(Difference between pages)

Page 1

Page 1Revision 1

Page 2

Page 2Revision 2

Content deleted Content addedVisualWikitext

Revision as of 17:16, 17 February 2012 editBeetstra (talk | contribs)Edit filter managers, Administrators172,031 edits Saving copy of the {{chembox}} taken from revid 476022757 of page FAD for the Chem/Drugbox validation project (updated: 'ChemSpiderID', 'ChEMBL', 'KEGG', 'StdInChI', 'StdInChIKey').		Latest revision as of 03:36, 23 December 2024 edit Graeme Bartlett (talk | contribs)Administrators249,686 edits chemspider; remove dead 3dmet
Line 1:		Line 1:
			{{short description|Redox-active coenzyme}}
	{{ambox | text = This page contains a copy of the infobox ({{tl|chembox}}) taken from revid of page ] with values updated to verified values.}}
			{{Redirect|FAD}}
	{{Chembox		{{Chembox
	| Verifiedfields = changed		| Verifiedfields = changed
	| verifiedrevid = 461098620		| verifiedrevid = 477393105
	| ImageFile = flavin adenine dinucleotide.png		| ImageFile = FAD.png
	| ImageFile_Ref = {{chemboximage|correct|??}}		| ImageFile_Ref = {{chemboximage|correct|??}}
	| ImageSize = 244		| ImageSize = 150
	| ImageName = Stereo, Kekulé, skeletal formula of a minor tautomer of FAD		| ImageName = Stereo, Kekulé, skeletal formula of FAD
			| ImageClass = skin-invert-image
	| ImageFile2 = FAD_Raswin.png		| ImageFile2 = FAD_Raswin.png
	| ImageFile2_Ref = {{chemboximage|correct|??}}		| ImageFile2_Ref = {{chemboximage|correct|??}}
	| ImageSize2 = 244		| ImageSize2 = 244
	| ImageName2 = Spacefill model of a minor tautomer of FAD		| ImageName2 = Spacefill model of FAD
			|Section1={{Chembox Identifiers
	| ImageFile3 = FAD_B&S.png
			| IUPHAR_ligand = 5184
	| ImageFile3_Ref = {{chemboximage|correct|??}}
	| ImageSize3 = 244		| CASNo = 146-14-5
			| CASNo_Ref = {{cascite|correct|CAS}}
	| ImageName3 = Ball and stick model of a minor tautomer of FAD
			| PubChem = 643975
	| Section1 = {{Chembox Identifiers
			| ChemSpiderID = 559059
	| CASNo = 146-14-5
			| ChemSpiderID_Ref =
	| CASNo_Ref = {{cascite|correct|CAS}}
			| UNII = ZC44YTI8KK
	| CASNo_Comment = <small></small>
			| UNII_Ref = {{fdacite|correct|FDA}}
	| PubChem = 703
			| EINECS = 205-663-1
	| PubChem_Ref = {{Pubchemcite|correct|pubchem}}
	| PubChem1 = 24868262		| DrugBank = DB03147
	| PubChem1_Ref = {{Pubchemcite|correct|pubchem}}		| DrugBank_Ref = {{drugbankcite|correct|drugbank}}
			| KEGG = C00016
	| PubChem1_Comment = <small></small>
			| KEGG_Ref = {{keggcite|changed|kegg}}
	| PubChem2 = 439154
			| MeSHName = Flavin-Adenine+Dinucleotide
	| PubChem2_Ref = {{Pubchemcite|correct|pubchem}}
			| ChEBI = 16238
	| PubChem2_Comment = <small></small>
			| ChEBI_Ref = {{ebicite|correct|EBI}}
	| PubChem3 = 25164505
			| ChEMBL = 1232653
	| PubChem3_Ref = {{Pubchemcite|correct|pubchem}}
			| ChEMBL_Ref = {{ebicite|changed|EBI}}
	| PubChem3_Comment = <small></small>
	| PubChem4 = 5288191		| Beilstein = 1208946
			| Gmelin = 108834
	| PubChem4_Ref = {{Pubchemcite|correct|pubchem}}
			| SMILES = c12cc(C)c(C)cc1N=C3C(=O)NC(=O)N=C3N2C(O)(O)(O)COP(=O)(O)OP(=O)(O)OC4(O)(O)(O4)n5cnc6c5ncnc6N
	| PubChem4_Comment = <small></small>
			| SMILES_Comment = ] form
	| PubChem5= 643975
			| SMILES1 = c12cc(C)c(C)cc1N3C(=O)NC(=O)N=C3N2C(O)(O)(O)COP(=O)()OP(=O)()OC4(O)(O)(O4)n5cnc6c5ncnc6N
	| PubChem5_Ref = {{Pubchemcite|correct|pubchem}}
			| SMILES1_Comment = ] form
	| PubChem5_Comment = <small></small>
			| SMILES2 = c12cc(C)c(C)cc1NC=3C(=O)NC(=O)NC=3N2C(O)(O)(O)COP(=O)()OP(=O)()OC4(O)(O)(O4)n5cnc6c5ncnc6N
	| ChemSpiderID = 559059
			| SMILES2_Comment = ] form
	| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}
			| SMILES3 = c12cc(C)c(C)cc1()=C3C(=O)NC(=O)N=C3N2C(O)(O)(O)COP(=O)(O)OP(=O)(O)OC4(O)(O)(O4)n5cnc6c5ncnc6N
	| ChemSpiderID1 = 388300
			| SMILES3_Comment = ] form
	| ChemSpiderID1_Ref = {{chemspidercite|correct|chemspider}}
			| InChI_Ref = {{stdinchicite|correct|PubChem}}
	| ChemSpiderID1_Comment = <small></small>
			| InChI = 1S/C27H33N9O15P2/c1-10-3-12-13(4-11(10)2)35(24-18(32-12)25(42)34-27(43)33-24)5-14(37)19(39)15(38)6-48-52(44,45)51-53(46,47)49-7-16-20(40)21(41)26(50-16)36-9-31-17-22(28)29-8-30-23(17)36/h3-4,8-9,14-16,19-21,26,37-41H,5-7H2,1-2H3,(H,44,45)(H,46,47)(H2,28,29,30)(H,34,42,43)/t14-,15+,16+,19-,20+,21+,26+/m0/s1
	| ChemSpiderID2 = 4450403
			| InChIKey = VWWQXMAJTJZDQX-UYBVJOGSSA-N
	| ChemSpiderID2_Ref = {{chemspidercite|correct|chemspider}}
			| InChIKey_Ref = {{stdinchicite|correct|PubChem}}
	| ChemSpiderID2_Comment = <small></small>
	| UNII = ZC44YTI8KK
	| UNII_Ref = {{fdacite|correct|FDA}}
	| EINECS = 205-663-1
	| DrugBank = DB03147
	| DrugBank_Ref = {{drugbankcite|correct|drugbank}}
	| KEGG = <!-- blanked - oldvalue: D00005 -->
	| KEGG_Ref = {{keggcite|correct|kegg}}
	| MeSHName = Flavin-Adenine+Dinucleotide
	| ChEBI = 16238
	| ChEBI_Ref = {{ebicite|correct|EBI}}
	| ChEMBL = <!-- blanked - oldvalue: 1232663 -->
	| ChEMBL_Ref = {{ebicite|changed|EBI}}
	| Beilstein = 1208946
	| Gmelin = 108834
	| 3DMet = B04619
	| SMILES = Cc1cc2nc3c(nc(O)nc3=O)n(CC(O)C(O)C(O)COP(O)(=O)OP(O)(=O)OCC3OC(C(O)C3O)n3cnc4c(N)ncnc34)c2cc1C
	| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
	| StdInChI = 1S/C27H33N9O15P2/c1-10-3-12-13(4-11(10)2)35(24-18(32-12)25(42)34-27(43)33-24)5-14(37)19(39)15(38)6-48-52(44,45)51-53(46,47)49-7-16-20(40)21(41)26(50-16)36-9-31-17-22(28)29-8-30-23(17)36/h3-4,8-9,14-16,19-21,26,37-41H,5-7H2,1-2H3,(H,44,45)(H,46,47)(H2,28,29,30)(H,34,42,43)/t14-,15+,16+,19-,20+,21+,26+/m0/s1
	| InChI = 1/C27H33N9O15P2/c1-10-3-12-13(4-11(10)2)35(24-18(32-12)25(42)34-27(43)33-24)5-14(37)19(39)15(38)6-48-52(44,45)51-53(46,47)49-7-16-20(40)21(41)26(50-16)36-9-31-17-22(28)29-8-30-23(17)36/h3-4,8-9,14-16,19-21,26,37-41H,5-7H2,1-2H3,(H,44,45)(H,46,47)(H2,28,29,30)(H,34,42,43)
	| StdInChIKey = VWWQXMAJTJZDQX-UYBVJOGSSA-N
	| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
	| InChIKey = VWWQXMAJTJZDQX-UYBVJOGSBL
	}}		}}
	| Section2 = {{Chembox Properties		|Section2={{Chembox Properties
	| Formula = {{Chem|C|27|H|33|P|2|N|9|O|15}}		| C=27 | H=33 | P=2 | N=9 | O=15
			| Appearance = White, vitreous crystals
	| MolarMass = 785.5497 g mol<sup>-1</sup>
			| LogP = -1.336
	| ExactMass = 785.157134455 g mol<sup>-1</sup>
			| pKa = 1.128
	| Appearance = White, vitreous crystals
	| LogP = -1.336		| pKb = 12.8689
	| pKa = 1.128
	| pKb = 12.8689
	}}		}}
	}}		}}
			In ], '''flavin adenine dinucleotide''' ('''FAD''') is a ]-active ] associated with various ]s, which is involved with several enzymatic reactions in ]. A ] is a protein that contains a ], which may be in the form of FAD or ] (FMN). Many flavoproteins are known: components of the succinate dehydrogenase complex, ], and a component of the ].
			FAD can exist in four redox states, which are the ], ], ], and ].<ref>{{Cite journal|last1=Teufel|first1=Robin|last2=Agarwal|first2=Vinayak|last3=Moore|first3=Bradley S.|date=2016-04-01|title=Unusual flavoenzyme catalysis in marine bacteria|journal=Current Opinion in Chemical Biology|volume=31|pages=31–39|doi=10.1016/j.cbpa.2016.01.001|issn=1879-0402|pmc=4870101|pmid=26803009}}</ref> FAD is converted between these states by accepting or donating electrons. FAD, in its fully oxidized form, or ] form, accepts two electrons and two protons to become FADH<sub>2</sub> (hydroquinone form). The semiquinone (FADH<sup>·</sup>) can be formed by either reduction of FAD or oxidation of FADH<sub>2</sub> 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.<ref name=":0">{{cite journal|last1=Teufel|first1=R|last2=Miyanaga|first2=A|last3=Michaudel|first3=Q|last4=Stull|first4=F|last5=Louie|first5=G|last6=Noel|first6=JP|last7=Baran|first7=PS|last8=Palfey|first8=B|last9=Moore|first9=BS|title=Flavin-mediated dual oxidation controls an enzymatic Favorskii-type rearrangement.|journal=Nature|date=28 November 2013|volume=503|issue=7477|pages=552–6|pmid=24162851|doi=10.1038/nature12643|pmc=3844076|bibcode=2013Natur.503..552T}}</ref><ref name=":1">{{Cite journal|last1=Teufel|first1=Robin|last2=Stull|first2=Frederick|last3=Meehan|first3=Michael J.|last4=Michaudel|first4=Quentin|last5=Dorrestein|first5=Pieter C.|last6=Palfey|first6=Bruce|last7=Moore|first7=Bradley S.|date=2015-07-01|title=Biochemical Establishment and Characterization of EncM's Flavin-N5-oxide Cofactor|journal=Journal of the American Chemical Society|volume=137|issue=25|pages=8078–8085|doi=10.1021/jacs.5b03983|issn=1520-5126|pmc=4720136|pmid=26067765}}</ref>
			== History ==
			] were first discovered in 1879 by separating components of cow's milk. They were initially called lactochrome due to their milky origin and yellow ].<ref name=GK10 /> It took 50 years for the scientific community to make any substantial progress in identifying the molecules responsible for the yellow pigment. The 1930s launched the field of ] research with the publication of many ] and ] derivative structures and their obligate roles in redox catalysis. German scientists ] and Walter Christian discovered a yeast derived yellow ] required for ] in 1932. Their colleague ] separated this yellow enzyme into ] and yellow pigment, and showed that neither the enzyme nor the pigment was capable of ] ] on their own, but mixing them together would restore activity. Theorell confirmed the pigment to be ]'s phosphate ester, ] (FMN) in 1937, which was the first direct evidence for ] ].<ref name="GM2">{{cite book|last1=Hayashi|first1=Hideyuki | name-list-style = vanc | title=B Vitamins and Folate: Chemistry, Analysis, Function and Effects|date=2013|publisher=The Royal Society of Chemistry|location=Cambridge, UK|isbn=978-1-84973-369-4|page=7}}</ref> Warburg and Christian then found FAD to be a cofactor of ] through similar experiments in 1938.<ref name="GM3">{{cite journal|last1=Warburg|first1=O|last2=Christian|first2=W | name-list-style = vanc | title=Isolation of the prosthetic group of the amino acid oxidase |journal=Biochemische Zeitschrift|date=1938|volume=298|pages=150–168}}</ref> Warburg's work with linking nicotinamide to hydride transfers and the discovery of flavins paved the way for many scientists in the 40s and 50s to discover copious amounts of redox biochemistry and link them together in pathways such as the ] and ] synthesis.
			== Properties ==
			Flavin adenine dinucleotide consists of two portions: the ] ] (]) and the ] (FMN) bridged together through their ] groups. Adenine is bound to a cyclic ] at the ] carbon, while phosphate is bound to the ribose at the ] carbon to form the adenine nucledotide. ] is formed by a carbon-nitrogen (C-N) bond between the ] and the ]. The phosphate group is then bound to the terminal ribose carbon, forming a FMN. Because the bond between the isoalloxazine and the ribitol is not considered to be a ], the flavin mononucleotide is not truly a nucleotide.<ref>{{cite book | last1 = Metzler | first1 = David E. | last2 = Metzler | first2 = Carol M. | last3 = Sauke | first3 = David J. | name-list-style = vanc | title = Biochemistry | date = 2003 | publisher = Harcourt, Academic Press | location = San Diego | isbn = 978-0-12-492541-0 | edition = 2nd | url-access = registration | url = https://archive.org/details/biochemistrychem0002metz }}</ref> This makes the dinucleotide name misleading; however, the flavin mononucleotide group is still very close to a nucleotide in its structure and chemical properties. ]
			]
			FAD can be ] to FADH<sub>2</sub> through the addition of 2 H<sup>+</sup> and 2 e<sup>−</sup>. FADH<sub>2</sub> can also be ] by the loss of 1 H<sup>+</sup> and 1 e<sup>−</sup> to form FADH. The FAD form can be recreated through the further loss of 1 H<sup>+</sup> and 1 e<sup>−</sup>. FAD formation can also occur through the reduction and dehydration of flavin-N(5)-oxide.<ref name="Ref 1">{{cite book | last1 = Devlin | first1 = Thomas M | name-list-style = vanc | title = Textbook of Biochemistry: with Clinical Correlations | date = 2011 | publisher = John Wiley & Sons | location = Hoboken, NJ | isbn = 978-0-470-28173-4 | edition = 7th }}</ref> Based on the oxidation state, flavins take specific colors when in ]. ] (superoxidized) is yellow-orange, FAD (fully oxidized) is yellow, FADH (half reduced) is either blue or red based on the ], and the fully reduced form is colorless.<ref name="Ref 3" /><ref>{{Cite journal|last1=Teufel|first1=Robin|last2=Miyanaga|first2=Akimasa|last3=Michaudel|first3=Quentin|last4=Stull|first4=Frederick|last5=Louie|first5=Gordon|last6=Noel|first6=Joseph P.|last7=Baran|first7=Phil S.|last8=Palfey|first8=Bruce|last9=Moore|first9=Bradley S.|date=2013-11-28|title=Flavin-mediated dual oxidation controls an enzymatic Favorskii-type rearrangement|journal=Nature|volume=503|issue=7477|pages=552–556|doi=10.1038/nature12643|issn=1476-4687|pmc=3844076|pmid=24162851|bibcode=2013Natur.503..552T}}</ref> Changing the form can have a large impact on other chemical properties. For example, FAD, the fully oxidized form is subject to ], the fully reduced form, FADH<sub>2</sub> has high ], while the half reduced form is unstable in aqueous solution.<ref name=DS2>{{cite journal | vauthors = Kim HJ, Winge DR | title = Emerging concepts in the flavinylation of succinate dehydrogenase | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1827 | issue = 5 | pages = 627–36 | date = May 2013 | pmid = 23380393 | doi = 10.1016/j.bbabio.2013.01.012 | pmc=3626088}}</ref> FAD is an ] ring system, whereas FADH<sub>2</sub> is not.<ref>{{cite book | last1 = Liu | first1 = Shijie | name-list-style = vanc | title = Bioprocess Engineering: Kinetics, Sustainability, and Reactor Design | date = 2012 | publisher = Newnes | isbn = 978-0-444-63783-3 | url = https://books.google.com/books?id=hxBXwEtYCOEC&q=fadh2+aromatic&pg=PA523
			}}</ref> This means that FADH<sub>2</sub> is significantly higher in energy, without the stabilization through ] that the aromatic structure provides. FADH<sub>2</sub> is an energy-carrying molecule, because, once oxidized it regains aromaticity and releases the energy represented by this stabilization.
			The ] properties of FAD and its variants allows for reaction monitoring by use of ] and ] spectroscopies. Each form of FAD has distinct absorbance spectra, making for easy observation of changes in oxidation state.<ref name=DS2 /> A major local absorbance maximum for FAD is observed at 450&nbsp;nm, with an extinction coefficient of 11,300 M<sup>−1</sup> cm<sup>−1</sup>.<ref name=DS3>{{cite journal | vauthors = Lewis JA, Escalante-Semerena JC | title = The FAD-dependent tricarballylate dehydrogenase (TcuA) enzyme of Salmonella enterica converts tricarballylate into cis-aconitate | journal = Journal of Bacteriology | volume = 188 | issue = 15 | pages = 5479–86 | date = Aug 2006 | pmid = 16855237 | doi = 10.1128/jb.00514-06 | pmc=1540016}}</ref> Flavins in general have fluorescent activity when unbound (proteins bound to flavin nucleic acid derivatives are called ]s). This property can be utilized when examining protein binding, observing loss of fluorescent activity when put into the bound state.<ref name=DS2 /> Oxidized flavins have high absorbances of about 450&nbsp;nm, and fluoresce at about 515-520&nbsp;nm.<ref name="Ref 3" />
			== Chemical states ==
			In biological systems, FAD acts as an acceptor of H<sup>+</sup> and e<sup>−</sup> in its fully oxidized form, an acceptor or donor in the FADH form, and a donor in the reduced FADH<sub>2</sub> 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 ]. Through ]s, 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 ] gain, in which the molecule gains what amounts to be one hydride ion. Mechanisms 3 and 4 ] 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 ] and a reaction using a carbon radical.
			<div class="skin-invert-image">
			{|
			|-
			|]
			|]
			|-
			|]
			|]
			|-
			|]
			|]
			|}
			</div>
			{{clear}}
			== Biosynthesis ==
			FAD plays a major role as an enzyme ] along with ], another molecule originating from riboflavin.<ref name="Ref 1"/> Bacteria, fungi and plants can produce ], but other ]s, such as humans, have lost the ability to make it.<ref name="Ref 3">{{cite journal | vauthors = Barile M, Giancaspero TA, Brizio C, Panebianco C, Indiveri C, Galluccio M, Vergani L, Eberini I, Gianazza E | title = Biosynthesis of flavin cofactors in man: implications in health and disease | journal = Current Pharmaceutical Design | volume = 19 | issue = 14 | pages = 2649–75 | date = 2013 | pmid = 23116402 | doi = 10.2174/1381612811319140014 }}</ref> Therefore, humans must obtain riboflavin, also known as vitamin B2, from dietary sources.<ref name="GC2"/> Riboflavin is generally ingested in the small intestine and then transported to cells via carrier proteins.<ref name="Ref 3"/> ] (EC 2.7.1.26) adds a phosphate group to riboflavin to produce flavin mononucleotide, and then ] attaches an adenine ]; both steps require ].<ref name="Ref 3" /> Bacteria generally have one bi-functional enzyme, but ] and eukaryotes usually employ two distinct enzymes.<ref name="Ref 3" /> Current research indicates that distinct ] exist in the ] and ].<ref name="Ref 3" /> It seems that FAD is synthesized in both locations and potentially transported where needed.<ref name=DS2 />
			:]
			== Function ==
			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 ] ions. The N5 and C4a of the fully oxidized flavin ring are also susceptible to ].<ref>{{cite book|last1=Monteira|first1=Mariana | name-list-style = vanc | title=B Vitamins and Folate: Chemistry, Analysis, Function and Effects|date=2013|publisher=The Royal Society of Chemistry|location=Cambridge, UK|isbn=978-1-84973-369-4|page=94}}</ref> 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).
			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.<ref name="GM1.1">{{cite journal | vauthors = Macheroux P, Kappes B, Ealick SE | title = Flavogenomics--a genomic and structural view of flavin-dependent proteins | journal = The FEBS Journal | volume = 278 | issue = 15 | pages = 2625–34 | date = Aug 2011 | pmid = 21635694 | doi = 10.1111/j.1742-4658.2011.08202.x | s2cid = 22220250 | doi-access = free }}</ref> FAD is the more complex and abundant form of flavin and is reported to bind to 75% of the total flavoproteome<ref name="GM1.1" /> and 84% of human encoded flavoproteins.<ref name="Ref 7">{{cite journal | vauthors = Lienhart WD, Gudipati V, Macheroux P | title = The human flavoproteome | journal = Archives of Biochemistry and Biophysics | volume = 535 | issue = 2 | pages = 150–62 | date = Jul 2013 | pmid = 23500531 | doi = 10.1016/j.abb.2013.02.015 | pmc=3684772}}</ref> 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).<ref>{{cite journal | vauthors = Hühner J, Ingles-Prieto Á, Neusüß C, Lämmerhofer M, Janovjak H | title = Quantification of riboflavin, flavin mononucleotide, and flavin adenine dinucleotide in mammalian model cells by CE with LED-induced fluorescence detection | journal = Electrophoresis | volume = 36 | issue = 4 | pages = 518–25 | date = Feb 2015 | pmid = 25488801 | doi = 10.1002/elps.201400451 | s2cid = 27285540 }}</ref>
			FAD has a more positive ] than ] 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 ]. FAD-dependent proteins function in a large variety of metabolic pathways including electron transport, DNA repair, nucleotide biosynthesis, ] of fatty acids, amino acid catabolism, as well as synthesis of other cofactors such as ], ] and ] groups. One well-known reaction is part of the ] (also known as the TCA or Krebs cycle); ] (complex II in the ]) requires covalently bound FAD to catalyze the oxidation of ] to ] by coupling it with the reduction of ] to ].<ref name=DS2 /> The high-energy electrons from this oxidation are stored momentarily by reducing FAD to FADH<sub>2</sub>. FADH<sub>2</sub> then reverts to FAD, sending its two high-energy electrons through the electron transport chain; the energy in FADH<sub>2</sub> is enough to produce 1.5 equivalents of ]<ref>{{cite book | last1 = Stryer | first1 = Lubert | last2 = Berg | first2 = Jeremy M. | last3 = Tymoczko | first3 = John L.| name-list-style = vanc | title = Biochemistry | url = https://archive.org/details/biochemistry0006berg | url-access = registration | date = 2007 | publisher = Freeman | location = New York | isbn = 978-0-7167-8724-2 | edition = 6th }}</ref> by ]. Some redox flavoproteins non-covalently bind to FAD like ] which are involved in ] of fatty acids and catabolism of amino acids like ] (]), ], (short/branched-chain acyl-CoA dehydrogenase), ] (isobutyryl-CoA dehydrogenase), and ] (]).<ref name="Mansoorabadi_2007">{{cite journal | vauthors = Mansoorabadi SO, Thibodeaux CJ, Liu HW | title = The diverse roles of flavin coenzymes--nature's most versatile thespians | journal = The Journal of Organic Chemistry | volume = 72 | issue = 17 | pages = 6329–42 | date = Aug 2007 | pmid = 17580897 | doi = 10.1021/jo0703092 | pmc=2519020}}</ref> Additional examples of FAD-dependent enzymes that regulate metabolism are ] (triglyceride synthesis) and ] involved in ] nucleotide catabolism.<ref>{{cite web|url=https://themedicalbiochemistrypage.org/vitamins.php|title=Vitamins, Minerals, Supplements|last1=King|first1=Michael W|website=The Medical Biochemistry Page|date=18 May 2020|name-list-style=vanc}}</ref> Noncatalytic functions that FAD can play in flavoproteins include as structural roles, or involved in blue-sensitive light ] that regulate ] and development, generation of light in ] bacteria.<ref name="Mansoorabadi_2007"/>
			== Flavoproteins ==
			] have either an ] or FAD molecule as a prosthetic group, this prosthetic group can be tightly bound or covalently linked. Only about 5-10% of flavoproteins have a covalently linked FAD, but these enzymes have stronger redox power.<ref name=DS2 /> In some instances, FAD can provide structural support for active sites or provide stabilization of intermediates during catalysis.<ref name="Mansoorabadi_2007"/> Based on the available structural data, the known FAD-binding sites can be divided into more than 200 types.<ref>{{Cite journal|last1=Garma|first1=Leonardo D.|last2=Medina|first2=Milagros|last3=Juffer|first3=André H.|date=2016-11-01|title=Structure-based classification of FAD binding sites: A comparative study of structural alignment tools|journal=Proteins: Structure, Function, and Bioinformatics|language=en|volume=84|issue=11|pages=1728–1747|doi=10.1002/prot.25158|pmid=27580869|s2cid=26066208|issn=1097-0134}}</ref>
			90 flavoproteins are encoded in the human genome; about 84% require FAD, and around 16% require FMN, whereas 5 proteins require both to be present.<ref name="Ref 7" /> Flavoproteins are mainly located in the ] because of their redox power.<ref name="Ref 7" /> Of all flavoproteins, 90% perform redox reactions and the other 10% are ], ], ], ].<ref name="GM1.1" />
			=== Oxidation of carbon-heteroatom bonds ===
			==== Carbon-nitrogen ====
			] (MAO) is an extensively studied flavoenzyme due to its biological importance with the ] of ], ] and ]. MAO oxidizes primary, secondary and tertiary amines, which nonenzymatically hydrolyze from the ] to ] or ]. Even though this class of enzyme has been extensively studied, its mechanism of action is still being debated. Two mechanisms have been proposed: a radical mechanism and a nucleophilic mechanism. The radical mechanism is less generally accepted because no spectral or ] evidence exists for the presence of a radical intermediate. The nucleophilic mechanism is more favored because it is supported by ] studies which mutated two tyrosine residues that were expected to increase the nucleophilicity of the substrates.<ref name="Elsevier flavo review">{{cite journal|last1=Fagan|first1=Rebecca L.|last2=Palfey|first2=Bruce A. | name-list-style = vanc | title = Flavin-Dependent Enzymes|journal=Comprehensive Natural Products II Chemistry and Biology|date=2010|volume=7|pages=37–113}}</ref>
			:]
			==== Carbon-oxygen ====
			] (GOX) catalyzes the oxidation of β-D-glucose to D-glucono-δ-lactone with the simultaneous reduction of enzyme-bound flavin. GOX exists as a homodimer, with each subunit binding one FAD molecule. Crystal structures show that FAD binds in a deep pocket of the enzyme near the dimer interface. Studies showed that upon replacement of FAD with 8-hydroxy-5-carba-5-deaza FAD, the stereochemistry of the reaction was determined by reacting with the ] of the flavin. During turnover, the neutral and anionic semiquinones are observed which indicates a radical mechanism.<ref name="Elsevier flavo review" />
			:]
			==== Carbon-sulfur ====
			Prenylcysteine lyase (PCLase) catalyzes the cleavage of prenylcysteine (a protein modification) to form an isoprenoid aldehyde and the freed cysteine residue on the protein target. The FAD is non-covalently bound to PCLase. Not many mechanistic studies have been done looking at the reactions of the flavin, but the proposed mechanism is shown below. A hydride transfer from the C1 of the prenyl moiety to FAD is proposed, resulting in the reduction of the flavin to FADH<sub>2</sub>. COformED IS a ] that is stabilized by the neighboring sulfur atom. FADH<sub>2</sub> then reacts with molecular oxygen to restore the oxidized enzyme.<ref name="Elsevier flavo review" />
			:]
			=== Carbon-carbon ===
			UDP-N-acetylenolpyruvylglucosamine Reductase (MurB) is an enzyme that catalyzes the ]-dependent reduction of enolpyruvyl-UDP-N-acetylglucosamine (substrate) to the corresponding D-lactyl compound UDP-N-acetylmuramic acid (product). MurB is a monomer and contains one FAD molecule. Before the substrate can be converted to product, NADPH must first reduce FAD. Once NADP<sup>+</sup> dissociates, the substrate can bind and the reduced flavin can reduce the product.<ref name="Elsevier flavo review" />
			:]
			=== Thiol/disulfide chemistry ===
			] (GR) catalyzes the reduction of glutathione disulfide (GSSG) to glutathione (GSH). GR requires FAD and NADPH to facilitate this reaction; first a hydride must be transferred from NADPH to FAD. The reduced flavin can then act as a ] to attack the disulfide, this forms the C4a-cysteine adduct. Elimination of this adduct results in a flavin-thiolate charge-transfer complex.<ref name="Elsevier flavo review" />
			:]
			=== Electron transfer reactions ===
			] type enzymes that catalyze monooxygenase (hydroxylation) reactions are dependent on the transfer of two electrons from FAD to the P450. Two types of P450 systems are found in eukaryotes. The P450 systems that are located in the endoplasmic reticulum are dependent on a ] (CPR) that contains both an FAD and an ]. The two electrons on reduced FAD (FADH<sub>2</sub>) are transferred one at a time to FMN and then a single electron is passed from FMN to the heme of the P450.<ref name="1996-Hanukoglu">{{cite journal | author = Hanukoglu I | title = Electron transfer proteins of cytochrome P450 systems | journal = Adv. Mol. Cell Biol. | series = Advances in Molecular and Cell Biology | year = 1996 | volume = 14 | pages= 29–55 | doi = 10.1016/S1569-2558(08)60339-2 | isbn = 9780762301133 | url=https://www.science.co.il/hi/pub/Electron-transfer-proteins-of-cytochrome-P450-systems.pdf}}</ref>
			The P450 systems that are located in the mitochondria are dependent on two electron transfer proteins: An FAD containing ] (AR) and a small iron-sulfur group containing protein named ]. FAD is embedded in the FAD-binding domain of AR.<ref name="1999-Ziegler">{{cite journal | vauthors = Ziegler GA, Vonrhein C, Hanukoglu I, Schulz GE | title = The structure of adrenodoxin reductase of mitochondrial P450 systems: electron transfer for steroid biosynthesis | journal = Journal of Molecular Biology | volume = 289 | issue = 4 | pages = 981–90 | date = Jun 1999 | pmid = 10369776 | doi = 10.1006/jmbi.1999.2807 }}</ref><ref name="2017-Hanukoglu-JME">{{cite journal | vauthors = Hanukoglu I | title = Conservation of the Enzyme-Coenzyme Interfaces in FAD and NADP Binding Adrenodoxin Reductase-A Ubiquitous Enzyme | journal = Journal of Molecular Evolution | volume = 85 | issue= 5 | pages= 205–218 | year= 2017 | pmid= 29177972 | doi= 10.1007/s00239-017-9821-9 | bibcode = 2017JMolE..85..205H | s2cid = 7120148 }}</ref> The FAD of AR is reduced to FADH<sub>2</sub> by transfer of two electrons from NADPH that binds in the NADP-binding domain of AR. The structure of this enzyme is highly conserved to maintain precisely the alignment of electron donor NADPH and acceptor FAD for efficient electron transfer.<ref name="2017-Hanukoglu-JME" /> The two electrons in reduced FAD are transferred one a time to adrenodoxin which in turn donates the single electron to the heme group of the mitochondrial P450.<ref name="1980-Hanukoglu">{{cite journal | vauthors = Hanukoglu I, Jefcoate CR | title = Mitochondrial cytochrome P-450scc. Mechanism of electron transport by adrenodoxin | journal = The Journal of Biological Chemistry | volume = 255 | issue = 7 | pages = 3057–61 | date = Apr 1980 | doi = 10.1016/S0021-9258(19)85851-9 | pmid = 6766943 | url = http://www.jbc.org/content/255/7/3057.full.pdf | doi-access = free }}</ref>
			The structures of the reductase of the microsomal versus reductase of the mitochondrial P450 systems are completely different and show no homology.<ref name="1996-Hanukoglu" />
			=== Redox ===
			''p''-Hydroxybenzoate hydroxylase (PHBH) catalyzes the oxygenation of ''p''-hydroxybenzoate (''p''OHB) to 3,4-dihyroxybenzoate (3,4-diOHB); FAD, NADPH and molecular oxygen are all required for this reaction. NADPH first transfers a hydride equivalent to FAD, creating FADH<sup>−</sup>, and then NADP<sup>+</sup> dissociates from the enzyme. Reduced PHBH then reacts with molecular oxygen to form the flavin-C(4a)-hydroperoxide. The flavin hydroperoxide quickly hydroxylates ''p''OHB, and then eliminates water to regenerate oxidized flavin.<ref name="Elsevier flavo review" /> An alternative flavin-mediated oxygenation mechanism involves the use of a ] rather than a flavin-C(4a)-(hydro)peroxide.<ref name=":0" /><ref name=":1" />
			:]
			=== Nonredox ===
			] (CS) catalyzes the last step in the ]—the formation of chorismate. Two classes of CS are known, both of which require ], but are divided on their need for ] as a reducing agent. The proposed mechanism for CS involves radical species. The radical flavin species has not been detected spectroscopically without using a substrate analogue, which suggests that it is short-lived. However, when using a fluorinated substrate, a neutral flavin semiquinone was detected.<ref name="Elsevier flavo review" />
			:]
			=== Complex flavoenzymes ===
			Glutamate synthase catalyzes the conversion of 2-oxoglutarate into L-glutamate with L-glutamine serving as the nitrogen source for the reaction. All glutamate syntheses are iron-sulfur flavoproteins containing an iron-sulfur cluster and FMN. The three classes of glutamate syntheses are categorized based on their sequences and biochemical properties. Even though there are three classes of this enzyme, it is believed that they all operate through the same mechanism, only differing by what first reduces the FMN. The enzyme produces two glutamate molecules: one by the hydrolysis of glutamine (forming glutamate and ammonia), and the second by the ammonia produced from the first reaction attacking 2-oxoglutarate, which is reduced by FMN to glutamate.<ref name="Elsevier flavo review" />
			== Clinical significance ==
			=== Flavoprotein-related diseases ===
			Due to the importance of ]s, it is unsurprising that approximately 60% of human flavoproteins cause human disease when mutated.<ref name="Ref 7"/> In some cases, this is due to a decreased ] for FAD or FMN and so excess riboflavin intake may lessen disease symptoms, such as for ].<ref name="Ref 3" /> In addition, riboflavin deficiency itself (and the resulting lack of FAD and FMN) can cause health issues.<ref name="Ref 3" /> For example, in ] patients, there are decreased levels of FAD synthesis.<ref name="Ref 3" /> Both of these paths can result in a variety of symptoms, including developmental or gastrointestinal abnormalities, faulty ], ], neurological problems, ] or ], ], worsened vision and skin lesions.<ref name="Ref 3" /> The pharmaceutical industry therefore produces riboflavin to supplement diet in certain cases. In 2008, the global need for riboflavin was 6,000 tons per year, with production capacity of 10,000 tons.<ref name=GK10>{{cite journal | vauthors = Abbas CA, Sibirny AA | title = Genetic control of biosynthesis and transport of riboflavin and flavin nucleotides and construction of robust biotechnological producers | journal = Microbiology and Molecular Biology Reviews | volume = 75 | issue = 2 | pages = 321–60 | date = Jun 2011 | pmid = 21646432 | doi = 10.1128/mmbr.00030-10 | pmc=3122625}}</ref> This $150 to 500 million market is not only for medical applications, but is also used as a supplement to animal food in the agricultural industry and as a ].<ref name=GK10 />
			=== Drug design ===
			New ] of anti-bacterial medications is of continuing importance in scientific research as bacterial antibiotic resistance to common antibiotics increases. A specific metabolic protein that uses FAD (]) is vital for bacterial virulence, and so targeting FAD synthesis or creating FAD analogs could be a useful area of investigation.<ref name=GK5>{{cite journal | vauthors = McNeil MB, Fineran PC | title = Prokaryotic assembly factors for the attachment of flavin to complex II | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1827 | issue = 5 | pages = 637–47 | date = May 2013 | pmid = 22985599 | doi = 10.1016/j.bbabio.2012.09.003 | doi-access = free }}</ref> Already, scientists have determined the two structures FAD usually assumes once bound: either an extended or a butterfly conformation, in which the molecule essentially folds in half, resulting in the stacking of the adenine and isoalloxazine rings.<ref name=GC2>{{cite journal | vauthors = Kuppuraj G, Kruise D, Yura K | title = Conformational behavior of flavin adenine dinucleotide: conserved stereochemistry in bound and free states | journal = The Journal of Physical Chemistry B | volume = 118 | issue = 47 | pages = 13486–97 | date = Nov 2014 | pmid = 25389798 | doi = 10.1021/jp507629n }}</ref> FAD imitators that are able to bind in a similar manner but do not permit protein function could be useful mechanisms of inhibiting bacterial infection.<ref name="GC2"/> Alternatively, drugs blocking FAD synthesis could achieve the same goal; this is especially intriguing because human and bacterial FAD synthesis relies on very different enzymes, meaning that a drug made to target bacterial FAD synthase would be unlikely to interfere with the human FAD synthase enzymes.<ref name=GK16>{{cite journal | vauthors = Serrano A, Ferreira P, Martínez-Júlvez M, Medina M | title = The prokaryotic FAD synthetase family: a potential drug target | journal = Current Pharmaceutical Design | volume = 19 | issue = 14 | pages = 2637–48 | date = 2013 | pmid = 23116401 | doi = 10.2174/1381612811319140013 }}</ref>
			=== Optogenetics ===
			] allows control of biological events in a non-invasive manner.<ref name=GK15>{{cite journal | vauthors = Christie JM, Gawthorne J, Young G, Fraser NJ, Roe AJ | title = LOV to BLUF: flavoprotein contributions to the optogenetic toolkit | journal = Molecular Plant | volume = 5 | issue = 3 | pages = 533–44 | date = May 2012 | pmid = 22431563 | doi = 10.1093/mp/sss020 | doi-access = free }}</ref> The field has advanced in recent years with a number of new tools, including those to trigger light sensitivity, such as the Blue-Light-Utilizing FAD domains (BLUF). BLUFs encode a 100 to 140 ] sequence that was derived from photoreceptors in plants and bacteria.<ref name="GK15"/> Similar to other ], the light causes structural changes in the BLUF domain that results in disruption of downstream interactions.<ref name=GK15 /> Current research investigates proteins with the appended BLUF domain and how different external factors can impact the proteins.<ref name=GK15 />
			=== Treatment monitoring ===
			There are a number of molecules in the body that have native ] including tryptophan, ], FAD, ] and ]s.<ref name=GK18>{{cite journal | vauthors = Sivabalan S, Vedeswari CP, Jayachandran S, Koteeswaran D, Pravda C, Aruna PR, Ganesan S | title = In vivo native fluorescence spectroscopy and nicotinamide adinine dinucleotide/flavin adenine dinucleotide reduction and oxidation states of oral submucous fibrosis for chemopreventive drug monitoring | journal = Journal of Biomedical Optics | volume = 15 | issue = 1 | pages = 017010–017010–11 | date = 2010 | pmid = 20210484 | doi = 10.1117/1.3324771 | bibcode = 2010JBO....15a7010S | s2cid = 40028193 | doi-access = free }}</ref> Scientists have taken advantage of this by using them to monitor disease progression or treatment effectiveness or aid in diagnosis. For instance, native fluorescence of a FAD and NADH is varied in normal tissue and ], which is an early sign of invasive ].<ref name=GK18 /> Doctors therefore have been employing fluorescence to assist in diagnosis and monitor treatment as opposed to the standard ].<ref name=GK18 />
			== Additional images ==
			<div class="skin-invert-image">
			<gallery>
			Image:Riboflavin v2.svg|]
			Image:FADH2.png|FADH<sub>2</sub>
			</gallery>
			</div>
			== See also ==
			*]
			*], flavin-containing monooxygenase
			*]
			== References ==
			{{reflist|33em}}
			== External links ==
			* in the ]
			* entry in the NIH Chemical Database
			{{Enzyme cofactors}}
			]
			]
			]