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Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism.<ref>Template:Lehninger4th</ref> Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine nucleobase and the other, nicotinamide. NAD exists in two forms: an oxidized and reduced form, abbreviated as NADTemplate:+ and NADH (H for hydrogen), respectively.
In cellular metabolism, NAD is involved in redox reactions, carrying electrons from one reaction to another, so it is found in two forms: NADTemplate:+ is an oxidizing agent, accepting electrons from other molecules and becoming reduced; with H+, 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 of enzymes in adding or removing chemical groups to or 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 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 NADTemplate:+, the 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 propertiesEdit
Template:Further Nicotinamide adenine dinucleotide consists of two nucleosides joined by pyrophosphate. The nucleosides each contain a ribose ring, one with adenine attached to the first carbon atom (the 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 diastereomers. It is the β-nicotinamide diastereomer of NADTemplate:+ that is found in nature.</ref><ref name=Pollak>Template:Cite journal</ref>
The compound accepts or donates the equivalent of H−.<ref name=Belenky>Template:Cite journal</ref> Such reactions (summarized in formula below) involve the removal of two hydrogen atoms from a reactant (R), in the form of a hydride ion (H−), and a proton (HTemplate:+). The proton is released into solution, while the reductant RH2 is oxidized and NADTemplate:+ reduced to NADH by transfer of the hydride to the nicotinamide ring.
- RH2 + NADTemplate:+ → NADH + HTemplate:+ + R;
From the electron pair of the hydride ion, one electron is attracted to the slightly more electronegative atom of the nicotinamide ring of NADTemplate:+, becoming part of the nicotinamide moiety. The remaining hydrogen atom is transferred to the carbon atom opposite the N atom. The midpoint potential of the NADTemplate:+/NADH redox pair is −0.32 volts, which makes NADH a moderately strong reducing agent.<ref name=Unden>Template:Cite journal</ref> The reaction is easily reversible, when NADH reduces another molecule and is re-oxidized to NADTemplate:+. This means the coenzyme can continuously cycle between the NADTemplate:+ and NADH forms without being consumed.<ref name=Pollak/>
In appearance, all forms of this coenzyme are white amorphous powders that are hygroscopic and highly water-soluble.<ref>Template:Cite book</ref> The solids are stable if stored dry and in the dark. Solutions of NADTemplate:+ are colorless and stable for about a week at 4 °C and neutral pH, but decompose rapidly in acidic or alkaline solutions. Upon decomposition, they form products that are enzyme inhibitors.<ref>Template:Cite journal</ref>
Both NADTemplate:+ and NADH strongly absorb ultraviolet light because of the adenine. For example, peak absorption of NADTemplate:+ 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.<ref name=Dawson>Template:Cite book</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 assays – by measuring the amount of UV absorption at 340 nm using a spectrophotometer.<ref name=Dawson/>
NADTemplate:+ and NADH also differ in their fluorescence. Freely diffusing NADH in aqueous solution, when excited at the nicotinamide absorbance of ~335 nm (near-UV), fluoresces at 445–460 nm (violet to blue) with a fluorescence lifetime of 0.4 nanoseconds, while NADTemplate:+ does not fluoresce.<ref name="Blacker Mann Gale Ziegler p. ">Template:Cite journal</ref><ref name=Lakowicz>Template:Cite journal</ref> 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.<ref name=Lakowicz/><ref>Template:Cite journal</ref> These changes in fluorescence are also used to measure changes in the redox state of living cells, through fluorescence microscopy.<ref name=Kasimova>Template:Cite journal</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>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Concentration and state in cellsEdit
In rat liver, the total amount of NADTemplate:+ and NADH is approximately 1 μmole per gram of wet weight, about 10 times the concentration of NADPTemplate:+ and NADPH in the same cells.<ref>Template:Cite journal</ref> The actual concentration of NADTemplate:+ in cell cytosol is harder to measure, with recent estimates in animal cells ranging around 0.3 mM,<ref>Template:Cite journal</ref><ref name=Yang>Template:Cite journal</ref> and approximately 1.0 to 2.0 mM in yeast.<ref name="Belenky2">Template:Cite journal</ref> However, more than 80% of NADH fluorescence in mitochondria is from bound form, so the concentration in solution is much lower.<ref>Template:Cite journal</ref>
NADTemplate:+ concentrations are highest in the mitochondria, constituting 40% to 70% of the total cellular NADTemplate:+.<ref name="pmid31412683">Template:Cite journal</ref> NADTemplate:+ in the cytosol is carried into the mitochondrion by a specific membrane transport protein, since the coenzyme cannot diffuse across membranes.<ref>Template:Cite journal</ref> The intracellular half-life of NAD+ was claimed to be between 1–2 hours by one review,<ref name="pmid27465020">Template:Cite journal</ref> whereas another review gave varying estimates based on compartment: intracellular 1–4 hours, cytoplasmic 2 hours, and mitochondrial 4–6 hours.<ref name="pmid29413178">Template:Cite book</ref>
The balance between the oxidized and reduced forms of nicotinamide adenine dinucleotide is called the NADTemplate:+/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>Template:Cite journal</ref> The effects of the NADTemplate:+/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 NADTemplate:+ to NADH in the cytoplasm typically lie around 700:1; the ratio is thus favorable for oxidative reactions.<ref name=Williamson>Template:Cite journal</ref><ref name=Zhang>Template:Cite journal</ref> The ratio of total NADTemplate:+/NADH is much lower, with estimates ranging from 3–10 in mammals.<ref>Template:Cite journal</ref> In contrast, the [[nicotinamide adenine dinucleotide phosphate|NADPTemplate:+/NADPH]] ratio is normally about 0.005, so NADPH is the dominant form of this coenzyme.<ref>Template:Cite journal</ref> These different ratios are key to the different metabolic roles of NADH and NADPH.
BiosynthesisEdit
NADTemplate:+ 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 NADTemplate:+. Although most tissues synthesize NADTemplate:+ by the salvage pathway in mammals, much more de novo synthesis occurs in the liver from tryptophan, and in the kidney and macrophages from nicotinic acid.<ref name="pmid32097708">Template:Cite journal</ref>
De novo productionEdit
Most organisms synthesize NADTemplate:+ 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 acidTemplate:Sndeither tryptophan (Trp) in animals and some bacteria, or aspartic acid (Asp) in some bacteria and plants.<ref>Template:Cite journal</ref><ref>Template:Cite journal</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 NADTemplate:+ is converted into NADPTemplate:+ by [[NAD+ kinase|NADTemplate:+ kinase]], which phosphorylates NADTemplate:+.<ref>Template:Cite journal</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 horikoshii, use inorganic polyphosphate as an alternative phosphoryl donor.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Salvage pathwaysEdit
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>Template:Cite journal</ref> This high requirement for NADTemplate:+ results from the constant consumption of the coenzyme in reactions such as posttranslational modifications, since the cycling of NADTemplate:+ between oxidized and reduced forms in redox reactions does not change the overall levels of the coenzyme.<ref name=Belenky/> The major source of NADTemplate:+ in mammals is the salvage pathway which recycles the nicotinamide produced by enzymes utilizing NADTemplate:+.<ref name="pmid29514064">Template:Cite journal</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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Besides assembling NADTemplate:+ 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 NADTemplate:+. 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 NADTemplate:+ in this organelle.<ref>Template:Cite journal</ref> There are some reports that mammalian cells can take up extracellular NADTemplate:+ from their surroundings,<ref>Template:Cite journal</ref> and both nicotinamide and nicotinamide riboside can be absorbed from the gut.<ref>Template:Cite journal</ref>
The salvage pathways used in microorganisms differ from those of mammals.<ref name=Rongvaux>Template:Cite journal</ref> Some pathogens, such as the yeast Candida glabrata and the bacterium Haemophilus influenzae are NADTemplate:+ auxotrophs – they cannot synthesize NADTemplate:+ – but possess salvage pathways and thus are dependent on external sources of NADTemplate:+ or its precursors.<ref>Template:Cite journal</ref><ref>Template:Cite journal</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 NADTemplate:+ and NADPTemplate:+, and must acquire these coenzymes from its host.<ref>Template:Cite journal</ref>
FunctionsEdit
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 NADTemplate:+ 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>Template:Cite journal</ref><ref name=Billington>Template:Cite journal</ref> and can therefore have important extracellular roles.<ref name=Billington/>
Oxidoreductase binding of NADEdit
The main role of NADTemplate:+ 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>{{#invoke:citation/CS1|citation |CitationClass=web }}</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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
There are many different superfamilies of enzymes that bind NADTemplate:+ / NADH. One of the most common superfamilies includes a structural motif known as the Rossmann fold.<ref name="2015-Hanukoglu">Template:Cite journal</ref><ref>Template:Cite journal</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>Template:Cite journal</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 (Template:PDB; Template:InterPro).<ref>Template:Cite journal</ref>
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 NADTemplate:+ by transferring deuterium rather than hydrogen. In this case, an enzyme can produce one of two stereoisomers of NADH.<ref name=bellamacina>Template:Cite journal</ref>
Despite the similarity in how proteins bind the two coenzymes, enzymes almost always show a high level of specificity for either NADTemplate:+ or NADPTemplate:+.<ref>Template:Cite journal</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 NADPTemplate:+. On the converse, in NAD-dependent enzymes the charge in this pocket is reversed, preventing NADPTemplate:+ 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>Template:Cite journal</ref>
Role in redox metabolismEdit
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 NADTemplate:+ 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 NADTemplate:+) by mitochondrial shuttles, such as the malate-aspartate shuttle.<ref>Template:Cite journal</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>Template:Cite journal</ref> These shuttle systems also have the same transport function in chloroplasts.<ref>Template:Cite journal</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 NADTemplate:+ and NADH, with the high NADTemplate:+/NADH ratio allowing this coenzyme to act as both an oxidizing and a reducing agent.<ref name=Nicholls>Template:Cite book</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 NADPTemplate:+/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>Template:Cite journal</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>Template:Cite journal</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>Template:Cite journal</ref>
Non-redox rolesEdit
The coenzyme NADTemplate:+ 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>Template:Cite journal</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>Template:Cite journal</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>Template:Cite journal</ref><ref>Template:Cite journal</ref> Poly(ADP-ribosyl)ation is carried out by the poly(ADP-ribose) polymerases.<ref name="Diefenbach"/><ref name=Burkle>Template:Cite journal</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>Template:Cite journal</ref> NADTemplate:+ may also be added onto cellular RNA as a 5'-terminal modification.<ref>Template:Cite journal</ref>
Another function of this coenzyme in cell signaling is as a precursor of cyclic ADP-ribose, which is produced from NADTemplate:+ by ADP-ribosyl cyclases, as part of a second messenger system.<ref>Template:Cite journal</ref> This molecule acts in calcium signaling by releasing calcium from intracellular stores.<ref>Template:Cite journal</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, and inducing the activation of the transcription factor NAFC3<ref>Template:Cite journal</ref>
NADTemplate:+ is also consumed by different NAD+-consuming enzymes, such as CD38, CD157, PARPs and the NAD-dependent deacetylases (sirtuins, such as Sir2.<ref>Template:Cite journal</ref>).<ref name=":0">Template:Cite journal</ref> These enzymes act by transferring an acetyl group from their substrate protein to the ADP-ribose moiety of NADTemplate:+; 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>Template:Cite journal</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>Template:Cite journal</ref><ref name=":1">Template:Cite journal</ref>
Other NAD-dependent enzymes include bacterial DNA ligases, which join two DNA ends by using NADTemplate:+ 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>Template:Cite journal</ref> This contrasts with eukaryotic DNA ligases, which use ATP to form the DNA-AMP intermediate.<ref>Template:Cite journal</ref>
Li et al. have found that NADTemplate:+ directly regulates protein-protein interactions.<ref name="Li 2017">Template:Cite journal</ref> They also show that one of the causes of age-related decline in DNA repair may be increased binding of the protein DBC1 (Deleted in Breast Cancer 1) to PARP1 (poly[ADP–ribose] polymerase 1) as NADTemplate:+ levels decline during aging.<ref name="Li 2017" /> The decline in cellular concentrations of NADTemplate:+ during aging likely contributes to the 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 NADTemplate:+ may protect against cancer, radiation, and aging.<ref name="Li 2017" />
Extracellular actions of NAD+Edit
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>Template:Cite journal</ref><ref name=Koch-Nolte>Template:Cite journal</ref> NAD+ is released from neurons in blood vessels,<ref name=Smyth/> urinary bladder,<ref name=Smyth/><ref name=Breen>Template:Cite journal</ref> large intestine,<ref name=Mutafova-Yambolieva>Template:Cite journal</ref><ref name=Hwang>Template:Cite journal</ref> from neurosecretory cells,<ref name=Yamboliev>Template:Cite journal</ref> and from brain synaptosomes,<ref name=Durnin>Template:Cite journal</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/> 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>Template:Cite journal</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 significanceEdit
The enzymes that make and use NADTemplate:+ and NADH are important in both pharmacology and the research into future treatments for disease.<ref>Template:Cite journal</ref> Drug design and drug development exploits NADTemplate:+ 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 NADTemplate:+ biosynthesis.<ref>Template:Cite journal</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">Template:Cite journal</ref><ref name="pmid32111066">Template:Cite journal</ref>
It has been studied for its potential use in the therapy of neurodegenerative diseases such as Alzheimer's and Parkinson's disease as well as multiple sclerosis.<ref name=Belenky/><ref name=":1" /><ref>Template:Cite journal</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>Template:Cite journal</ref>
NADTemplate:+ 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>Template:Cite journal</ref> This radical then reacts with NADH, to produce adducts that are very potent inhibitors of the enzymes enoyl-acyl carrier protein reductase,<ref>Template:Cite journal</ref> and dihydrofolate reductase.<ref>Template:Cite journal</ref>
Since many oxidoreductases use NADTemplate:+ and NADH as substrates, and bind them using a highly conserved structural motif, the idea that inhibitors based on NADTemplate:+ could be specific to one enzyme is surprising.<ref name="Pankiewicz">Template:Cite journal</ref> However, this can be possible: for example, inhibitors based on the compounds mycophenolic acid and tiazofurin inhibit IMP dehydrogenase at the NADTemplate:+ 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>Template:Cite journal</ref> Other drugs are not enzyme inhibitors, but instead activate enzymes involved in NADTemplate:+ metabolism. Sirtuins are a particularly interesting target for such drugs, since activation of these NAD-dependent deacetylases extends lifespan in some animal models.<ref name="Kim">Template:Cite journal</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>Template:Cite journal</ref> and invertebrate model organisms.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In one experiment, mice given NAD for one week had improved nuclear-mitochrondrial communication.<ref>Template:Cite journal</ref>
Because of the differences in the metabolic pathways of NADTemplate:+ biosynthesis between organisms, such as between bacteria and humans, this area of metabolism is a promising area for the development of new antibiotics.<ref>Template:Cite journal</ref><ref>Template:Cite book</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 as a supplement to culture media for some fastidious bacteria.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</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 licensed for medical use in the UK; there are likely breaches 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 rats 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>Template:Cite news</ref>
HistoryEdit
The coenzyme NADTemplate:+ was first discovered by the British biochemists Arthur Harden and William John Young in 1906.<ref>Template:Cite journal</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>{{#invoke:citation/CS1|citation |CitationClass=web }}</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>Template:Cite journal</ref>
Vitamin precursors of NADTemplate:+ were first identified in 1938, when Conrad Elvehjem showed that liver has an "anti-black tongue" activity in the form of nicotinamide.<ref>Template:Cite journal</ref> Then, in 1939, he provided the first strong evidence that nicotinic acid is used to synthesize NADTemplate:+.<ref>Template:Cite journal</ref> In the early 1940s, Arthur Kornberg was the first to detect an enzyme in the biosynthetic pathway.<ref>Template:Cite journal</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>Template:Cite journal</ref> In 1958, Jack Preiss and Philip Handler discovered the intermediates and enzymes involved in the biosynthesis of NADTemplate:+;<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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 NADTemplate:+.<ref name="Bieganowski, P, Brenner, C 2004 495–502">Template:Cite journal</ref>
The non-redox roles of NAD(P) were discovered later.<ref name=Pollak/> The first to be identified was the use of NADTemplate:+ as the ADP-ribose donor in ADP-ribosylation reactions, observed in the early 1960s.<ref>Template:Cite journal</ref> Studies in the 1980s and 1990s revealed the activities of NADTemplate:+ and NADPTemplate:+ metabolites in cell signaling – such as the action of cyclic ADP-ribose, which was discovered in 1987.<ref>Template:Cite journal</ref>
The metabolism of NADTemplate:+ remained an area of intense research into the 21st century, with interest heightened after the discovery of the NADTemplate:+-dependent protein deacetylases called sirtuins in 2000, by Shin-ichiro Imai and coworkers in the laboratory of Leonard P. Guarente.<ref>Template:Cite journal</ref> In 2009 Imai proposed the "NAD World" hypothesis that key regulators of aging and longevity in mammals are sirtuin 1 and the primary NADTemplate:+ synthesizing enzyme nicotinamide phosphoribosyltransferase (NAMPT).<ref name="pmid19130305">Template:Cite journal</ref> In 2016 Imai expanded his hypothesis to "NAD World 2.0", which postulates that extracellular NAMPT from adipose tissue maintains NADTemplate:+ in the hypothalamus (the control center) in conjunction with myokines from skeletal muscle cells.<ref name="pmid28725474">Template:Cite journal</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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
See alsoEdit
ReferencesEdit
Further readingEdit
FunctionEdit
- Template:Cite book
- Template:Cite book
- Template:Cite book
- {{#invoke:citation/CS1|citation
|CitationClass=web }}
HistoryEdit
- Template:Cite book, A history of early enzymology.
- Template:Cite book, a textbook from the 19th century.
External linksEdit
- NAD bound to proteins in the Protein Data Bank
- NAD Animation (Flash Required)
- β-Nicotinamide adenine dinucleotide (NADTemplate:+, oxidized) and NADH (reduced) Chemical data sheet from Sigma-Aldrich
- NADTemplate:+, NADH and NAD synthesis pathway at the MetaCyc database
- List of oxidoreductases Template:Webarchive at the SWISS-PROT database
- NAD+
- NAD+ The Molecule of Youth