View source for Nicotinamide adenine dinucleotide

{{short description|Chemical compound which is reduced and oxidized}}
{{Redirect2|NAD(P)+|NAD(P)H|the phosphates (NADP{{+}}/NADPH)|Nicotinamide adenine dinucleotide phosphate}}
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{{Use dmy dates|date=July 2019}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 464362739
| ImageFile = NAD+.svg
| ImageSize = 180px
| ImageName = Skeletal formula of the oxidized form
| ImageClass = skin-invert
| ImageFile1 = NAD+-from-xtal-2003-3D-balls.png
| ImageClass1 = bg-transparent  
| ImageSize1 =
| ImageName1 = Ball-and-stick model of the oxidized form
| IUPACName =
| OtherNames = Diphosphopyridine nucleotide (DPN{{+}}), Coenzyme I
|Section1={{Chembox Identifiers
| UNII = 0U46U6E8UK
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII1_Ref = {{fdacite|correct|FDA}}
| UNII1 = BY8P107XEP
| UNII1_Comment = (phosphate)
| UNII2_Ref = {{fdacite|correct|FDA}}
| UNII2 = 4J24DQ0916
| UNII2_Comment = (NADH)<!--also verified-->
| InChI = 1/C21H27N7O14P2/c22-17-12-19(25-7-24-17)28(8-26-12)21-16(32)14(30)11(41-21)6-39-44(36,37)42-43(34,35)38-5-10-13(29)15(31)20(40-10)27-3-1-2-9(4-27)18(23)33/h1-4,7-8,10-11,13-16,20-21,29-32H,5-6H2,(H5-,22,23,24,25,33,34,35,36,37)/t10-,11-,13-,14-,15-,16-,20-,21-/m1/s1
| InChIKey = BAWFJGJZGIEFAR-NNYOXOHSBR
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C21H27N7O14P2/c22-17-12-19(25-7-24-17)28(8-26-12)21-16(32)14(30)11(41-21)6-39-44(36,37)42-43(34,35)38-5-10-13(29)15(31)20(40-10)27-3-1-2-9(4-27)18(23)33/h1-4,7-8,10-11,13-16,20-21,29-32H,5-6H2,(H5-,22,23,24,25,33,34,35,36,37)/t10-,11-,13-,14-,15-,16-,20-,21-/m1/s1
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = BAWFJGJZGIEFAR-NNYOXOHSSA-N
| CASNo = 53-84-9
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo1_Ref = {{cascite|correct|CAS}}
| CASNo1 = 53-59-8
| CASNo1_Comment = (phosphate)
| CASNo2_Ref = {{cascite|correct|CAS}}
| CASNo2 = 58-68-4
| CASNo2_Comment = (NADH)<!--also verified-->
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 5681
| PubChem = 925
| ChEMBL_Ref = {{ebicite|changed|EBI}}
| ChEMBL = 1628272
| IUPHAR_ligand = 2451
| DrugBank_Ref = {{drugbankcite|correct|drugbank}}
| DrugBank = DB00157
| SMILES = O=C(N)c1ccc[n+](c1)[C@@H]2O[C@@H]([C@@H](O)[C@H]2O)COP([O-])(=O)OP(=O)([O-])OC[C@H]5O[C@@H](n4cnc3c(ncnc34)N)[C@H](O)[C@@H]5O
| SMILES_Comment = NAD<sup>+</sup>
| SMILES1 = O=C(N)C1CC=C[N](C=1)[C@@H]2O[C@@H]([C@@H](O)[C@H]2O)COP([O-])(=O)OP(=O)([O-])OC[C@H]5O[C@@H](n4cnc3c(ncnc34)N)[C@H](O)[C@@H]5O
| SMILES1_Comment = NADH
| MeSHName =
| KEGG_Ref = {{keggcite|changed|kegg}}
| KEGG = C00003
| ChEBI_Ref = {{ebicite|changed|EBI}}
| ChEBI = 16908
| RTECS = UU3450000
  }}
|Section2={{Chembox Properties
| Formula=C<sub>21</sub>H<sub>28</sub>N<sub>7</sub>O<sub>14</sub>P<sub>2</sub><sup>+</sup> (oxidized)<br />C<sub>21</sub>H<sub>29</sub>N<sub>7</sub>O<sub>14</sub>P<sub>2</sub> (reduced)
| MolarMass=664.4 g/mol (oxidized)<br />665.4 g/mol (reduced)
| Appearance = White powder
| Density =
| MeltingPtC = 160
| Solubility =
  }}
|Section7={{Chembox Hazards
| MainHazards = Not hazardous
| NFPA-H = 1
| NFPA-F = 1
| NFPA-R = 0
| FlashPt =
| AutoignitionPt =
  }}
}}

'''Nicotinamide adenine dinucleotide''' ('''NAD''') is a [[Cofactor (biochemistry)|coenzyme]] central to [[metabolism]].<ref>{{Lehninger4th}}</ref> Found in all living [[cell (biology)|cell]]s, NAD is called a dinucleotide because it consists of two [[nucleotide]]s joined through their [[phosphate]] groups. One nucleotide contains an [[adenine]] [[nucleobase]] and the other, [[nicotinamide]]. NAD exists in two forms: an [[Redox|oxidized and reduced]] form, abbreviated as '''NAD{{+}}''' and '''NADH''' (H for [[hydrogen]]), respectively.

In cellular metabolism, NAD is involved in redox reactions, carrying [[electron]]s from one reaction to another, so it is found in two forms: NAD{{+}} is an [[oxidizing agent]], accepting electrons from other molecules and becoming reduced; with H<sup>+</sup>, this reaction forms NADH, which can be used as a [[reducing agent]] to donate electrons. These [[electron transfer]] reactions are the main function of NAD. It is also used in other cellular processes, most notably as a [[substrate (biochemistry)|substrate]] of [[enzyme]]s in adding or removing [[chemical group]]s to or from [[protein]]s, in [[posttranslational modification]]s. Because of the importance of these functions, the enzymes involved in NAD metabolism are targets for [[drug discovery]].

In organisms, NAD can be [[Organic synthesis|synthesized]] from simple building-blocks ([[de novo synthesis|''de novo'']]) from either [[tryptophan]] or [[aspartic acid]], each a case of an [[amino acid]]. Alternatively, more complex components of the coenzymes are taken up from nutritive compounds such as [[nicotinic acid]]; similar compounds are produced by reactions that break down the structure of NAD, providing a [[salvage pathway]] that recycles them back into their respective active form.

In the name NAD{{+}}, the [[Subscript and superscript|superscripted]] plus sign indicates the positive [[formal charge]] on one of its nitrogen atoms.
A biological coenzyme that acts as an electron carrier in enzymatic reactions.

Some NAD is converted into the coenzyme [[nicotinamide adenine dinucleotide phosphate]] (NADP), whose chemistry largely parallels that of NAD, though its predominant role is as a coenzyme in [[anabolic]] metabolism.
NADP is a reducing agent in anabolic reactions like the Calvin cycle and lipid and nucleic acid syntheses. NADP exists in two forms: NADP+, the oxidized form, and NADPH, the reduced form. NADP is similar to nicotinamide adenine dinucleotide (NAD), but NADP has a phosphate group at the C-2′ position of the adenosyl.

==Physical and chemical properties==
{{Further|Redox}}
Nicotinamide adenine dinucleotide consists of two [[nucleoside]]s joined by [[pyrophosphate]]. The nucleosides each contain a [[ribose]] ring, one with [[adenine]] attached to the first carbon atom (the [[Nucleic acid nomenclature|1']] position) ([[adenosine diphosphate ribose]]) and the other with [[nicotinamide]] at this position.<ref>The nicotinamide group can be attached in two orientations to the anomeric ribose carbon atom. Because of these two possible structures, the NAD could exists as either of two [[diastereomer]]s. It is the β-nicotinamide diastereomer of NAD{{+}} that is found in nature.</ref><ref name=Pollak>{{cite journal |vauthors=Pollak N, Dölle C, Ziegler M |title= The power to reduce: pyridine nucleotides&nbsp;– small molecules with a multitude of functions |journal= Biochem. J. |volume= 402 |issue= 2 |pages= 205–218 |year= 2007 |pmid= 17295611 |pmc= 1798440 |doi= 10.1042/BJ20061638 }}</ref>

[[File:NAD oxidation reduction.svg|thumb|left|class=skin-invert-image|The [[redox]] reactions of nicotinamide adenine dinucleotide]]

The compound accepts or donates the equivalent of H<sup>−</sup>.<ref name=Belenky>{{cite journal |last1=Belenky |first1=Peter |last2=Bogan |first2=Katrina L. |last3=Brenner |first3=Charles |title=NAD+ metabolism in health and disease |journal=Trends in Biochemical Sciences |date=January 2007 |volume=32 |issue=1 |pages=12–19 |doi=10.1016/j.tibs.2006.11.006 |pmid=17161604 }}</ref> Such reactions (summarized in formula below) involve the removal of two hydrogen atoms from a reactant (R), in the form of a [[hydride|hydride ion]] (H<sup>−</sup>), and a [[proton]] (H{{+}}). The proton is released into solution, while the reductant RH{{sub|2}} is oxidized and NAD{{+}} reduced to NADH by transfer of the hydride to the nicotinamide ring.

:RH{{sub|2}} + NAD{{+}} → NADH + H{{+}} + R;

From the electron pair of the hydride ion, one electron is attracted to the slightly more electronegative atom of the nicotinamide ring of NAD{{+}},<ref>{{Cite web |title=What is NAD? |url=https://ignitepeptides.com/what-is-nad-the-molecule-behind-energy-aging-and-brain-health/ |access-date=2025-11-04 |website=Ignite Peptides}}</ref> becoming part of the nicotinamide moiety. The remaining hydrogen atom is transferred to the carbon atom opposite the N atom. The [[standard electrode potential#Non-standard condition|midpoint potential]] of the NAD{{+}}/NADH redox pair is −0.32&nbsp;[[volt]]s, which makes NADH a moderately strong ''reducing'' agent.<ref name=Unden>{{cite journal |vauthors=Unden G, Bongaerts J |title= Alternative respiratory pathways of ''Escherichia coli'': energetics and transcriptional regulation in response to electron acceptors |journal= Biochim. Biophys. Acta |volume= 1320 |issue= 3 |pages= 217–234 |year= 1997 |pmid= 9230919 |doi= 10.1016/S0005-2728(97)00034-0|doi-access= free }}</ref> The reaction is easily reversible, when NADH reduces another molecule and is re-oxidized to NAD{{+}}. This means the coenzyme can continuously cycle between the NAD{{+}} and NADH forms without being consumed.<ref name=Pollak/>

In appearance, all forms of this coenzyme are white [[amorphous solid|amorphous]] powders that are [[hygroscopy|hygroscopic]] and highly water-soluble.<ref>{{cite book |author=Windholz, Martha |title=The Merck Index: an encyclopedia of chemicals, drugs, and biologicals |publisher=Merck |location=Rahway NJ |year=1983 |page=[https://archive.org/details/merckindexencycl00wind/page/909 909] |isbn=978-0-911910-27-8 |edition=10th |title-link=The Merck Index }}</ref> The solids are stable if stored dry and in the dark. Solutions of NAD{{+}} are colorless and stable for about a week at 4&nbsp;[[Celsius|°C]] and neutral [[pH]], but decompose rapidly in acidic or alkaline solutions. Upon decomposition, they form products that are [[enzyme inhibitor]]s.<ref>{{cite journal |vauthors=Biellmann JF, Lapinte C, Haid E, Weimann G |title= Structure of lactate dehydrogenase inhibitor generated from coenzyme |journal= Biochemistry |volume= 18 |issue= 7 |pages= 1212–1217 |year= 1979 |pmid= 218616 |doi= 10.1021/bi00574a015}}</ref>

[[File:NADNADH.svg|thumb|class=skin-invert-image|[[ultraviolet|UV]] absorption spectra of NAD{{+}} and NADH{{image reference needed|date=December 2022}}]]

Both NAD{{+}} and NADH strongly absorb [[ultraviolet]] light because of the adenine. For example, peak absorption of NAD{{+}} is at a [[wavelength]] of 259&nbsp;[[nanometer]]s (nm), with an [[Molar absorptivity|extinction coefficient]] of 16,900&nbsp;[[Concentration#Molarity|M]]<sup>−1</sup>[[Centimetre|cm]]<sup>−1</sup>. NADH also absorbs at higher wavelengths, with a second peak in UV absorption at 339&nbsp;nm with an extinction coefficient of 6,220&nbsp;M<sup>−1</sup>cm<sup>−1</sup>.<ref name=Dawson>{{cite book|author=Dawson, R. Ben |title=Data for biochemical research |publisher=Clarendon Press |location=Oxford |year=1985 |page=122 |isbn=978-0-19-855358-8 |edition=3rd}}</ref> This difference in the ultraviolet [[absorption spectra]] between the oxidized and reduced forms of the coenzymes at higher wavelengths makes it simple to measure the conversion of one to another in [[enzyme assay]]s&nbsp;– by measuring the amount of UV absorption at 340&nbsp;nm using a [[spectrophotometry|spectrophotometer]].<ref name=Dawson/>

NAD{{+}} and NADH also differ in their [[fluorescence]]. Freely diffusing NADH in aqueous solution, when excited at the nicotinamide absorbance of ~335&nbsp;nm (near-UV), fluoresces at 445–460&nbsp;nm (violet to blue) with a [[fluorescence#Lifetime|fluorescence lifetime]] of 0.4&nbsp;[[nanosecond]]s, while NAD{{+}} does not fluoresce.<ref name="Blacker Mann Gale Ziegler p. ">{{cite journal | last1=Blacker | first1=Thomas S. | last2=Mann | first2=Zoe F. | last3=Gale | first3=Jonathan E. | last4=Ziegler | first4=Mathias | last5=Bain | first5=Angus J. | last6=Szabadkai | first6=Gyorgy | last7=Duchen | first7=Michael R. | title=Separating NADH and NADPH fluorescence in live cells and tissues using FLIM | journal=Nature Communications | publisher=Springer Science and Business Media LLC | volume=5 | issue=1 | date=2014-05-29 | issn=2041-1723 | doi=10.1038/ncomms4936 | page=3936| pmid=24874098 | pmc=4046109 | bibcode=2014NatCo...5.3936B }}</ref><ref name=Lakowicz>{{cite journal |vauthors=Lakowicz JR, Szmacinski H, Nowaczyk K, Johnson ML |title= Fluorescence lifetime imaging of free and protein-bound NADH |journal= Proc. Natl. Acad. Sci. U.S.A. |volume= 89 |issue= 4 |pages= 1271–1275 |year= 1992 |pmid= 1741380 |pmc= 48431 |doi= 10.1073/pnas.89.4.1271 |bibcode= 1992PNAS...89.1271L|doi-access= free }}</ref> The properties of the fluorescence signal changes when NADH binds to [[protein]]s, so these changes can be used to measure [[dissociation constant]]s, which are useful in the study of [[enzyme kinetics]].<ref name=Lakowicz/><ref>{{cite journal |vauthors=Jameson DM, Thomas V, Zhou DM |title= Time-resolved fluorescence studies on NADH bound to mitochondrial malate dehydrogenase |journal= Biochim. Biophys. Acta |volume= 994 |issue= 2 |pages= 187–190 |year= 1989 |pmid= 2910350 |doi= 10.1016/0167-4838(89)90159-3}}</ref> These changes in fluorescence are also used to measure changes in the redox state of living cells, through [[fluorescence microscope|fluorescence microscopy]].<ref name=Kasimova>{{cite journal |vauthors=Kasimova MR, Grigiene J, Krab K, Hagedorn PH, Flyvbjerg H, Andersen PE, Møller IM |title= The Free NADH Concentration Is Kept Constant in Plant Mitochondria under Different Metabolic Conditions |journal= Plant Cell |volume= 18 |issue= 3 |pages= 688–698 |year= 2006 |pmid= 16461578 |pmc= 1383643 |doi= 10.1105/tpc.105.039354|bibcode= 2006PlanC..18..688K }}</ref>

NADH can be converted to NAD+ in a reaction catalysed by copper, which requires hydrogen peroxide. Thus, the supply of NAD+ in cells requires mitochondrial copper(II).<ref>{{cite journal |last1=Chan |first1=PC |last2=Kesner |first2=L |title=Copper (II) complex-catalyzed oxidation of NADH by hydrogen peroxide |journal=Biol Trace Elem Res |date=September 1980 |volume=2 |issue=3 |pages=159–174 |doi=10.1007/BF02785352 |pmid=24271266|bibcode=1980BTER....2..159C |s2cid=24264851 }}</ref><ref>{{cite journal |last1=Solier |first1=Stéphanie |last2=Müller |first2=Sebastian |last3=Tatiana |first3=Cañeque |last4=Antoine |first4=Versini |last5=Arnaud |first5=Mansart |last6=Fabien |first6=Sindikubwabo |last7=Leeroy |first7=Baron |last8=Laila |first8=Emam |last9=Pierre |first9=Gestraud |last10=G. Dan |first10=Pantoș |last11=Vincent |first11=Gandon |last12=Christine |first12=Gaillet |last13=Ting-Di |first13=Wu |last14=Florent |first14=Dingli |last15=Damarys |first15=Loew |last16=Sylvain |first16=Baulande |last17=Sylvère |first17=Durand |last18=Valentin |first18=Sencio |last19=Cyril |first19=Robil |last20=François |first20=Trottein |last21=David |first21=Péricat |last22=Emmanuelle |first22=Näser |last23=Céline |first23=Cougoule |last24=Etienne |first24=Meunier |last25=Anne-Laure |first25=Bègue |last26=Hélène |first26=Salmon |last27=Nicolas |first27=Manel |last28=Alain |first28=Puisieux |last29=Sarah |first29=Watson |last30=Mark A. |first30=Dawson |last31=Nicolas |first31=Servant |last32=Guido |first32=Kroemer |last33=Djillali |first33=Annane |last34=Raphaël |first34=Rodriguez |title=A druggable copper-signalling pathway that drives inflammation |journal=Nature |date=2023 |volume=617 |issue=7960 |pages=386–394 |doi=10.1038/s41586-023-06017-4 |pmid=37100912 |pmc=10131557 |bibcode=2023Natur.617..386S |s2cid=258353949 }}</ref>

==Concentration and state in cells==
In rat liver, the total amount of NAD{{+}} and NADH is approximately 1&nbsp;[[micromole|μmole]] per [[gram]] of wet weight, about 10 times the concentration of NADP{{+}} and NADPH in the same cells.<ref>{{cite journal |vauthors=Reiss PD, Zuurendonk PF, Veech RL |title= Measurement of tissue purine, pyrimidine, and other nucleotides by radial compression high-performance liquid chromatography |journal= Anal. Biochem. |volume= 140 |issue= 1 |pages= 162–71 |year= 1984 |pmid= 6486402 |doi= 10.1016/0003-2697(84)90148-9}}</ref> The actual concentration of NAD{{+}} in cell [[cytosol]] is harder to measure, with recent estimates in animal cells ranging around 0.3&nbsp;[[molar concentration|mM]],<ref>{{cite journal |vauthors=Yamada K, Hara N, Shibata T, Osago H, Tsuchiya M |title= The simultaneous measurement of nicotinamide adenine dinucleotide and related compounds by liquid chromatography/electrospray ionization tandem mass spectrometry |journal= Anal. Biochem. |volume= 352 |issue= 2 |pages= 282–5 |year= 2006 |pmid= 16574057 |doi= 10.1016/j.ab.2006.02.017}}</ref><ref name=Yang>{{cite journal |vauthors=Yang H, Yang T, Baur JA, Perez E, Matsui T, Carmona JJ, Lamming DW, Souza-Pinto NC, Bohr VA, Rosenzweig A, de Cabo R, Sauve AA, Sinclair DA |title= Nutrient-Sensitive Mitochondrial NAD<sup>+</sup> Levels Dictate Cell Survival |journal= Cell |volume= 130 |issue= 6 |pages= 1095–107 |year= 2007 |pmid= 17889652 |pmc= 3366687 |doi= 10.1016/j.cell.2007.07.035}}</ref> and approximately 1.0 to 2.0&nbsp;mM in [[yeast]].<ref name="Belenky2"><!-- was Belenky, exists another <ref name=Belenky>; someone needs to determine which one is intended by each other <ref name=Belenky/> -->{{cite journal|vauthors=Belenky P, Racette FG, Bogan KL, McClure JM, Smith JS, Brenner C|s2cid=4661723|year=2007|title=Nicotinamide riboside promotes Sir2 silencing and extends lifespan via Nrk and Urh1/Pnp1/Meu1 pathways to NAD<sup>+</sup>|journal=Cell|volume=129|issue=3|pages=473–84|doi=10.1016/j.cell.2007.03.024|pmid=17482543|doi-access=free}}</ref> However, more than 80% of NADH fluorescence in mitochondria is from bound form, so the concentration in solution is much lower.<ref>{{cite journal |vauthors=Blinova K, Carroll S, Bose S, Smirnov AV, Harvey JJ, Knutson JR, Balaban RS |title= Distribution of mitochondrial NADH fluorescence lifetimes: steady-state kinetics of matrix NADH interactions |journal= Biochemistry |volume= 44 |issue= 7 |pages= 2585–94 |year= 2005 |pmid= 15709771 |doi= 10.1021/bi0485124}}</ref>

NAD{{+}} concentrations are highest in the mitochondria, constituting 40% to 70% of the total cellular NAD{{+}}.<ref name="pmid31412683">{{cite journal | vauthors = Hopp A, Grüter P, Hottiger MO | title = Regulation of Glucose Metabolism by NAD + and ADP-Ribosylation | journal = [[Cells (journal)|Cells]] | volume = 8 | issue=8 | page = 890 | date=2019 | doi = 10.3390/cells8080890 | pmc=6721828 | pmid = 31412683| doi-access = free }}</ref> NAD{{+}} in the cytosol is carried into the mitochondrion by a specific [[membrane transport protein]], since the coenzyme cannot [[diffusion|diffuse]] across membranes.<ref>{{cite journal |vauthors=Todisco S, Agrimi G, Castegna A, Palmieri F |title= Identification of the mitochondrial NAD<sup>+</sup> transporter in ''Saccharomyces cerevisiae'' |journal= J. Biol. Chem. |volume= 281 |issue= 3 |pages= 1524–31 |year= 2006 |pmid= 16291748 |doi= 10.1074/jbc.M510425200|doi-access= free }}</ref> The intracellular [[Biological half-life|half-life]] of NAD<sup>+</sup> was claimed to be between 1–2 hours by one review,<ref name="pmid27465020">{{cite journal | author = Srivastava S | title = Emerging therapeutic roles for NAD(+) metabolism in mitochondrial and age-related disorders | journal = Clinical and Translational Medicine | volume = 5 | issue=1 | page = 25 | date=2016 | article-number = e25 | doi = 10.1186/s40169-016-0104-7 | pmc=4963347 | pmid = 27465020 | doi-access = free }}</ref> whereas another review gave varying estimates based on compartment: intracellular 1–4&nbsp;hours, cytoplasmic 2&nbsp;hours, and mitochondrial 4–6&nbsp;hours.<ref name="pmid29413178">{{cite book |doi=10.1016/bs.pmbts.2017.11.012 |chapter=Regulatory Effects of NAD + Metabolic Pathways on Sirtuin Activity |title=Sirtuins in Health and Disease |series=Progress in Molecular Biology and Translational Science |year=2018 |last1=Zhang |first1=Ning |last2=Sauve |first2=Anthony A. |volume=154 |pages=71–104 |pmid=29413178 |isbn=978-0-12-812261-7 }}</ref>

The balance between the oxidized and reduced forms of nicotinamide adenine dinucleotide is called the NAD{{+}}/NADH ratio. This ratio is an important component of what is called the ''redox state'' of a cell, a measurement that reflects both the metabolic activities and the health of cells.<ref>{{cite journal |vauthors=Schafer FQ, Buettner GR |title= Redox environment of the cell as viewed through the redox state of the glutathione disulfide/glutathione couple |journal= Free Radic Biol Med |volume= 30 |issue= 11 |pages= 1191–212 |year= 2001 |pmid= 11368918 |doi= 10.1016/S0891-5849(01)00480-4}}</ref> The effects of the NAD{{+}}/NADH ratio are complex, controlling the activity of several key enzymes, including [[glyceraldehyde 3-phosphate dehydrogenase]] and [[pyruvate dehydrogenase]]. In healthy mammalian tissues, estimates of the ratio of free NAD{{+}} to NADH in the cytoplasm typically lie around 700:1; the ratio is thus favorable for oxidative reactions.<ref name=Williamson>{{cite journal |vauthors=Williamson DH, Lund P, Krebs HA |title= The redox state of free nicotinamide-adenine dinucleotide in the cytoplasm and mitochondria of rat liver |journal= Biochem. J. |volume= 103 |issue= 2 |pages= 514–27 |year= 1967 |pmid= 4291787 |pmc= 1270436|doi= 10.1042/bj1030514 }}</ref><ref name=Zhang>{{cite journal |vauthors=Zhang Q, Piston DW, Goodman RH |s2cid= 31268989 |title= Regulation of corepressor function by nuclear NADH |journal= Science |volume= 295 |issue= 5561 |pages= 1895–7 |year= 2002 |pmid= 11847309 |doi= 10.1126/science.1069300|doi-access= free }}</ref> The ratio of total NAD{{+}}/NADH is much lower, with estimates ranging from 3–10 in mammals.<ref>{{cite journal |vauthors=Lin SJ, Guarente L |title= Nicotinamide adenine dinucleotide, a metabolic regulator of transcription, longevity and disease |journal= Curr. Opin. Cell Biol. |volume= 15 |issue= 2 |pages= 241–6 |date= April 2003 |pmid= 12648681 |doi= 10.1016/S0955-0674(03)00006-1}}</ref> In contrast, the [[nicotinamide adenine dinucleotide phosphate|NADP{{+}}/NADPH]] ratio is normally about 0.005, so NADPH is the dominant form of this coenzyme.<ref>{{cite journal |vauthors=Veech RL, Eggleston LV, Krebs HA |title= The redox state of free nicotinamide–adenine dinucleotide phosphate in the cytoplasm of rat liver |journal= Biochem. J. |volume= 115 |issue= 4 |pages= 609–19 |year= 1969 |pmid= 4391039 |pmc= 1185185 |doi=10.1042/bj1150609a}}</ref> These different ratios are key to the different metabolic roles of NADH and NADPH.

==Biosynthesis==
NAD{{+}} is synthesized through two metabolic pathways. It is produced either in a [[de novo synthesis|''de novo'']] pathway from amino acids or in salvage pathways by recycling preformed components such as [[nicotinamide]] back to NAD{{+}}. Although most tissues synthesize NAD{{+}} by the salvage pathway in mammals, much more ''de novo'' synthesis occurs in the liver from tryptophan, and in the kidney and [[macrophage]]s from [[nicotinic acid]].<ref name="pmid32097708">{{cite journal | vauthors = McReynolds MR, Chellappa K, Baur JA | title = Age-related NAD + decline | journal = [[Experimental Gerontology]] | volume = 134 | article-number = 110888 |date=2020 | doi = 10.1016/j.exger.2020.110888 | pmc =7442590 | pmid = 32097708}}</ref>

===''De novo'' production===
[[File:NAD metabolism.svg|thumb|upright=1.5|class=skin-invert-image|Some [[metabolic pathway]]s that synthesize and consume NAD{{+}} in [[vertebrate]]s.{{image reference needed|date=December 2022}} The abbreviations are defined in the text.]]

Most organisms synthesize NAD{{+}} from simple components.<ref name=Belenky/> The specific set of reactions differs among organisms, but a common feature is the generation of [[quinolinic acid]] (QA) from an amino acid{{snd}}either [[tryptophan]] (Trp) in animals and some bacteria, or [[aspartic acid]] (Asp) in some bacteria and plants.<ref>{{cite journal |vauthors=Katoh A, Uenohara K, Akita M, Hashimoto T |title= Early Steps in the Biosynthesis of NAD in Arabidopsis Start with Aspartate and Occur in the Plastid |journal= Plant Physiol. |volume= 141 |issue= 3 |pages= 851–857 |year= 2006 |pmid= 16698895 |pmc= 1489895 |doi= 10.1104/pp.106.081091}}</ref><ref>{{cite journal |vauthors=Foster JW, Moat AG |title= Nicotinamide adenine dinucleotide biosynthesis and pyridine nucleotide cycle metabolism in microbial systems |journal= Microbiol. Rev. |volume= 44 |issue= 1 |pages= 83–105 |date= 1 March 1980 |doi= 10.1128/MMBR.44.1.83-105.1980 |pmid= 6997723 |pmc= 373235}}</ref> The quinolinic acid is converted to nicotinic acid mononucleotide (NaMN) by transfer of a phosphoribose moiety. An adenylate moiety is then transferred to form nicotinic acid adenine dinucleotide (NaAD). Finally, the nicotinic acid moiety in NaAD is [[amide|amidated]] to a nicotinamide (Nam) moiety, forming nicotinamide adenine dinucleotide.<ref name=Belenky/>

In a further step, some NAD{{+}} is converted into NADP{{+}} by [[NAD+ kinase|NAD{{+}} kinase]], which [[phosphorylates]] NAD{{+}}.<ref>{{cite journal |vauthors=Magni G, Orsomando G, Raffaelli N |title= Structural and functional properties of NAD kinase, a key enzyme in NADP biosynthesis |journal= Mini Reviews in Medicinal Chemistry |volume= 6 |issue= 7 |pages= 739–746 |year= 2006 |pmid= 16842123 |doi= 10.2174/138955706777698688}}</ref> In most organisms, this enzyme uses [[adenosine triphosphate]] (ATP) as the source of the phosphate group, although several bacteria such as ''[[Mycobacterium tuberculosis]]'' and a hyperthermophilic [[archaeon]] ''[[Pyrococcus|Pyrococcus horikoshii]]'', use inorganic [[polyphosphate]] as an alternative phosphoryl donor.<ref>{{cite journal |vauthors=Sakuraba H, Kawakami R, Ohshima T |title= First Archaeal Inorganic Polyphosphate/ATP-Dependent NAD Kinase, from Hyperthermophilic Archaeon Pyrococcus horikoshii: Cloning, Expression, and Characterization |journal= Appl. Environ. Microbiol. |volume= 71 |issue= 8 |pages= 4352–4358 |year= 2005 |pmid= 16085824 |pmc= 1183369 |doi= 10.1128/AEM.71.8.4352-4358.2005|bibcode= 2005ApEnM..71.4352S }}</ref><ref>{{cite journal |vauthors=Raffaelli N, Finaurini L, Mazzola F, Pucci L, Sorci L, Amici A, Magni G |title= Characterization of Mycobacterium tuberculosis NAD kinase: functional analysis of the full-length enzyme by site-directed mutagenesis |journal= Biochemistry |volume= 43 |issue= 23 |pages= 7610–7617 |year= 2004 |pmid= 15182203 |doi= 10.1021/bi049650w}}</ref>

[[File:NA, N and NR.svg|thumb|left|upright=1.3|class=skin-invert-image|Salvage pathways use three precursors for NAD<sup>+</sup>.]]

===Salvage pathways===
Despite the presence of the ''de novo'' pathway, the salvage reactions are essential in humans; a lack of [[vitamin B3]] in the diet causes the [[vitamin deficiency]] disease [[pellagra]].<ref>{{cite journal |author= Henderson LM |title= Niacin |journal= Annu. Rev. Nutr. |volume= 3 |pages= 289–307 |year= 1983 |pmid= 6357238 |doi= 10.1146/annurev.nu.03.070183.001445}}</ref> This high requirement for NAD{{+}} results from the constant consumption of the coenzyme in reactions such as posttranslational modifications, since the cycling of NAD{{+}} between oxidized and reduced forms in redox reactions does not change the overall levels of the coenzyme.<ref name=Belenky/>
The major source of NAD{{+}} in mammals is the salvage pathway which recycles the [[nicotinamide]] produced by enzymes utilizing NAD{{+}}.<ref name="pmid29514064">{{cite journal | vauthors=Rajman L, Chwalek K, Sinclair DA | title=Therapeutic Potential of NAD-Boosting Molecules: The In Vivo Evidence| journal=[[Cell Metabolism]] | volume=27 | issue=3 | pages=529–547 | year=2018 | doi = 10.1016/j.cmet.2018.02.011 | pmc = 6342515 | pmid=29514064}}</ref> The first step, and the rate-limiting enzyme in the salvage pathway is [[nicotinamide phosphoribosyltransferase]] (NAMPT), which produces [[nicotinamide mononucleotide]] (NMN).<ref name="pmid29514064" /> NMN is the immediate precursor to NAD+ in the salvage pathway.<ref>{{Cite web|title=What is NMN?|url=https://www.nmn.com/precursors/what-is-nmn|access-date=2021-01-08|website=www.nmn.com}}</ref>

Besides assembling NAD{{+}} ''de novo'' from simple amino acid precursors, cells also salvage preformed compounds containing a pyridine base. The three vitamin precursors used in these salvage metabolic pathways are [[nicotinic acid]] (NA), [[nicotinamide]] (Nam) and [[nicotinamide riboside]] (NR).<ref name=Belenky/> These compounds can be taken up from the diet and are termed vitamin B{{sub|3}} or ''niacin''. However, these compounds are also produced within cells and by digestion of cellular NAD{{+}}. Some of the enzymes involved in these salvage pathways appear to be concentrated in the [[cell nucleus]], which may compensate for the high level of reactions that consume NAD{{+}} in this [[organelle]].<ref>{{cite journal |vauthors=Anderson RM, Bitterman KJ, Wood JG, Medvedik O, Cohen H, Lin SS, Manchester JK, Gordon JI, Sinclair DA |title= Manipulation of a nuclear NAD<sup>+</sup> salvage pathway delays aging without altering steady-state NAD<sup>+</sup> levels |journal= J. Biol. Chem. |volume= 277 |issue= 21 |pages= 18881–18890 |year= 2002 |pmid= 11884393 |doi= 10.1074/jbc.M111773200 |doi-access= free |pmc= 3745358 }}</ref> There are some reports that mammalian cells can take up extracellular NAD{{+}} from their surroundings,<ref>{{cite journal |vauthors=Billington RA, Travelli C, Ercolano E, Galli U, Roman CB, Grolla AA, Canonico PL, Condorelli F, Genazzani AA |title= Characterization of NAD Uptake in Mammalian Cells |journal= J. Biol. Chem. |volume= 283 |issue= 10 |pages= 6367–6374 |year= 2008 |pmid= 18180302 |doi= 10.1074/jbc.M706204200|doi-access= free }}</ref> and both nicotinamide and nicotinamide riboside can be absorbed from the gut.<ref>{{cite journal|vauthors=Trammell SA, Schmidt MS, Weidemann BJ, Redpath P, Jaksch F, Dellinger RW, Li Z, Abel ED, Migaud ME, Brenner C|title=Nicotinamide riboside is uniquely and orally bioavailable in mice and humans|journal=Nature Communications|year=2016|volume=7|article-number=12948|doi=10.1038/ncomms12948|pmid=27721479|pmc=5062546|bibcode=2016NatCo...712948T}}</ref>

The salvage pathways used in [[microorganism]]s differ from those of [[mammal]]s.<ref name=Rongvaux>{{cite journal |vauthors=Rongvaux A, Andris F, Van Gool F, Leo O |title= Reconstructing eukaryotic NAD metabolism |journal= BioEssays |volume= 25 |issue= 7 |pages= 683–690 |year= 2003 |pmid= 12815723 |doi= 10.1002/bies.10297}}</ref> Some pathogens, such as the yeast ''[[Candida glabrata]]'' and the bacterium ''[[Haemophilus influenzae]]'' are NAD{{+}} [[auxotroph]]s&nbsp;– they cannot synthesize NAD{{+}}&nbsp;– but possess salvage pathways and thus are dependent on external sources of NAD{{+}} or its precursors.<ref>{{cite journal |vauthors=Ma B, Pan SJ, Zupancic ML, Cormack BP |title= Assimilation of NAD<sup>+</sup> precursors in ''Candida glabrata'' |journal= Mol. Microbiol. |volume= 66 |issue= 1 |pages= 14–25 |year= 2007 |pmid= 17725566 |doi= 10.1111/j.1365-2958.2007.05886.x|s2cid= 22282128 |doi-access= free }}</ref><ref>{{cite journal |vauthors=Reidl J, Schlör S, Kraiss A, Schmidt-Brauns J, Kemmer G, Soleva E |title= NADP and NAD utilization in ''Haemophilus influenzae'' |journal= Mol. Microbiol. |volume= 35 |issue= 6 |pages= 1573–1581 |year= 2000 |pmid= 10760156 |doi= 10.1046/j.1365-2958.2000.01829.x|s2cid= 29776509 }}</ref> Even more surprising is the intracellular [[pathogen]] ''[[Chlamydia trachomatis]]'', which lacks recognizable candidates for any genes involved in the biosynthesis or salvage of both NAD{{+}} and NADP{{+}}, and must acquire these coenzymes from its [[host (biology)|host]].<ref>{{cite journal |vauthors=Gerdes SY, Scholle MD, D'Souza M, Bernal A, Baev MV, Farrell M, Kurnasov OV, Daugherty MD, Mseeh F, Polanuyer BM, Campbell JW, Anantha S, Shatalin KY, Chowdhury SA, Fonstein MY, Osterman AL |title= From Genetic Footprinting to Antimicrobial Drug Targets: Examples in Cofactor Biosynthetic Pathways |journal= J. Bacteriol. |volume= 184 |issue= 16 |pages= 4555–4572 |year= 2002 |pmid= 12142426 |pmc= 135229 |doi= 10.1128/JB.184.16.4555-4572.2002}}</ref>

==Functions==
[[File:Rossman fold.png|thumb|[[Rossmann fold]] in part of the [[lactate dehydrogenase]] of ''[[Cryptosporidium parvum]]'', showing NAD{{+}} in red, beta sheets in yellow, and alpha helices in purple<ref>{{cite journal |vauthors=Senkovich O, Speed H, Grigorian A, etal |title=Crystallization of three key glycolytic enzymes of the opportunistic pathogen ''Cryptosporidium parvum'' |journal=Biochim. Biophys. Acta |volume=1750 |issue=2 |pages=166–72 |year=2005 |pmid=15953771 |doi=10.1016/j.bbapap.2005.04.009}}</ref>]]

Nicotinamide adenine dinucleotide has several essential roles in [[metabolism]]. It acts as a [[coenzyme]] in [[redox]] reactions, as a donor of ADP-ribose moieties in [[ADP-ribosylation]] reactions, as a precursor of the [[second messenger]] molecule [[cyclic ADP-ribose]], as well as acting as a substrate for bacterial [[DNA ligase]]s and a group of enzymes called [[sirtuin]]s that use NAD{{+}} to remove [[acetyl]] groups from proteins. In addition to these metabolic functions, NAD<sup>+</sup> emerges as an adenine nucleotide that can be released from cells spontaneously and by regulated mechanisms,<ref name=Smyth>{{cite journal |vauthors=Smyth LM, Bobalova J, Mendoza MG, Lew C, Mutafova-Yambolieva VN |title= Release of beta-nicotinamide adenine dinucleotide upon stimulation of postganglionic nerve terminals in blood vessels and urinary bladder |journal= J Biol Chem |volume= 279 |issue= 47 |pages= 48893–903 |year= 2004 |pmid= 15364945 |doi= 10.1074/jbc.M407266200|doi-access= free }}</ref><ref name=Billington>{{cite journal |vauthors=Billington RA, Bruzzone S, De Flora A, Genazzani AA, Koch-Nolte F, Ziegler M, Zocchi E |title= Emerging functions of extracellular pyridine nucleotides |journal= Mol. Med. |volume= 12 |issue= 11–12 |pages= 324–7 |year= 2006 |pmid= 17380199 |pmc= 1829198 |doi= 10.2119/2006-00075.Billington}}</ref> and can therefore have important [[extracellular]] roles.<ref name=Billington/>

===Oxidoreductase binding of NAD===
{{Further|Protein structure|Oxidoreductase}}

The main role of NAD{{+}} in metabolism is the transfer of electrons from one molecule to another. Reactions of this type are catalyzed by a large group of enzymes called [[oxidoreductase]]s. The correct names for these enzymes contain the names of both their substrates: for example [[NADH dehydrogenase|NADH-ubiquinone oxidoreductase]] catalyzes the oxidation of NADH by [[coenzyme Q]].<ref>{{cite web |url= http://www.chem.qmul.ac.uk/iubmb/enzyme |title= Enzyme Nomenclature, Recommendations for enzyme names from the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology |access-date= 6 December 2007 |archive-url= https://web.archive.org/web/20071205181246/http://www.chem.qmul.ac.uk/iubmb/enzyme/ |archive-date= 5 December 2007 }}</ref> However, these enzymes are also referred to as ''dehydrogenases'' or ''reductases'', with NADH-ubiquinone oxidoreductase commonly being called ''NADH dehydrogenase'' or sometimes ''coenzyme Q reductase''.<ref>{{cite web|url=https://enzyme.expasy.org/EC/1.6.5.3 |title=NiceZyme View of ENZYME: EC 1.6.5.3 |access-date=2007-12-16 |publisher=Expasy}}</ref>

There are many different superfamilies of enzymes that bind NAD{{+}} / NADH. One of the most common superfamilies includes a [[structural motif]] known as the [[Rossmann fold]].<ref name="2015-Hanukoglu">{{cite journal |vauthors= Hanukoglu I |title= Proteopedia: Rossmann fold: A beta-alpha-beta fold at dinucleotide binding sites |journal= Biochem Mol Biol Educ |volume=43 |issue=3 |pages=206–209 |year=2015 |pmid=25704928 |doi=10.1002/bmb.20849 |s2cid= 11857160 |doi-access=free }}</ref><ref>{{cite journal |author= Lesk AM |title= NAD-binding domains of dehydrogenases |journal= Curr. Opin. Struct. Biol. |volume= 5 |issue= 6 |pages= 775–83 |year= 1995 |pmid= 8749365 |doi= 10.1016/0959-440X(95)80010-7}}</ref> The motif is named after [[Michael Rossmann]], who was the first scientist to notice how common this structure is within nucleotide-binding proteins.<ref name=Rao>{{cite journal |vauthors=Rao ST, Rossmann MG |title= Comparison of super-secondary structures in proteins |journal= J Mol Biol |volume= 76 |issue= 2 |pages= 241–56 |year= 1973 |pmid= 4737475 |doi= 10.1016/0022-2836(73)90388-4}}</ref>

An example of a NAD-binding bacterial enzyme involved in [[amino acid]] metabolism that does not have the Rossmann fold is found in ''[[Pseudomonas syringae]]'' pv. tomato ({{PDB|2CWH}}; {{InterPro|IPR003767}}).<ref>{{cite journal |vauthors=Goto M, Muramatsu H, Mihara H, Kurihara T, Esaki N, Omi R, Miyahara I, Hirotsu K |title= Crystal structures of Delta1-piperideine-2-carboxylate/Delta1-pyrroline-2-carboxylate reductase belonging to a new family of NAD(P)H-dependent oxidoreductases: conformational change, substrate recognition, and stereochemistry of the reaction |journal= J. Biol. Chem. |volume= 280 |issue= 49 |pages= 40875–84 |year= 2005 |pmid= 16192274 |doi= 10.1074/jbc.M507399200 |doi-access= free }}</ref>

[[File:NAD+ phys alt.svg|thumb|left|class=skin-invert-image|In this diagram, the hydride acceptor C4 carbon is shown at the top. When the nicotinamide ring lies in the plane of the page with the carboxy-amide to the right, as shown, the hydride donor lies either "above" or "below" the plane of the page. If "above" hydride transfer is class A, if "below" hydride transfer is class B.<ref name=bellamacina/>]]

When bound in the active site of an oxidoreductase, the nicotinamide ring of the coenzyme is positioned so that it can accept a hydride from the other substrate. Depending on the enzyme, the hydride donor is positioned either "above" or "below" the plane of the planar C4 carbon, as defined in the figure. Class A oxidoreductases transfer the atom from above; class B enzymes transfer it from below. Since the C4 carbon that accepts the hydrogen is [[prochiral]], this can be exploited in [[enzyme kinetics]] to give information about the enzyme's mechanism. This is done by mixing an enzyme with a substrate that has [[deuterium]] atoms substituted for the hydrogens, so the enzyme will reduce NAD{{+}} by transferring deuterium rather than hydrogen. In this case, an enzyme can produce one of two [[stereoisomerism|stereoisomer]]s of NADH.<ref name=bellamacina>{{cite journal |author= Bellamacina CR |title= The nicotinamide dinucleotide binding motif: a comparison of nucleotide binding proteins |journal= FASEB J. |volume= 10 |issue= 11 |pages= 1257–69 |date= 1 September 1996 |pmid= 8836039|doi= 10.1096/fasebj.10.11.8836039 |doi-access= free |s2cid= 24189316 }}</ref>

Despite the similarity in how proteins bind the two coenzymes, enzymes almost always show a high level of specificity for either NAD{{+}} or NADP{{+}}.<ref>{{cite journal |vauthors=Carugo O, Argos P |title= NADP-dependent enzymes. I: Conserved stereochemistry of cofactor binding |journal= Proteins |volume= 28 |issue= 1 |pages= 10–28 |year= 1997 |pmid= 9144787 |doi= 10.1002/(SICI)1097-0134(199705)28:1<10::AID-PROT2>3.0.CO;2-N|s2cid= 23969986 }}</ref> This specificity reflects the distinct metabolic roles of the respective coenzymes, and is the result of distinct sets of [[amino acid]] residues in the two types of coenzyme-binding pocket. For instance, in the active site of NADP-dependent enzymes, an [[ionic bond]] is formed between a basic amino acid side-chain and the acidic phosphate group of NADP{{+}}. On the converse, in NAD-dependent enzymes the charge in this pocket is reversed, preventing NADP{{+}} from binding. However, there are a few exceptions to this general rule, and enzymes such as [[aldose reductase]], [[glucose-6-phosphate dehydrogenase]], and [[methylenetetrahydrofolate reductase]] can use both coenzymes in some species.<ref>{{cite journal |vauthors=Vickers TJ, Orsomando G, de la Garza RD, Scott DA, Kang SO, Hanson AD, Beverley SM |title= Biochemical and genetic analysis of methylenetetrahydrofolate reductase in Leishmania metabolism and virulence |journal= J. Biol. Chem. |volume= 281 |issue= 50 |pages= 38150–8 |year= 2006 |pmid= 17032644 |doi= 10.1074/jbc.M608387200|doi-access= free }}</ref>

===Role in redox metabolism===
[[File:Catabolism schematic.svg|thumb|class=skin-invert-image|A simplified outline of redox [[metabolism]], showing how NAD{{+}} and NADH link the [[citric acid cycle]] and [[oxidative phosphorylation]]{{image reference needed|date=December 2022}}]]

{{Further|Cellular respiration|Oxidative phosphorylation}}

The redox reactions catalyzed by oxidoreductases are vital in all parts of metabolism, but one particularly important area where these reactions occur is in the release of energy from nutrients. Here, reduced compounds such as [[glucose]] and [[fatty acid]]s are oxidized, thereby releasing energy. This energy is transferred to NAD{{+}} by reduction to NADH, as part of [[beta oxidation]], [[glycolysis]], and the [[citric acid cycle]]. In [[eukaryote]]s the electrons carried by the NADH that is produced in the [[cytoplasm]] are transferred into the [[mitochondrion]] (to reduce mitochondrial NAD{{+}}) by [[mitochondrial shuttle]]s, such as the [[malate-aspartate shuttle]].<ref>{{cite journal |vauthors=Bakker BM, Overkamp KM, Kötter P, Luttik MA, Pronk JT |title= Stoichiometry and compartmentation of NADH metabolism in ''Saccharomyces cerevisiae'' |journal= FEMS Microbiol. Rev. |volume= 25 |issue= 1 |pages= 15–37 |year= 2001 |pmid= 11152939 |doi= 10.1111/j.1574-6976.2001.tb00570.x|doi-access= free }}</ref> The mitochondrial NADH is then oxidized in turn by the [[electron transport chain]], which pumps protons across a membrane and generates ATP through [[oxidative phosphorylation]].<ref>{{cite journal |last1=Rich |first1=P.R. |title=The molecular machinery of Keilin's respiratory chain |journal=Biochemical Society Transactions |date=1 December 2003 |volume=31 |issue=6 |pages=1095–1105 |doi=10.1042/bst0311095 |pmid=14641005 |s2cid=32361233 }}</ref> These shuttle systems also have the same transport function in [[chloroplast]]s.<ref>{{cite journal |vauthors=Heineke D, Riens B, Grosse H, Hoferichter P, Peter U, Flügge UI, Heldt HW |title= Redox Transfer across the Inner Chloroplast Envelope Membrane |journal= Plant Physiol |volume= 95 |issue= 4 |pages= 1131–1137 |year= 1991 |pmid= 16668101 |pmc= 1077662 |doi= 10.1104/pp.95.4.1131}}</ref>

Since both the oxidized and reduced forms of nicotinamide adenine dinucleotide are used in these linked sets of reactions, the cell maintains significant concentrations of both NAD{{+}} and NADH, with the high NAD{{+}}/NADH ratio allowing this coenzyme to act as both an oxidizing and a reducing agent.<ref name=Nicholls>{{cite book|author=Nicholls DG|author2=Ferguson SJ |title=Bioenergetics 3 |edition=1st|publisher=Academic Press|year=2002 |isbn=978-0-12-518121-1}}</ref> In contrast, the main function of NADPH is as a reducing agent in [[anabolism]], with this coenzyme being involved in pathways such as [[fatty acid synthesis]] and [[photosynthesis]]. Since NADPH is needed to drive redox reactions as a strong reducing agent, the NADP{{+}}/NADPH ratio is kept very low.<ref name=Nicholls/>

Although it is important in catabolism, NADH is also used in anabolic reactions, such as [[gluconeogenesis]].<ref>{{cite journal |last1=Sistare |first1=F D |last2=Haynes |first2=R C |title=The interaction between the cytosolic pyridine nucleotide redox potential and gluconeogenesis from lactate/pyruvate in isolated rat hepatocytes. Implications for investigations of hormone action. |journal=Journal of Biological Chemistry |date=October 1985 |volume=260 |issue=23 |pages=12748–12753 |doi=10.1016/S0021-9258(17)38940-8 |pmid=4044607 |doi-access=free }}</ref> This need for NADH in anabolism poses a problem for prokaryotes growing on nutrients that release only a small amount of energy. For example, [[nitrification|nitrifying]] bacteria such as ''[[Nitrobacter]]'' oxidize nitrite to nitrate, which releases sufficient energy to pump protons and generate ATP, but not enough to produce NADH directly.<ref>{{cite journal|vauthors=Freitag A, Bock E |year=1990 |title=Energy conservation in ''Nitrobacter'' |journal=FEMS Microbiology Letters |volume=66 |issue=1–3 |pages=157–62 |doi=10.1111/j.1574-6968.1990.tb03989.x|doi-access=free }}</ref> As NADH is still needed for anabolic reactions, these bacteria use a [[nitrite oxidoreductase]] to produce enough [[chemiosmosis|proton-motive force]] to run part of the electron transport chain in reverse, generating NADH.<ref>{{cite journal |vauthors=Starkenburg SR, Chain PS, Sayavedra-Soto LA, Hauser L, Land ML, Larimer FW, Malfatti SA, Klotz MG, Bottomley PJ, Arp DJ, Hickey WJ |title= Genome Sequence of the Chemolithoautotrophic Nitrite-Oxidizing Bacterium ''Nitrobacter winogradskyi'' Nb-255 |journal= Appl. Environ. Microbiol. |volume= 72 |issue= 3 |pages= 2050–63 |year= 2006 |pmid= 16517654 |pmc= 1393235 |doi= 10.1128/AEM.72.3.2050-2063.2006 |bibcode= 2006ApEnM..72.2050S }}</ref>

===Non-redox roles===
The coenzyme NAD{{+}} is also consumed in ADP-ribose transfer reactions. For example, enzymes called [[Glycosyltransferase|ADP-ribosyltransferases]] add the ADP-ribose moiety of this molecule to proteins, in a [[posttranslational modification]] called [[ADP-ribosylation]].<ref>{{cite journal |author= Ziegler M |title= New functions of a long-known molecule. Emerging roles of NAD in cellular signaling |journal= Eur. J. Biochem. |volume= 267 |issue= 6 |pages= 1550–64 |year= 2000 |pmid= 10712584 |doi= 10.1046/j.1432-1327.2000.01187.x|doi-access= free }}</ref> ADP-ribosylation involves either the addition of a single ADP-ribose moiety, in ''mono-ADP-ribosylation'', or the transferral of ADP-ribose to proteins in long branched chains, which is called ''poly(ADP-ribosyl)ation''.<ref name=Diefenbach>{{cite journal |vauthors=Diefenbach J, Bürkle A |title= Introduction to poly(ADP-ribose) metabolism |journal= Cell. Mol. Life Sci. |volume= 62 |issue= 7–8 |pages= 721–30 |year= 2005 |pmid= 15868397 |doi= 10.1007/s00018-004-4503-3|pmc= 11924567 }}</ref> Mono-ADP-ribosylation was first identified as the mechanism of a group of bacterial [[toxin]]s, notably [[cholera toxin]], but it is also involved in normal [[cell signaling]].<ref>{{cite journal |vauthors=Berger F, Ramírez-Hernández MH, Ziegler M |title= The new life of a centenarian: signaling functions of NAD(P) |journal= Trends Biochem. Sci. |volume= 29 |issue= 3 |pages= 111–8 |year= 2004 |pmid= 15003268 |doi= 10.1016/j.tibs.2004.01.007|s2cid= 8820773 }}</ref><ref>{{cite journal |vauthors=Corda D, Di Girolamo M |title= New Embo Member's Review: Functional aspects of protein mono-ADP-ribosylation |journal= EMBO J. |volume= 22 |issue= 9 |pages= 1953–8 |year= 2003 |pmid= 12727863 |pmc= 156081 |doi= 10.1093/emboj/cdg209}}</ref> Poly(ADP-ribosyl)ation is carried out by the [[poly ADP ribose polymerase|poly(ADP-ribose) polymerase]]s.<ref name="Diefenbach"/><ref name=Burkle>{{cite journal |author= Bürkle A |title= Poly(ADP-ribose). The most elaborate metabolite of NAD<sup>+</sup> |journal= FEBS J. |volume= 272 |issue= 18 |pages= 4576–89 |year= 2005 |pmid= 16156780 |doi= 10.1111/j.1742-4658.2005.04864.x|s2cid= 22975714 |doi-access= free }}</ref> The poly(ADP-ribose) structure is involved in the regulation of several cellular events and is most important in the [[cell nucleus]], in processes such as [[DNA repair]] and [[telomere]] maintenance.<ref name=Burkle/> In addition to these functions within the cell, a group of [[extracellular]] ADP-ribosyltransferases has recently been discovered, but their functions remain obscure.<ref>{{cite journal |vauthors=Seman M, Adriouch S, Haag F, Koch-Nolte F |title= Ecto-ADP-ribosyltransferases (ARTs): emerging actors in cell communication and signaling |journal= Curr. Med. Chem. |volume= 11 |issue= 7 |pages= 857–72 |year= 2004 |pmid= 15078170 |doi= 10.2174/0929867043455611}}</ref>
NAD{{+}} may also be added onto cellular [[RNA]] as a 5'-terminal modification.<ref>{{cite journal |vauthors=Chen YG, Kowtoniuk WE, Agarwal I, Shen Y, Liu DR |title= LC/MS analysis of cellular RNA reveals NAD-linked RNA |journal= Nat Chem Biol |volume= 5 |issue= 12 |pages= 879–881 |date= December 2009 |pmid= 19820715 |pmc= 2842606 |doi= 10.1038/nchembio.235}}</ref>

[[File:Cyclic ADP ribose.svg|thumb|left|upright=1.1|class=skin-invert-image|The structure of [[cyclic ADP-ribose]]]]

Another function of this coenzyme in cell signaling is as a precursor of [[cyclic ADP-ribose]], which is produced from NAD{{+}} by ADP-ribosyl cyclases, as part of a [[second messenger system]].<ref>{{cite journal |author= Guse AH |title= Biochemistry, biology, and pharmacology of cyclic adenosine diphosphoribose (cADPR) |journal= Curr. Med. Chem. |volume= 11 |issue= 7 |pages= 847–55 |year= 2004 |pmid= 15078169 |doi= 10.2174/0929867043455602}}</ref> This molecule acts in [[calcium signaling]] by releasing calcium from intracellular stores.<ref>{{cite journal |author= Guse AH |title= Regulation of calcium signaling by the second messenger cyclic adenosine diphosphoribose (cADPR) |journal= Curr. Mol. Med. |volume= 4 |issue= 3 |pages= 239–48 |year= 2004 |pmid= 15101682 |doi= 10.2174/1566524043360771}}</ref> It does this by binding to and opening a class of calcium channels called [[ryanodine receptor]]s, which are located in the membranes of [[organelle]]s, such as the [[endoplasmic reticulum]], and inducing the activation of the [[transcription factor]] [[NFATC3|NAFC3]]<ref>{{cite journal |author= Guse AH |title= Second messenger function and the structure-activity relationship of cyclic adenosine diphosphoribose (cADPR) |journal= FEBS J. |volume= 272 |issue= 18 |pages= 4590–7 |year= 2005 |pmid= 16156781 |doi= 10.1111/j.1742-4658.2005.04863.x|s2cid= 21509962 |doi-access= free }}</ref>

NAD{{+}} is also consumed by different NAD+-consuming enzymes, such as [[CD38]], [[BST1|CD157]], [[Poly (ADP-ribose) polymerase|PARPs]] and the NAD-dependent [[histone deacetylase|deacetylases]] ([[sirtuin]]s, such as [[Sir2]].<ref>{{cite journal |vauthors=North BJ, Verdin E |title= Sirtuins: Sir2-related NAD-dependent protein deacetylases |journal= Genome Biol |volume= 5 |issue= 5 |page= 224 |year= 2004 |pmid= 15128440 |pmc= 416462 |doi= 10.1186/gb-2004-5-5-224 |doi-access= free }}</ref>).<ref name=":0">{{Cite journal |last=Verdin |first=Eric |date=2015-12-04 |title=NAD⁺ in aging, metabolism, and neurodegeneration |journal=Science |volume=350 |issue=6265 |pages=1208–1213 |doi=10.1126/science.aac4854 |issn=1095-9203 |pmid=26785480|bibcode=2015Sci...350.1208V |s2cid=27313960 }}</ref> These enzymes act by transferring an [[acetyl]] group from their substrate protein to the ADP-ribose moiety of NAD{{+}}; this cleaves the coenzyme and releases nicotinamide and O-acetyl-ADP-ribose. The sirtuins mainly seem to be involved in regulating [[transcription (genetics)|transcription]] through deacetylating histones and altering [[nucleosome]] structure.<ref>{{cite journal |last1=Blander |first1=Gil |last2=Guarente |first2=Leonard |title=The Sir2 Family of Protein Deacetylases |journal=Annual Review of Biochemistry |date=June 2004 |volume=73 |issue=1 |pages=417–435 |doi=10.1146/annurev.biochem.73.011303.073651 |pmid=15189148 |s2cid=27494475 }}</ref> However, non-histone proteins can be deacetylated by sirtuins as well. These activities of sirtuins are particularly interesting because of their importance in the regulation of [[aging]].<ref>{{cite journal |vauthors=Trapp J, Jung M |title= The role of NAD+ dependent histone deacetylases (sirtuins) in ageing |journal= Curr Drug Targets |volume= 7 |issue= 11 |pages= 1553–60 |year= 2006 |pmid= 17100594 |doi= 10.2174/1389450110607011553}}</ref><ref name=":1">{{Cite journal |last1=Meyer |first1=Tom |last2=Shimon |first2=Dor |last3=Youssef |first3=Sawsan |last4=Yankovitz |first4=Gal |last5=Tessler |first5=Adi |last6=Chernobylsky |first6=Tom |last7=Gaoni-Yogev |first7=Anat |last8=Perelroizen |first8=Rita |last9=Budick-Harmelin |first9=Noga |last10=Steinman |first10=Lawrence |last11=Mayo |first11=Lior |date=2022-08-30 |title=NAD+ metabolism drives astrocyte proinflammatory reprogramming in central nervous system autoimmunity |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=119 |issue=35 |article-number=e2211310119 |doi=10.1073/pnas.2211310119 |doi-access=free |issn=1091-6490 |pmc=9436380 |pmid=35994674|bibcode=2022PNAS..11911310M }}</ref>

Other NAD-dependent enzymes include bacterial [[DNA ligase]]s, which join two DNA ends by using NAD{{+}} as a substrate to donate an [[adenosine monophosphate]] (AMP) moiety to the 5' phosphate of one DNA end. This intermediate is then attacked by the 3' hydroxyl group of the other DNA end, forming a new [[phosphodiester bond]].<ref>{{cite journal |vauthors=Wilkinson A, Day J, Bowater R |title= Bacterial DNA ligases |journal= Mol. Microbiol. |volume= 40 |issue= 6 |pages= 1241–8 |year= 2001 |pmid= 11442824 |doi= 10.1046/j.1365-2958.2001.02479.x|s2cid= 19909818 |doi-access= free }}</ref> This contrasts with [[eukaryotic]] DNA ligases, which use ATP to form the DNA-AMP intermediate.<ref>{{cite journal |vauthors=Schär P, Herrmann G, Daly G, Lindahl T |title= A newly identified DNA ligase of ''Saccharomyces cerevisiae'' involved in RAD52-independent repair of DNA double-strand breaks |journal= Genes & Development |volume= 11 |issue= 15 |pages= 1912–24 |year= 1997 |pmid= 9271115 |pmc= 316416 |doi= 10.1101/gad.11.15.1912}}</ref>

Li et al. have found that NAD{{+}} directly regulates protein-protein interactions.<ref name="Li 2017">{{cite journal |last1=Li |first1=Jun |last2=Bonkowski |first2=Michael S. |last3=Moniot |first3=Sébastien |last4=Zhang |first4=Dapeng |last5=Hubbard |first5=Basil P. |last6=Ling |first6=Alvin J. Y. |last7=Rajman |first7=Luis A. |last8=Qin |first8=Bo |last9=Lou |first9=Zhenkun |last10=Gorbunova |first10=Vera |last11=Aravind |first11=L. |last12=Steegborn |first12=Clemens |last13=Sinclair |first13=David A. |title=A conserved NAD binding pocket that regulates protein-protein interactions during aging |journal=Science |date=23 March 2017 |volume=355 |issue=6331 |pages=1312–1317 |doi=10.1126/science.aad8242|pmc=5456119 |pmid=28336669|bibcode=2017Sci...355.1312L }}</ref> They also show that one of the causes of age-related decline in DNA repair may be increased binding of the protein [[KIAA1967|DBC1]] (Deleted in Breast Cancer 1) to [[PARP1]] (poly[ADP–ribose] polymerase 1) as NAD{{+}} levels decline during aging.<ref name="Li 2017" />  The decline in cellular concentrations of NAD{{+}}  during aging likely contributes to the [[ageing|aging]] process and to the [[pathogenesis]] of the chronic diseases of aging.<ref>Verdin E. NAD⁺ in aging, metabolism, and neurodegeneration. Science. 2015 Dec 4;350(6265):1208-13. doi: 10.1126/science.aac4854. PMID 26785480</ref>  Thus, the modulation of NAD{{+}} may protect against cancer, radiation, and aging.<ref name="Li 2017" />

===Extracellular actions of NAD<sup>+</sup>===
In recent years, NAD<sup>+</sup> has also been recognized as an [[extracellular]] signaling molecule involved in cell-to-cell communication.<ref name=Billington/><ref name=Ziegler>{{cite journal |vauthors=Ziegler M, Niere M |title= NAD<sup>+</sup> surfaces again |journal= Biochem. J. |volume= 382 |issue= Pt 3 |pages= e5–6 |year= 2004 |article-number= E5 |pmid= 15352307 |pmc= 1133982 |doi= 10.1042/BJ20041217}}</ref><ref name=Koch-Nolte>{{cite journal |vauthors=Koch-Nolte F, Fischer S, Haag F, Ziegler M |title= Compartmentation of NAD<sup>+</sup>-dependent signalling |journal= FEBS Lett. |volume= 585 |issue= 11 |pages= 1651–6 |year= 2011 |pmid= 21443875 |doi= 10.1016/j.febslet.2011.03.045|s2cid= 4333147 |doi-access= free |bibcode= 2011FEBSL.585.1651K }}</ref> NAD<sup>+</sup> is released from [[neuron]]s in [[blood vessel]]s,<ref name=Smyth/> [[urinary bladder]],<ref name=Smyth/><ref name=Breen>{{cite journal |last1=Breen |first1=Leanne T. |last2=Smyth |first2=Lisa M. |last3=Yamboliev |first3=Ilia A. |last4=Mutafova-Yambolieva |first4=Violeta N. |title=β-NAD is a novel nucleotide released on stimulation of nerve terminals in human urinary bladder detrusor muscle |journal=American Journal of Physiology. Renal Physiology |date=February 2006 |volume=290 |issue=2 |pages=F486–F495 |doi=10.1152/ajprenal.00314.2005 |pmid=16189287 |s2cid=11400206 }}</ref> [[large intestine]],<ref name=Mutafova-Yambolieva>{{cite journal |vauthors=Mutafova-Yambolieva VN, Hwang SJ, Hao X, Chen H, Zhu MX, Wood JD, Ward SM, Sanders KM |title= Beta-nicotinamide adenine dinucleotide is an inhibitory neurotransmitter in visceral smooth muscle |journal= Proc. Natl. Acad. Sci. U.S.A. |volume= 104 |issue= 41 |pages= 16359–64 |year= 2007 |pmid= 17913880 |pmc= 2042211 |doi= 10.1073/pnas.0705510104 |bibcode= 2007PNAS..10416359M|doi-access= free }}</ref><ref name=Hwang>{{cite journal |vauthors=Hwang SJ, Durnin L, Dwyer L, Rhee PL, Ward SM, Koh SD, Sanders KM, Mutafova-Yambolieva VN |title= β-nicotinamide adenine dinucleotide is an enteric inhibitory neurotransmitter in human and nonhuman primate colons |journal= Gastroenterology |volume= 140 |issue= 2 |pages= 608–617.e6 |year= 2011 |pmid= 20875415 |pmc= 3031738 |doi= 10.1053/j.gastro.2010.09.039}}</ref> from neurosecretory cells,<ref name=Yamboliev>{{cite journal |vauthors=Yamboliev IA, Smyth LM, Durnin L, Dai Y, Mutafova-Yambolieva VN |title= Storage and secretion of beta-NAD, ATP and dopamine in NGF-differentiated rat pheochromocytoma PC12 cells |journal= Eur. J. Neurosci. |volume= 30 |issue= 5 |pages= 756–68 |year= 2009 |pmid= 19712094 |pmc= 2774892 |doi= 10.1111/j.1460-9568.2009.06869.x}}</ref> and from brain [[synaptosome]]s,<ref name=Durnin>{{cite journal |vauthors=Durnin L, Dai Y, Aiba I, Shuttleworth CW, Yamboliev IA, Mutafova-Yambolieva VN |title= Release, neuronal effects and removal of extracellular β-nicotinamide adenine dinucleotide (β-NAD<sup>+</sup>) in the rat brain |journal= Eur. J. Neurosci. |volume= 35 |issue= 3 |pages= 423–35 |year= 2012 |pmid= 22276961 |pmc= 3270379 |doi= 10.1111/j.1460-9568.2011.07957.x}}</ref> and is proposed to be a novel [[neurotransmitter]] that transmits information from [[nerve]]s to effector cells in [[smooth muscle]] organs.<ref name=Mutafova-Yambolieva/><ref name=Hwang/> In plants, the extracellular nicotinamide adenine dinucleotide induces resistance to pathogen infection and the first extracellular NAD receptor has been identified.<ref name=Zhou&Wang>{{cite journal |vauthors=Wang C, Zhou M, Zhang X, Yao J, Zhang Y, Mou Z |title= A lectin receptor kinase as a potential sensor for extracellular nicotinamide adenine dinucleotide in Arabidopsis thaliana|journal= eLife |volume= 6 |article-number= e25474|year= 2017 |doi= 10.7554/eLife.25474|pmc= 5560858 |pmid=28722654|doi-access= free}}</ref> Further studies are needed to determine the underlying mechanisms of its extracellular actions and their importance for human health and life processes in other organisms.

==Clinical significance==
The enzymes that make and use NAD{{+}} and NADH are important in both [[pharmacology]] and the research into future treatments for disease.<ref>{{cite journal |author= Sauve AA |s2cid= 875753 |title= NAD<sup>+</sup> and vitamin B3: from metabolism to therapies |journal= The Journal of Pharmacology and Experimental Therapeutics |volume= 324 |issue= 3 |pages= 883–893 |date=March 2008 |pmid= 18165311 |doi= 10.1124/jpet.107.120758}}</ref> [[Drug design]] and drug development exploits NAD{{+}} in three ways: as a direct target of drugs, by designing [[enzyme inhibitor]]s or activators based on its structure that change the activity of NAD-dependent enzymes, and by trying to inhibit NAD{{+}} biosynthesis.<ref>{{cite journal |vauthors=Khan JA, Forouhar F, Tao X, Tong L |s2cid= 6490887 |title= Nicotinamide adenine dinucleotide metabolism as an attractive target for drug discovery |journal= Expert Opin. Ther. Targets |volume= 11 |issue= 5 |pages= 695–705 |year= 2007 |pmid= 17465726 |doi= 10.1517/14728222.11.5.695}}</ref>

Because cancer cells utilize increased [[glycolysis]], and because NAD enhances glycolysis, nicotinamide phosphoribosyltransferase (NAD salvage pathway) is often amplified in cancer cells.<ref name="pmid30631755">{{cite journal | vauthors = Yaku K, Okabe K, Hikosaka K, Nakagawa T | title = NAD Metabolism in Cancer Therapeutics | journal = [[Frontiers in Microbiology]] | volume = 8  | article-number = 622 | date=2018 | doi = 10.3389/fonc.2018.00622 | pmc=6315198 | pmid = 30631755| doi-access = free }}</ref><ref name="pmid32111066">{{cite journal | vauthors = Pramono AA, Rather GM, Herman H | title = NAD- and NADPH-Contributing Enzymes as Therapeutic Targets in Cancer: An Overview | journal = [[Biomolecules (journal)|Biomolecules]] | volume = 10 | issue=3 | page = 358 | date=2020 | doi = 10.3390/biom10030358 | pmc=7175141 | pmid = 32111066| doi-access = free }}</ref>

It has been studied for its potential use in the therapy of [[neurodegenerative disease]]s such as [[Alzheimer's]] and [[Parkinson's disease]] as well as [[multiple sclerosis]].<ref name=Belenky/><ref name=":1" /><ref>{{Cite journal |last1=Penberthy |first1=W. Todd |last2=Tsunoda |first2=Ikuo |date=2009 |title=The importance of NAD in multiple sclerosis |journal=Current Pharmaceutical Design |volume=15 |issue=1 |pages=64–99 |doi=10.2174/138161209787185751 |issn=1873-4286 |pmc=2651433 |pmid=19149604}}</ref><ref name=":0" /> A placebo-controlled clinical trial of NADH (which excluded NADH precursors) in people with Parkinson's failed to show any effect.<ref>{{cite journal |author= Swerdlow RH |s2cid= 10683162 |title= Is NADH effective in the treatment of Parkinson's disease? |journal= Drugs Aging |volume= 13 |issue= 4 |pages= 263–268 |year= 1998 |pmid= 9805207 |doi= 10.2165/00002512-199813040-00002}}</ref>

NAD{{+}} is also a direct [[drug target|target]] of the drug [[isoniazid]], which is used in the treatment of [[tuberculosis]], an infection caused by ''[[Mycobacterium tuberculosis]]''. Isoniazid is a [[prodrug]] and once it has entered the bacteria, it is activated by a [[peroxidase]] enzyme, which oxidizes the compound into a [[free radical]] form.<ref>{{cite journal |vauthors=Timmins GS, Deretic V |title= Mechanisms of action of isoniazid |journal= Mol. Microbiol. |volume= 62 |issue= 5 |pages= 1220–1227 |year= 2006 |pmid= 17074073 |doi= 10.1111/j.1365-2958.2006.05467.x|s2cid= 43379861 |doi-access= free }}</ref> This radical then reacts with NADH, to produce adducts that are very potent inhibitors of the enzymes [[enoyl-acyl carrier protein reductase]],<ref>{{cite journal |vauthors=Rawat R, Whitty A, Tonge PJ |title= The isoniazid-NAD adduct is a slow, tight-binding inhibitor of InhA, the Mycobacterium tuberculosis enoyl reductase: Adduct affinity and drug resistance |journal= Proc. Natl. Acad. Sci. U.S.A. |volume= 100 |issue= 24 |pages= 13881–13886 |year= 2003 |pmid= 14623976 |pmc= 283515 |doi= 10.1073/pnas.2235848100 |bibcode= 2003PNAS..10013881R|doi-access= free }}</ref> and [[dihydrofolate reductase]].<ref>{{cite journal |vauthors=Argyrou A, Vetting MW, Aladegbami B, Blanchard JS |s2cid= 7721666 |title= Mycobacterium tuberculosis dihydrofolate reductase is a target for isoniazid |journal= Nat. Struct. Mol. Biol. |volume= 13 |issue= 5 |pages= 408–413 |year= 2006 |pmid= 16648861 |doi= 10.1038/nsmb1089}}</ref>

Since many oxidoreductases use NAD{{+}} and NADH as substrates, and bind them using a highly conserved structural motif, the idea that inhibitors based on NAD{{+}} could be specific to one enzyme is surprising.<ref name="Pankiewicz">{{cite journal |vauthors=Pankiewicz KW, Patterson SE, Black PL, Jayaram HN, Risal D, Goldstein BM, Stuyver LJ, Schinazi RF |title= Cofactor mimics as selective inhibitors of NAD-dependent inosine monophosphate dehydrogenase (IMPDH){{snd}}the major therapeutic target |journal= Curr. Med. Chem. |volume= 11 |issue= 7 |pages= 887–900 |year= 2004 |pmid= 15083807 |doi= 10.2174/0929867043455648}}</ref> However, this can be possible: for example, inhibitors based on the compounds [[mycophenolic acid]] and [[tiazofurin]] inhibit [[IMP dehydrogenase]] at the NAD{{+}} binding site. Because of the importance of this enzyme in [[purine metabolism]], these compounds may be useful as anti-cancer, anti-viral, or [[immunosuppressive drug]]s.<ref name="Pankiewicz"/><ref>{{cite journal |vauthors=Franchetti P, Grifantini M |title= Nucleoside and non-nucleoside IMP dehydrogenase inhibitors as antitumor and antiviral agents |journal= Curr. Med. Chem. |volume= 6 |issue= 7 |pages= 599–614 |year= 1999 |doi= 10.2174/092986730607220401123801 |pmid= 10390603|s2cid= 247868867 }}</ref> Other drugs are not enzyme inhibitors, but instead activate enzymes involved in NAD{{+}} metabolism. [[Sirtuin]]s are a particularly interesting target for such drugs, since activation of these NAD-dependent deacetylases extends lifespan in some animal models.<ref name="Kim">{{cite journal |vauthors=Kim EJ, Um SJ |title= SIRT1: roles in aging and cancer |journal= BMB Rep |volume= 41 |issue= 11 |pages= 751–756 |year= 2008 |pmid= 19017485 |doi= 10.5483/BMBRep.2008.41.11.751|doi-access= free }}</ref> Compounds such as [[resveratrol]] increase the activity of these enzymes, which may be important in their ability to delay aging in both vertebrate,<ref>{{cite journal |vauthors=Valenzano DR, Terzibasi E, Genade T, Cattaneo A, Domenici L, Cellerino A |s2cid= 1662390 |title= Resveratrol prolongs lifespan and retards the onset of age-related markers in a short-lived vertebrate |journal= Curr. Biol. |volume= 16 |issue= 3 |pages= 296–300 |year= 2006 |pmid= 16461283 |doi= 10.1016/j.cub.2005.12.038|doi-access= free |bibcode= 2006CBio...16..296V |hdl= 11384/14713 |hdl-access= free }}</ref> and invertebrate [[model organism]]s.<ref>{{cite journal |vauthors=Howitz KT, Bitterman KJ, Cohen HY, Lamming DW, Lavu S, Wood JG, Zipkin RE, Chung P, Kisielewski A, Zhang LL, Scherer B, Sinclair DA |s2cid= 4395572 |title= Small molecule activators of sirtuins extend ''Saccharomyces cerevisiae'' lifespan |journal= Nature |volume= 425 |issue= 6954 |pages= 191–196 |year= 2003 |pmid= 12939617 |doi= 10.1038/nature01960 |bibcode= 2003Natur.425..191H}}</ref><ref>{{cite journal |vauthors=Wood JG, Rogina B, Lavu S, Howitz K, Helfand SL, Tatar M, Sinclair D |s2cid= 52851999 |title= Sirtuin activators mimic caloric restriction and delay ageing in metazoans |journal= Nature |volume= 430 |issue= 7000 |pages= 686–689 |year= 2004 |pmid= 15254550 |doi= 10.1038/nature02789 |bibcode= 2004Natur.430..686W}}</ref> In one experiment, mice given NAD for one week had improved nuclear-mitochrondrial communication.<ref>{{cite journal |vauthors=Gomes AP, Price NL, Ling AJ, Moslehi JJ, Montgomery MK, Rajman L, White JP, Teodoro JS, Wrann CD, Hubbard BP, Mercken EM, Palmeira CM, de Cabo R, Rolo AP, Turner N, Bell EL, Sinclair DA |title= Declining NAD+ Induces a Pseudohypoxic State Disrupting Nuclear-Mitochondrial Communication during Aging |journal= Cell |volume= 155 |issue= 7 |pages= 1624–1638 |date= 19 December 2013 |pmid= 24360282 |doi= 10.1016/j.cell.2013.11.037 |pmc=4076149}}</ref>

Because of the differences in the [[metabolic pathway]]s of NAD{{+}} biosynthesis between organisms, such as between bacteria and humans, this area of metabolism is a promising area for the development of new [[antibiotic]]s.<ref>{{cite journal |vauthors=Rizzi M, Schindelin H |title= Structural biology of enzymes involved in NAD and molybdenum cofactor biosynthesis |journal= Curr. Opin. Struct. Biol. |volume= 12 |issue= 6 |pages= 709–720 |year= 2002 |pmid= 12504674 |doi= 10.1016/S0959-440X(02)00385-8}}</ref><ref>{{cite book |doi=10.1016/S0083-6729(01)61003-3 |chapter=The biosynthesis of nicotinamide adenine dinucleotides in bacteria |title=Cofactor Biosynthesis |series=Vitamins & Hormones |year=2001 |last1=Begley |first1=Tadhg P. |last2=Kinsland |first2=Cynthia |last3=Mehl |first3=Ryan A. |last4=Osterman |first4=Andrei |last5=Dorrestein |first5=Pieter |volume=61 |pages=103–119 |pmid=11153263 |isbn=978-0-12-709861-6 }}</ref> For example, the enzyme [[nicotinamidase]], which converts nicotinamide to nicotinic acid, is a target for drug design because this enzyme is absent in humans but present in yeast and bacteria.<ref name=Rongvaux/>

In bacteriology, NAD, sometimes referred to factor V, is used as a supplement to culture media for some [[fastidious organism|fastidious]] bacteria.<ref>{{Cite web|url=https://www.cdc.gov/meningitis/lab-manual/chpt09-id-characterization-hi.html|title=Meningitis Lab Manual: ID and Characterization of Hib|publisher=CDC|date=30 March 2021}}</ref>

High-cost unlicensed infusions of NAD+ have been claimed in the UK to be  "clinically proven" and "effective" treatment for [[alcoholism]] and [[drug abuse]]. NAD+ is not approved or [[Medical license|licensed for medical use]] in the UK; there are likely breaches [[Advertising Standards Authority (United Kingdom)|of advertising]] and medicines rules, and no proof that treatments work. Medical experts say "It's complete nonsense" ... "It's untested and unproven. We don't know anything about its efficacy or long-term safety". A November 2024 study, cited 700 times, claiming that NAD+ levels in [[lab rat]]s decreased with age was withdrawn after images were found to have been manipulated, and underlying data was not provided to the publishers on request.<ref>{{cite news| last=Das | first=Shanti | title='It's not ethical and it's not medical': how UK rehab clinics are cashing in on NAD+ |newspaper=The Observer| date=23 February 2025 | url=https://www.theguardian.com/society/2025/feb/23/its-not-ethical-and-its-not-medical-how-uk-rehab-clinics-are-cashing-in-on-nad }}</ref>

==History==
[[File:ArthurHarden.jpg|thumb|[[Arthur Harden]], co-discoverer of NAD]]
{{Further|History of biochemistry}}

The coenzyme NAD{{+}} was first discovered by the British biochemists [[Arthur Harden]] and [[William John Young (biochemist)|William John Young]] in 1906.<ref>{{cite journal |first1= A |last1= Harden |last2= Young |first2=WJ |title= The alcoholic ferment of yeast-juice Part II.--The coferment of yeast-juice |journal= Proceedings of the Royal Society of London |series= Series B, Containing Papers of a Biological Character |volume= 78 |date= 24 October 1906 |pages= 369–375 |issue= 526 |jstor=80144 |doi=10.1098/rspb.1906.0070|doi-access= free }}</ref> They noticed that adding boiled and filtered [[yeast]] extract greatly accelerated [[alcoholic fermentation]] in unboiled yeast extracts. They called the unidentified factor responsible for this effect a ''coferment''. Through a long and difficult purification from yeast extracts, this heat-stable factor was identified as a [[nucleotide]] sugar phosphate by [[Hans von Euler-Chelpin]].<ref>{{cite web |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1929/euler-chelpin-lecture.pdf |title=Fermentation of sugars and fermentative enzymes |work=Nobel Lecture, 23 May 1930 |access-date=2007-09-30 |publisher=Nobel Foundation |archive-url=https://web.archive.org/web/20070927170330/http://nobelprize.org/nobel_prizes/chemistry/laureates/1929/euler-chelpin-lecture.pdf |archive-date=27 September 2007 }}</ref> In 1936, the German scientist [[Otto Heinrich Warburg]] showed the function of the nucleotide coenzyme in hydride transfer and identified the nicotinamide portion as the site of redox reactions.<ref>{{cite journal |vauthors=Warburg O, Christian W |title=Pyridin, der wasserstoffübertragende bestandteil von gärungsfermenten (pyridin-nucleotide) |language=de |trans-title=Pyridin, the hydrogen-transferring component of the fermentation enzymes (pyridine nucleotide) |journal=Biochemische Zeitschrift |volume=287 |year=1936 |page=291 |doi=10.1002/hlca.193601901199}}</ref>

Vitamin precursors of NAD{{+}} were first identified in 1938, when [[Conrad Elvehjem]] showed that liver has an "anti-black tongue" activity in the form of nicotinamide.<ref>{{cite journal |vauthors=Elvehjem CA, Madden RJ, Strong FM, Woolley DW |title=The isolation and identification of the anti-black tongue factor |journal=J. Biol. Chem. |volume=123 |issue=1 |pages=137–49 |year=1938 |doi=10.1016/S0021-9258(18)74164-1 |doi-access=free }}</ref> Then, in 1939, he provided the first strong evidence that [[nicotinic acid]] is used to synthesize NAD{{+}}.<ref>{{cite journal|vauthors=Axelrod AE, Madden RJ, Elvehjem CA |title=The effect of a nicotinic acid deficiency upon the coenzyme I content of animal tissues |journal=J. Biol. Chem. |volume=131 |issue=1 |pages=85–93 |year=1939 |doi=10.1016/S0021-9258(18)73482-0 |doi-access=free }}</ref> In the early 1940s, [[Arthur Kornberg]] was the first to detect an enzyme in the biosynthetic pathway.<ref>{{cite journal |author= Kornberg A |title= The participation of inorganic pyrophosphate in the reversible enzymatic synthesis of diphosphopyridine nucleotide |journal= J. Biol. Chem. |volume= 176 |issue= 3 |pages= 1475–76 |year= 1948 |doi= 10.1016/S0021-9258(18)57167-2 |pmid=18098602 |doi-access= free }}</ref> In 1949, the American biochemists Morris Friedkin and [[Albert L. Lehninger]] proved that NADH linked metabolic pathways such as the [[citric acid cycle]] with the synthesis of ATP in oxidative phosphorylation.<ref>{{cite journal |vauthors=Friedkin M, Lehninger AL |title= Esterification of inorganic phosphate coupled to electron transport between dihydrodiphosphopyridine nucleotide and oxygen |journal= J. Biol. Chem. |volume= 178 |issue= 2 |pages= 611–23 |date= 1 April 1949 |doi= 10.1016/S0021-9258(18)56879-4 |pmid= 18116985 |doi-access= free }}</ref> In 1958, Jack Preiss and Philip Handler discovered the intermediates and enzymes involved in the biosynthesis of NAD{{+}};<ref>{{cite journal |vauthors=Preiss J, Handler P |title= Biosynthesis of diphosphopyridine nucleotide. I. Identification of intermediates |journal= J. Biol. Chem. |volume= 233 |issue= 2 |pages= 488–92 |year= 1958 |doi= 10.1016/S0021-9258(18)64789-1 |pmid= 13563526 |doi-access= free }}</ref><ref>{{cite journal |vauthors=Preiss J, Handler P |title= Biosynthesis of diphosphopyridine nucleotide. II. Enzymatic aspects |journal= J. Biol. Chem. |volume= 233 |issue= 2 |pages= 493–500 |year= 1958 |doi= 10.1016/S0021-9258(18)64790-8 |pmid= 13563527|doi-access= free }}</ref> salvage synthesis from nicotinic acid is termed the Preiss-Handler pathway. In 2004, [[Charles Brenner (biochemist)|Charles Brenner]] and co-workers uncovered the [[nicotinamide riboside]] kinase pathway to NAD{{+}}.<ref name="Bieganowski, P, Brenner, C 2004 495–502">{{cite journal |doi= 10.1016/S0092-8674(04)00416-7 |author= Bieganowski, P |author2= Brenner, C |s2cid= 4642295 |title= Discoveries of Nicotinamide Riboside as a Nutrient and Conserved NRK Genes Establish a Preiss-Handler Independent Route to NAD+ in Fungi and Humans |journal= Cell |volume= 117 |pages= 495&ndash;502 |year= 2004 |pmid= 15137942 |issue= 4|doi-access= free }}</ref>

The non-redox roles of NAD(P) were discovered later.<ref name=Pollak/> The first to be identified was the use of NAD{{+}} as the ADP-ribose donor in ADP-ribosylation reactions, observed in the early 1960s.<ref>{{cite journal |vauthors=Chambon P, Weill JD, Mandel P |title= Nicotinamide mononucleotide activation of new DNA-dependent polyadenylic acid synthesizing nuclear enzyme |journal= Biochem. Biophys. Res. Commun. |volume= 11 |pages= 39–43 |year= 1963 |issue= 1 |pmid= 14019961 |doi= 10.1016/0006-291X(63)90024-X |bibcode= 1963BBRC...11...39C }}</ref> Studies in the 1980s and 1990s revealed the activities of NAD{{+}} and NADP{{+}} metabolites in cell signaling&nbsp;– such as the action of [[cyclic ADP-ribose]], which was discovered in 1987.<ref>{{cite journal |vauthors=Clapper DL, Walseth TF, Dargie PJ, Lee HC |title= Pyridine nucleotide metabolites stimulate calcium release from sea urchin egg microsomes desensitized to inositol trisphosphate |journal= J. Biol. Chem. |volume= 262 |issue= 20 |pages= 9561–8 |date= 15 July 1987 |doi= 10.1016/S0021-9258(18)47970-7 |pmid= 3496336 |doi-access= free }}</ref>

The metabolism of NAD{{+}} remained an area of intense research into the 21st century, with interest heightened after the discovery of the NAD{{+}}-dependent protein deacetylases called [[sirtuin]]s in 2000, by Shin-ichiro Imai and coworkers in the laboratory of [[Leonard P. Guarente]].<ref>{{cite journal |vauthors=Imai S, Armstrong CM, Kaeberlein M, Guarente L |s2cid= 2967911 |title= Transcriptional silencing and longevity protein Sir2 is an NAD-dependent histone deacetylase |journal= Nature |volume= 403 |issue= 6771 |pages= 795–800 |year= 2000 |pmid= 10693811 |doi= 10.1038/35001622 |bibcode= 2000Natur.403..795I}}</ref> In 2009 Imai proposed the "NAD World" hypothesis that key regulators of aging and longevity in mammals are [[sirtuin 1|sirtuin&nbsp;1]] and the primary NAD{{+}} synthesizing enzyme [[nicotinamide phosphoribosyltransferase]] (NAMPT).<ref name="pmid19130305">{{cite journal | author = Imai S  | title = The NAD World: a new systemic regulatory network for metabolism and aging--Sirt1, systemic NAD biosynthesis, and their importance | journal = [[Cell Biochemistry and Biophysics]] | volume = 53 | issue = 2 | pages = 65–74| date=2009 | doi = 10.1007/s12013-008-9041-4 | pmc=2734380 | pmid = 19130305}}</ref> In 2016 Imai expanded his hypothesis to "NAD World 2.0", which postulates that extracellular NAMPT from [[adipose tissue]] maintains NAD{{+}} in the [[hypothalamus]] (the control center) in conjunction with [[myokine]]s from [[skeletal muscle]] cells.<ref name="pmid28725474">{{cite journal | author = Imai S  | title = The NAD World 2.0: the importance of the inter-tissue communication mediated by NAMPT/NAD +/SIRT1 in mammalian aging and longevity control | journal = [[List of Nature Research journals#N|npj Systems Biology and Applications]] | volume = 2 | article-number = 16018 | date=2016 | doi = 10.1038/npjsba.2016.18 | pmc=5516857 | pmid = 28725474}}</ref> In 2018, Napa Therapeutics was formed to develop drugs against a novel aging-related target based on the research in NAD metabolism conducted in the lab of [[Eric Verdin]].<ref>{{Cite web |date=2018-08-17 |title=Napa Therapeutics Formed to Develop Drugs to Influence NAD Metabolism |url=https://www.fightaging.org/archives/2018/08/napa-therapeutics-formed-to-develop-drugs-to-influence-nad-metabolism/ |access-date=2023-11-29 |website=Fight Aging! |language=en-US}}</ref>

==See also==
*[[Enzyme catalysis]]
*[[List of EC numbers (EC 1)|List of oxidoreductases]]

==References==
{{Reflist|colwidth=30em}}

==Further reading==

===Function===
*{{cite book|author=Nelson DL|author2=Cox MM|title=Lehninger Principles of Biochemistry|edition=4th|publisher=W. H. Freeman|year=2004|isbn=978-0-7167-4339-2|url=https://archive.org/details/lehningerprincip00lehn_0}}
*{{cite book|author=Bugg T |title=Introduction to Enzyme and Coenzyme Chemistry |year=2004 |publisher=Blackwell Publishing Limited |edition=2nd |isbn=978-1-4051-1452-3}}
*{{cite book|author=Lee HC |title=Cyclic ADP-Ribose and NAADP: Structure, Metabolism and Functions |year=2002 |publisher=Kluwer Academic Publishers |isbn=978-1-4020-7281-9}}
*{{cite web |vauthors=Levine OS, Schuchat A, Schwartz B, Wenger JD, Elliott J |agency= Centers for Disease Control |title= Generic protocol for population-based surveillance of Haemophilus influenzae type B |year= 1997 |publisher= World Health Organization |id= WHO/VRD/GEN/95.05 |url=https://www.who.int/vaccine_research/documents/en/hinfluenzaeb_surveillance.pdf |archive-url=https://web.archive.org/web/20040701215032/http://www.who.int/vaccine_research/documents/en/hinfluenzaeb_surveillance.pdf |archive-date=1 July 2004 |page=13}}
*{{cite journal |last1=Kim |first1=Jinhyun |last2=Lee |first2=Sahng Ha |last3=Tieves |first3=Florian |last4=Paul |first4=Caroline E. |last5=Hollmann |first5=Frank |last6=Park |first6=Chan Beum |title=Nicotinamide adenine dinucleotide as a photocatalyst |journal=Science Advances |date=5 July 2019 |volume=5 |issue=7 |article-number=eaax0501 |doi=10.1126/sciadv.aax0501|pmid=31334353 |pmc=6641943 |bibcode=2019SciA....5..501K }}

===History===
*{{cite book |author-link1=Athel Cornish-Bowden |last1=Cornish-Bowden |first1=Athel |title=New Beer in an Old Bottle. Eduard Buchner and the Growth of Biochemical Knowledge. |publisher=Universitat de Valencia |location=Valencia |year=1997 |isbn=978-84-370-3328-0 |url=http://bip.cnrs-mrs.fr/bip10/buchner.htm |access-date=8 December 2007 |archive-date=13 December 2010 |archive-url=https://web.archive.org/web/20101213084345/http://bip.cnrs-mrs.fr/bip10/buchner.htm }}, A history of early enzymology.
*{{cite book|series=A History of Science: in Five Volumes |author=Williams, Henry Smith |year=1904 |publisher=Harper and Brothers |location=New York |url=http://etext.lib.virginia.edu/toc/modeng/public/Wil4Sci.html |volume=IV |title=Modern Development of the Chemical and Biological Sciences}}, a textbook from the 19th century.

==External links==
{{Commons category}}
*[http://www.ebi.ac.uk/pdbe-srv/PDBeXplore/ligand/?ligand=NAD NAD bound to proteins] in the [[Protein Data Bank]]
*[https://web.archive.org/web/20110707060740/http://www.1lecture.com/Biochemistry/How%20the%20NAD%20Works/index.html NAD Animation] (Flash Required)
*[http://www.sigmaaldrich.com/etc/medialib/docs/Sigma/Product_Information_Sheet/2/n8285pis.Par.0001.File.tmp/n8285pis.pdf β-Nicotinamide adenine dinucleotide (NAD{{+}}, oxidized)] and [http://www.sigmaaldrich.com/etc/medialib/docs/Sigma/Product_Information_Sheet/2/n4505pis.Par.0001.File.tmp/n4505pis.pdf NADH (reduced)] Chemical data sheet from [[Sigma-Aldrich]]
*[http://biocyc.org/META/NEW-IMAGE?type=COMPOUND&object=NAD NAD{{+}}], [http://biocyc.org/META/NEW-IMAGE?type=COMPOUND&object=NADH NADH] and [http://biocyc.org/META/NEW-IMAGE?type=PATHWAY&object=PWY-3502 NAD synthesis pathway] at the [[MetaCyc]] database
*[http://www.expasy.org/enzyme/1.-.-.- List of oxidoreductases] {{Webarchive|url=https://web.archive.org/web/20101130025807/http://www.expasy.org/enzyme/1.-.-.- |date=30 November 2010 }} at the [[Swiss-Prot|SWISS-PROT]] database
* [https://en.longevitywiki.org/wiki/NAD+ NAD<sup>+</sup>]
* [https://taheebo-tea.com/blogs/taheebo-tea-research/taheebo-tea-the-molecule-of-youth NAD<sup>+</sup> The Molecule of Youth]

{{Enzyme cofactors}}

[[Category:Anti-aging substances]]
[[Category:Cellular respiration]]
[[Category:Coenzymes]]
[[Category:Nucleotides]]
[[Category:Photosynthesis]]
[[Category:Pyridinium compounds]]