobutyryl - CoA dehydrogenase), and lysine (glutaryl - CoA dehydrogenase). Additional examples of FAD - dependent enzymes that regulate metabolism are glycerol - 3 - phosphate dehydrogenase (triglyceride synthesis) and xanthine oxidase involved in purine nucleotide catabolism . There are other noncatalytic roles that FAD can play in flavoproteins such as structural roles, or involved in blue - sensitive light photoreceptors that regulate biological clocks and development, generation of light in bioluminescent bacteria . </P>

Fad and fadh2 are derivatives of the b-vitamin