Nicotinamide adenine dinucleotide

Nicotinamide adenine dinucleotide
Names
Other names
Diphosphopyridine nucleotide (DPN+), Coenzyme I
Identifiers
53-84-9 YesY
58-68-4 (NADH) N
ChEBI CHEBI:16908 N
ChEMBL ChEMBL1628272 N
ChemSpider 5681 YesY
DrugBank DB00157 YesY
2451
Jmol 3D model Interactive image
KEGG C00003 N
PubChem 925
RTECS number UU3450000
UNII 0U46U6E8UK YesY
Properties
C21H27N7O14P2
Molar mass 663.43 g/mol
Appearance White powder
Melting point 160 °C (320 °F; 433 K)
Hazards
Main hazards Not hazardous
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
1
1
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells. The compound is a dinucleotide, because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base and the other nicotinamide. Nicotinamide adenine dinucleotide exists in two forms, an oxidized and reduced form abbreviated as NAD+ and NADH respectively.

In metabolism, nicotinamide adenine dinucleotide is involved in redox reactions, carrying electrons from one reaction to another. The coenzyme is, therefore, found in two forms in cells: NAD+ is an oxidizing agent – it accepts electrons from other molecules and becomes reduced. This reaction forms NADH, which can then be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD. However, it is also used in other cellular processes, the most notable one being a substrate of enzymes that add or remove chemical groups from proteins, in posttranslational modifications. Because of the importance of these functions, the enzymes involved in NAD metabolism are targets for drug discovery.

In organisms, NAD can be synthesized from simple building-blocks (de novo) from the amino acids tryptophan or aspartic acid. In an alternative fashion, more complex components of the coenzymes are taken up from food as the vitamin called niacin. Similar compounds are released by reactions that break down the structure of NAD. These preformed components then pass through a salvage pathway that recycles them back into the active form. Some NAD is also converted into nicotinamide adenine dinucleotide phosphate (NADP); the chemistry of this related coenzyme is similar to that of NAD, but it has different roles in metabolism.

Although NAD+ is written with a superscript plus sign because of the formal charge on a particular nitrogen atom, at physiological pH for the most part it is actually a singly charged anion (charge of minus 1), while NADH is a doubly charged anion.

Physical and chemical properties

Further information: Redox

Nicotinamide adenine dinucleotide, like all dinucleotides, consists of two nucleotides joined by a pair of bridging phosphate groups. The nucleotides consist of ribose rings, one with adenine attached to the first carbon atom (the 1' position) and the other with nicotinamide at this position. The nicotinamide moiety can be attached in two orientations to this anomeric carbon atom. Because of these two possible structures, the compound exists as two diastereomers. It is the β-nicotinamide diastereomer of NAD+ that is found in organisms. These nucleotides are joined together by a bridge of two phosphate groups through the 5' carbons.[1]

The redox reactions of nicotinamide adenine dinucleotide.

In metabolism, the compound accepts or donates electrons in redox reactions.[2] Such reactions (summarized in formula below) involve the removal of two hydrogen atoms from the reactant (R), in the form of a hydride ion (H−), and a proton (H+). The proton is released into solution, while the reductant RH2 is oxidized and NAD+ reduced to NADH by transfer of the hydride to the nicotinamide ring.

RH2 + NAD+ → NADH + H+ + R;

From the hydride electron pair, one electron is transferred to the positively charged nitrogen of the nicotinamide ring of NAD+, and the second hydrogen atom transferred to the C4 carbon atom opposite this nitrogen. The midpoint potential of the NAD+/NADH redox pair is −0.32 volts, which makes NADH a strong reducing agent.[3] 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.[1]

In appearance, all forms of this coenzyme are white amorphous powders that are hygroscopic and highly water-soluble.[4] The solids are stable if stored dry and in the dark. Solutions of NAD+ are colorless and stable for about a week at 4 °C and neutral pH, but decompose rapidly in acids or alkalis. Upon decomposition, they form products that are enzyme inhibitors.[5]

UV absorption spectra of NAD+ and NADH.

Both NAD+ and NADH strongly absorb ultraviolet light because of the adenine. For example, peak absorption of NAD+ is at a wavelength of 259 nanometers (nm), with an extinction coefficient of 16,900 M−1cm−1. NADH also absorbs at higher wavelengths, with a second peak in UV absorption at 339 nm with an extinction coefficient of 6,220 M−1cm−1.[6] 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 assays – by measuring the amount of UV absorption at 340 nm using a spectrophotometer.[6]

NAD+ and NADH also differ in their fluorescence. NADH in solution has an emission peak at 460 nm and a fluorescence lifetime of 0.4 nanoseconds, while the oxidized form of the coenzyme does not fluoresce.[7] The properties of the fluorescence signal changes when NADH binds to proteins, so these changes can be used to measure dissociation constants, which are useful in the study of enzyme kinetics.[7][8] These changes in fluorescence are also used to measure changes in the redox state of living cells, through fluorescence microscopy.[9]

Concentration and state in cells

In rat liver, the total amount of NAD+ and NADH is approximately 1 μmole per gram of wet weight, about 10 times the concentration of NADP+ and NADPH in the same cells.[10] The actual concentration of NAD+ in cell cytosol is harder to measure, with recent estimates in animal cells, ranging around 0.3 mM,[11][12] and approximately 1.0 to 2.0 mM in yeast.[13]

See also

References

  1. 1 2 Pollak N, Dölle C, Ziegler M (2007). "The power to reduce: pyridine nucleotides – small molecules with a multitude of functions". Biochem. J. 402 (2): 205–18. doi:10.1042/BJ20061638. PMC 1798440. PMID 17295611.
  2. ↑ Belenky P, Bogan KL, Brenner C (2007). "NAD+ metabolism in health and disease" (PDF). Trends Biochem. Sci. 32 (1): 12–9. doi:10.1016/j.tibs.2006.11.006. PMID 17161604. Retrieved 2007-12-23.
  3. ↑ Unden G, Bongaerts J (1997). "Alternative respiratory pathways of Escherichia coli: energetics and transcriptional regulation in response to electron acceptors". Biochim. Biophys. Acta 1320 (3): 217–34. doi:10.1016/S0005-2728(97)00034-0. PMID 9230919.
  4. ↑ Windholz, Martha (1983). The Merck Index: an encyclopedia of chemicals, drugs, and biologicals (10th ed.). Rahway NJ, US: Merck. p. 909. ISBN 0-911910-27-1.
  5. ↑ Biellmann JF, Lapinte C, Haid E, Weimann G (1979). "Structure of lactate dehydrogenase inhibitor generated from coenzyme". Biochemistry 18 (7): 1212–7. doi:10.1021/bi00574a015. PMID 218616.
  6. 1 2 Dawson, R. Ben (1985). Data for biochemical research (3rd ed.). Oxford: Clarendon Press. p. 122. ISBN 0-19-855358-7.
  7. 1 2 Lakowicz JR, Szmacinski H, Nowaczyk K, Johnson ML (1992). "Fluorescence lifetime imaging of free and protein-bound NADH". Proc. Natl. Acad. Sci. U.S.A. 89 (4): 1271–5. Bibcode:1992PNAS...89.1271L. doi:10.1073/pnas.89.4.1271. PMC 48431. PMID 1741380.
  8. ↑ Jameson DM, Thomas V, Zhou DM (1989). "Time-resolved fluorescence studies on NADH bound to mitochondrial malate dehydrogenase". Biochim. Biophys. Acta 994 (2): 187–90. doi:10.1016/0167-4838(89)90159-3. PMID 2910350.
  9. ↑ Kasimova MR, Grigiene J, Krab K, Hagedorn PH, Flyvbjerg H, Andersen PE, Møller IM (2006). "The Free NADH Concentration Is Kept Constant in Plant Mitochondria under Different Metabolic Conditions". Plant Cell 18 (3): 688–98. doi:10.1105/tpc.105.039354. PMC 1383643. PMID 16461578.
  10. ↑ Reiss PD, Zuurendonk PF, Veech RL (1984). "Measurement of tissue purine, pyrimidine, and other nucleotides by radial compression high-performance liquid chromatography". Anal. Biochem. 140 (1): 162–71. doi:10.1016/0003-2697(84)90148-9. PMID 6486402.
  11. ↑ Yamada K, Hara N, Shibata T, Osago H, Tsuchiya M (2006). "The simultaneous measurement of nicotinamide adenine dinucleotide and related compounds by liquid chromatography/electrospray ionization tandem mass spectrometry". Anal. Biochem. 352 (2): 282–5. doi:10.1016/j.ab.2006.02.017. PMID 16574057.
  12. ↑ 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 (2007). "Nutrient-Sensitive Mitochondrial NAD+ Levels Dictate Cell Survival". Cell 130 (6): 1095–107. doi:10.1016/j.cell.2007.07.035. PMC 3366687. PMID 17889652.
  13. ↑ Belenky P, Racette FG, Bogan KL, McClure JM, Smith JS, Brenner C (2007). "Nicotinamide riboside promotes Sir2 silencing and extends lifespan via Nrk and Urh1/Pnp1/Meu1 pathways to NAD+". Cell 129 (3): 473–84. doi:10.1016/j.cell.2007.03.024. PMID 17482543.</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 | author = 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> Data for other compartments in the cell are limited, although, in the mitochondrion the concentration of NAD+ is similar to that in the cytosol.<ref name=Yang/> This NAD+ is carried into the mitochondrion by a specific membrane transport protein, since the coenzyme cannot diffuse across membranes.<ref>{{cite journal | author = Todisco S, Agrimi G, Castegna A, Palmieri F | title = Identification of the mitochondrial NAD{{+}} transporter in ''Saccharomyces cerevisiae'' | journal = J. Biol. Chem. | volume = 281 | issue = 3 | pages = 1524–31 | year = 2006 | pmid = 16291748 | doi = 10.1074/jbc.M510425200 }}</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 | author = 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 between free NAD+ and NADH in the cytoplasm typically lie around 700; the ratio is thus favourable for oxidative reactions.<ref name=Williamson>{{cite journal | author = 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 }}</ref><ref name=Zhang>{{cite journal | author = Zhang Q, Piston DW, Goodman RH | 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 }}</ref> The ratio of total NAD+/NADH is much lower, with estimates ranging from 3–10 in mammals.<ref>{{cite journal | author = 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 NADP+/NADPH ratio is normally about 0.005, so NADPH is the dominant form of this coenzyme.<ref>{{cite journal | author = 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 }}</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 pathway from amino acids or in salvage pathways by recycling preformed components such as nicotinamide back to NAD+.

    De novo production

    Some metabolic pathways that synthesize and consume NAD+ in vertebrates. 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—either tryptophan (Trp) in animals and some bacteria, or aspartic acid in some bacteria and plants.<ref>{{cite journal | author = 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 = 851–7 | year = 2006 | pmid = 16698895 | pmc = 1489895 | doi = 10.1104/pp.106.081091 }}</ref><ref>{{cite journal | author = 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 | 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 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, which phosphorylates NAD+.<ref>{{cite journal | author = 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 = 739–46 | year = 2006 | pmid = 16842123 | doi = 10.2174/138955706777698688 }}</ref> In most organisms, this enzyme uses ATP as the source of the phosphate group, although several bacteria such as Mycobacterium tuberculosis and a hyperthermophilic archaeon Pyrococcus horikoshii, use inorganic polyphosphate as an alternative phosphoryl donor.<ref>{{cite journal | author = 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 = 4352–8 | year = 2005 | pmid = 16085824 | pmc = 1183369 | doi = 10.1128/AEM.71.8.4352-4358.2005 }}</ref><ref>{{cite journal | author = 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 = 7610–7 | year = 2004 | pmid = 15182203 | doi = 10.1021/bi049650w }}</ref>
    Salvage pathways use three precursors for NAD+.

    Salvage pathways

    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 B3 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 | author = Anderson RM, Bitterman KJ, Wood JG, Medvedik O, Cohen H, Lin SS, Manchester JK, Gordon JI, Sinclair DA | title = Manipulation of a nuclear NAD{{+}} salvage pathway delays aging without altering steady-state NAD{{+}} levels | journal = J. Biol. Chem. | volume = 277 | issue = 21 | pages = 18881–90 | year = 2002 | pmid = 11884393 | doi = 10.1074/jbc.M111773200 }}</ref> There are some reports that mammalian cells can take up extracellular NAD+ from their surroundings.<ref>{{cite journal | author = 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 = 6367–74 | year = 2008 | pmid = 18180302 | doi = 10.1074/jbc.M706204200 }}</ref> Despite the presence of the de novo pathway, the salvage reactions are essential in humans; a lack of niacin 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 salvage pathways used in microorganisms differ from those of mammals.<ref name=Rongvaux>{{cite journal | author = Rongvaux A, Andris F, Van Gool F, Leo O | title = Reconstructing eukaryotic NAD metabolism | journal = BioEssays | volume = 25 | issue = 7 | pages = 683–90 | 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+ auxotrophs – they cannot synthesize NAD+ – but possess salvage pathways and thus are dependent on external sources of NAD+ or its precursors.<ref>{{cite journal | author = Ma B, Pan SJ, Zupancic ML, Cormack BP | title = Assimilation of NAD{{+}} 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 }}</ref><ref>{{cite journal | author = 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 = 1573–81 | year = 2000 | pmid = 10760156 | doi = 10.1046/j.1365-2958.2000.01829.x }}</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.<ref>{{cite journal | author = 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 = 4555–72 | year = 2002 | pmid = 12142426 | pmc = 135229 | doi = 10.1128/JB.184.16.4555-4572.2002 }}</ref>

    Functions

    Rossmann fold in part of the lactate dehydrogenase of Cryptosporidium parvum, showing NAD+ in red, beta sheets in yellow, and alpha helices in purple.[14]
    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 ligases and a group of enzymes called sirtuins that use NAD+ to remove acetyl groups from proteins. In addition to these metabolic functions, NAD+ emerges as an adenine nucleotide that can be released from cells spontaneously and by regulated mechanisms,<ref name=Smyth>{{cite journal | author = 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 }}</ref><ref name=Billington>{{cite journal | author = 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 information: Protein structure and 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 oxidoreductases. The correct names for these enzymes contain the names of both their substrates: for example 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 | accessdate = 2007-12-06}}</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=http://www.expasy.org/enzyme/1.6.5.3 |title=NiceZyme View of ENZYME: EC 1.6.5.3 |accessdate=2007-12-16 |publisher=Expasy}}</ref> When bound to a protein, NAD+ and NADH are usually held within a structural motif known as the Rossmann fold.<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 | author = 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> This fold contains three or more parallel beta strands linked by two alpha helices in the order beta-alpha-beta-alpha-beta. This forms a beta sheet flanked by a layer of alpha helices on each side. Because each Rossmann fold binds one nucleotide, binding domains for NAD+ consist of two paired Rossmann folds, with each fold binding one of the two nucleotides of the cofactor.<ref name=Rao/> However, this fold is not universal among NAD-dependent enzymes, since a class of bacterial enzymes involved in amino acid metabolism have recently been discovered that bind the coenzyme, but lack this motif.<ref>{{cite journal | author = 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 | url = http://www.jbc.org/cgi/content/full/280/49/40875 }}</ref>
    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.[15]
    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 the 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 stereoisomers 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 }}</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 | author = 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 }}</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 | author = 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 }}</ref>

    Role in redox metabolism

    A simplified outline of redox metabolism, showing how NAD+ and NADH link the citric acid cycle and 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 acids 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 eukaryotes the electrons carried by the NADH that is produced in the cytoplasm are transferred into the mitochondrion (to reduce mitochondrial NAD+) by mitochondrial shuttles, such as the malate-aspartate shuttle.<ref>{{cite journal | author = 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 }}</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 | author = Rich PR | title = The molecular machinery of Keilin's respiratory chain | journal = Biochem. Soc. Trans. | volume = 31 | issue = Pt 6 | pages = 1095–105 | year = 2003 | pmid = 14641005 | doi = 10.1042/BST0311095 }}</ref> These shuttle systems also have the same transport function in chloroplasts.<ref>{{cite journal | author = 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=0-12-518121-3}}</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 | author = Sistare FD, Haynes RC | 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 = J. Biol. Chem. | volume = 260 | issue = 23 | pages = 12748–53 | date = 15 October 1985 | pmid = 4044607 | url = http://www.jbc.org/cgi/reprint/260/23/12748 }}</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, 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|author=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}}</ref> As NADH is still needed for anabolic reactions, these bacteria use a nitrite oxidoreductase to produce enough proton-motive force to run part of the electron transport chain in reverse, generating NADH.<ref>{{cite journal | author = 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 | url = http://aem.asm.org/cgi/content/full/72/3/2050?view=long&pmid=16517654 }}</ref>

    Non-redox roles

    The coenzyme NAD+ is also consumed in ADP-ribose transfer reactions. For example, enzymes called 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 }}</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 | author = 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 }}</ref> Mono-ADP-ribosylation was first identified as the mechanism of a group of bacterial toxins, notably cholera toxin, but it is also involved in normal cell signaling.<ref>{{cite journal | author = 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 }}</ref><ref>{{cite journal | author = 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) polymerases.<ref name="Diefenbach"/><ref name=Burkle>{{cite journal | author = Bürkle A | title = Poly(ADP-ribose). The most elaborate metabolite of NAD{{+}} | journal = FEBS J. | volume = 272 | issue = 18 | pages = 4576–89 | year = 2005 | pmid = 16156780 | doi = 10.1111/j.1742-4658.2005.04864.x }}</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 | author = 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 | author = 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>
    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 receptors, which are located in the membranes of organelles, such as the endoplasmic reticulum.<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 }}</ref> NAD+ is also consumed by sirtuins, which are NAD-dependent deacetylases, such as Sir2.<ref>{{cite journal | author = 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 }}</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 through deacetylating histones and altering nucleosome structure.<ref>{{cite journal | author = Blander G, Guarente L | title = The Sir2 family of protein deacetylases | journal = Annu. Rev. Biochem. | volume = 73 | pages = 417–35 | year = 2004 | pmid = 15189148 | doi = 10.1146/annurev.biochem.73.011303.073651 }}</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 | author = 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> Other NAD-dependent enzymes include bacterial DNA ligases, 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 | author = 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 }}</ref> This contrasts with eukaryotic DNA ligases, which use ATP to form the DNA-AMP intermediate.<ref>{{cite journal | author = 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>

    Extracellular actions of NAD+

    In recent years, NAD+ has also been recognized as an extracellular signaling molecule involved in cell-to-cell communication.<ref name=Billington/><ref name=Ziegler>{{cite journal | author = Ziegler M, Niere M | title = NAD{{+}} surfaces again | journal = Biochem. J. | volume = 382 | issue = Pt 3 | pages = e5–6 | year = 2004 | pmid = 15352307 | pmc = 1133982 | doi = 10.1042/BJ20041217 }}</ref><ref name=Koch-Nolte>{{cite journal | author = Koch-Nolte F, Fischer S, Haag F, Ziegler M | title = Compartmentation of NAD{{+}}-dependent signalling | journal = FEBS Lett. | volume = 585 | issue = 11 | pages = 1651–6 | year = 2011 | pmid = 21443875 | doi = 10.1016/j.febslet.2011.03.045 }}</ref> NAD+ is released from neurons in blood vessels,<ref name=Smyth/> urinary bladder,<ref name=Smyth/><ref name=Breen>{{cite journal | author = Breen LT, Smyth LM, Yamboliev IA, Mutafova-Yambolieva VN | title = beta-NAD is a novel nucleotide released on stimulation of nerve terminals in human urinary bladder detrusor muscle | journal = Am. J. Physiol. Renal Physiol. | volume = 290 | issue = 2 | pages = F486–95 | year = 2006 | pmid = 16189287 | doi = 10.1152/ajprenal.00314.2005 }}</ref> large intestine,<ref name=Mutafova-Yambolieva>{{cite journal | author = 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 }}</ref><ref name=Hwang>{{cite journal | author = 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 | author = 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 synaptosomes,<ref name=Durnin>{{cite journal | author = 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{{+}}) 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 nerves to effector cells in smooth muscle organs.<ref name=Mutafova-Yambolieva/><ref name=Hwang/> Further studies are needed to determine the underlying mechanisms of its extracellular actions and their importance for human health and diseases.

    Pharmacology and medical uses

    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 | title = NAD{{+}} and vitamin B3: from metabolism to therapies | journal = The Journal of Pharmacology and Experimental Therapeutics | volume = 324 | issue = 3 | pages = 883–93 |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 inhibitors or activators based on its structure that change the activity of NAD-dependent enzymes, and by trying to inhibit NAD+ biosynthesis.<ref>{{cite journal | author = Khan JA, Forouhar F, Tao X, Tong L | 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> The coenzyme NAD+ is not itself currently used as a treatment for any disease. However, it is being studied for its potential use in the therapy of neurodegenerative diseases such as Alzheimer's and Parkinson disease.<ref name=Belenky/> Evidence on the benefit of NAD+ in neurodegeneration is mixed; some studies in mice have produced promising results<ref>{{cite journal | author = Kaneko S, Wang J, Kaneko M, Yiu G, Hurrell JM, Chitnis T, Khoury SJ, He Z | title = Protecting axonal degeneration by increasing nicotinamide adenine dinucleotide levels in experimental autoimmune encephalomyelitis models | journal = J. Neurosci. | volume = 26 | issue = 38 | pages = 9794–804 | year = 2006 | pmid = 16988050 | doi = 10.1523/JNEUROSCI.2116-06.2006 }}</ref> whereas a placebo-controlled clinical trial in humans failed to show any effect.<ref>{{cite journal | author = Swerdlow RH | title = Is NADH effective in the treatment of Parkinson's disease? | journal = Drugs Aging | volume = 13 | issue = 4 | pages = 263–8 | year = 1998 | pmid = 9805207 | doi = 10.2165/00002512-199813040-00002 }}</ref> NAD+ is also a direct 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 | author = Timmins GS, Deretic V | title = Mechanisms of action of isoniazid | journal = Mol. Microbiol. | volume = 62 | issue = 5 | pages = 1220–7 | year = 2006 | pmid = 17074073 | doi = 10.1111/j.1365-2958.2006.05467.x }}</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 | author = 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 = 13881–6 | year = 2003 | pmid = 14623976 | pmc = 283515 | doi = 10.1073/pnas.2235848100 | bibcode = 2003PNAS..10013881R }}</ref> and dihydrofolate reductase.<ref>{{cite journal | author = Argyrou A, Vetting MW, Aladegbami B, Blanchard JS | title = Mycobacterium tuberculosis dihydrofolate reductase is a target for isoniazid | journal = Nat. Struct. Mol. Biol. | volume = 13 | issue = 5 | pages = 408–13 | year = 2006 | pmid = 16648861 | doi = 10.1038/nsmb1089 }}</ref> In one experiment, mice given NAD for one week had improved nuclear-mitochrondrial communication.<ref>{{cite journal | author = 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 = December 19, 2013 | pmid = 24360282 | doi = 10.1016/j.cell.2013.11.037 }}</ref> Since a large number of 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 | author = 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)—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 drugs.<ref name=Pankiewicz/><ref>{{cite journal | author = 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 | pmid = 10390603 }}</ref> Other drugs are not enzyme inhibitors, but instead activate enzymes involved in NAD+ metabolism. Sirtuins are a particularly interesting target for such drugs, since activation of these NAD-dependent deacetylases extends lifespan.<ref name=Kim>{{cite journal | author = Kim EJ, Um SJ | title = SIRT1: roles in aging and cancer | journal = BMB Rep | volume = 41 | issue = 11 | pages = 751–6 | year = 2008 | pmid = 19017485 | doi = 10.5483/BMBRep.2008.41.11.751 }}</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 | author = Valenzano DR, Terzibasi E, Genade T, Cattaneo A, Domenici L, Cellerino A | 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 }}</ref> and invertebrate model organisms.<ref>{{cite journal | author = Howitz KT, Bitterman KJ, Cohen HY, Lamming DW, Lavu S, Wood JG, Zipkin RE, Chung P, Kisielewski A, Zhang LL, Scherer B, Sinclair DA | title = Small molecule activators of sirtuins extend ''Saccharomyces cerevisiae'' lifespan | journal = Nature | volume = 425 | issue = 6954 | pages = 191–6 | year = 2003 | pmid = 12939617 | doi = 10.1038/nature01960 | bibcode = 2003Natur.425..191H }}</ref><ref>{{cite journal | author = Wood JG, Rogina B, Lavu S, Howitz K, Helfand SL, Tatar M, Sinclair D | title = Sirtuin activators mimic caloric restriction and delay ageing in metazoans | journal = Nature | volume = 430 | issue = 7000 | pages = 686–9 | year = 2004 | pmid = 15254550 | doi = 10.1038/nature02789 | bibcode = 2004Natur.430..686W }}</ref> Because of the differences in the metabolic pathways of NAD+ biosynthesis between organisms, such as between bacteria and humans, this area of metabolism is a promising area for the development of new antibiotics.<ref>{{cite journal | author = 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 = 709–20 | year = 2002 | pmid = 12504674 | doi = 10.1016/S0959-440X(02)00385-8 }}</ref><ref>{{cite journal | author = Begley TP, Kinsland C, Mehl RA, Osterman A, Dorrestein P | title = The biosynthesis of nicotinamide adenine dinucleotides in bacteria | journal = Vitam. Horm. | volume = 61 | pages = 103–19 | year = 2001 | pmid = 11153263 | doi = 10.1016/S0083-6729(01)61003-3 | isbn = 978-0-12-709861-6 | series = Vitamins & Hormones }}</ref> For example, the enzyme nicotinamidase, which converts nicotinamide to nicotinic acid, is a target for drug design, as 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 a supplement to culture media for some fastidious bacteria.<ref>http://www.cdc.gov/meningitis/lab-manual/chpt09-id-characterization-hi.html</ref>

    History

    Arthur Harden, co-discoverer of NAD.
    Further information: History of biochemistry
    The coenzyme NAD+ was first discovered by the British biochemists Arthur Harden and William John Young in 1906.<ref>{{Cite journal| first = A | last = Harden | last2 = Young |first2=WJ | title = The alcoholic ferment of yeast-juice Part II.--The coferment of yeast-juice |work = 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}}</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 | accessdate=2007-09-30 | publisher = Nobel Foundation|format=PDF}}</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 |author=Warburg O, Christian W. |title=Pyridin, der wasserstoffübertragende bestandteil von gärungsfermenten (pyridin-nucleotide) |language=German |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|author=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 |url=http://www.jbc.org/cgi/reprint/123/1/137.pdf|format=PDF}}</ref> Then, in 1939, he provided the first strong evidence that niacin is used to synthesize NAD+.<ref>{{Cite journal|author=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 |url=http://www.jbc.org/cgi/reprint/131/1/85.pdf |format=PDF }}</ref> In the early 1940s, Arthur Kornberg made another important contribution towards understanding NAD+ metabolism, by being 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 | pmid = 18098602 | url = http://www.jbc.org/cgi/reprint/176/3/1475.pdf | format = PDF }}</ref> Subsequently, 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 | author = 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 | pmid = 18116985 | url = http://www.jbc.org/cgi/reprint/178/2/611 }}</ref> In 1958, Jack Preiss and Philip Handler discovered the intermediates and enzymes involved in the biosynthesis of NAD+;<ref>{{cite journal | author = 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 | pmid = 13563526 | url = http://www.jbc.org/cgi/reprint/233/2/488 }}</ref><ref>{{cite journal | author = 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 | pmid = 13563527 }}</ref> consequently, salvage synthesis from nicotinic acid is termed the Preiss-Handler pathway. In 2004, 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, Brenner, C | 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 }}</ref> The non-redox roles of NAD(P) are a recent discovery.<ref name=Pollak/> The first of these functions 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 | author = 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 | pmid = 14019961 | doi = 10.1016/0006-291X(63)90024-X }}</ref> Later studies in the 1980s and 1990s revealed the activities of NAD+ and NADP+ metabolites in cell signaling – such as the action of cyclic ADP-ribose, which was discovered in 1987.<ref>{{cite journal | author = 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 | pmid = 3496336 | url = http://www.jbc.org/cgi/reprint/262/20/9561 }}</ref> The metabolism of NAD+ has remained an area of intense research into the 21st century, with interest being heightened after the discovery of the NAD+-dependent protein deacetylases called sirtuins in 2000, by Shin-ichiro Imai and coworkers at the Massachusetts Institute of Technology.<ref>Imai S, Armstrong CM, Kaeberlein M, Guarente L (2000). "Transcriptional silencing and longevity protein Sir2 is an NAD-dependent histone deacetylase". Nature 403 (6771): 795–800. Bibcode:2000Natur.403..795I. doi:10.1038/35001622. PMID 10693811.
  14. ↑ Senkovich O, Speed H, Grigorian A, et al. (2005). "Crystallization of three key glycolytic enzymes of the opportunistic pathogen Cryptosporidium parvum". Biochim. Biophys. Acta 1750 (2): 166–72. doi:10.1016/j.bbapap.2005.04.009. PMID 15953771.
  15. ↑

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