Nicotinamide adenine dinucleotide

Nicotinamide adenine dinucleotide, abbreviated NAD⁺, is a adenosine ring, and the other containing nicotinamide.

In drug discovery.

In organisms, NAD⁺ can be synthesized from scratch (de novo) from the amino acids nicotinamide adenine dinucleotide phosphate (NADP⁺); this related coenzyme has similar chemistry to NAD⁺, but has different roles in metabolism.

Physical and chemical properties

Further information: Redox

Nicotinamide adenine dinucleotide is a dinucleotide since it consists of two phosphate groups through the 5' carbons.^[1]

  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 proton (H⁺). The proton is released into solution, while the reductant RH₂ is oxidized and NAD⁺ reduced to NADH by transfer of the hydride.

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 carbon atom opposite this nitrogen. The 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 enzyme inhibitors.^[5]   Both NAD⁺ and NADH absorb strongly in the spectrophotometer.^[6]

NAD⁺ and NADH also differ in their fluorescence microscopy.^[9]

Concentration and state in cells

In rat liver, the total amount of NAD⁺ and NADH is approximately 1 μmole per yeast.^[2] However, over 80% is bound to proteins, so the concentration in solution is much lower.^[13]

Data for other compartments in the cell are limited, although, in the diffuse across membranes.^[14]

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.^[15] The effects of the NAD⁺/NADH ratio are complex, controling the activity of several key enzymes, including NADP⁺/NADPH ratio is normally about 0.005, around 200 times lower than the NAD⁺/NADH ratio, so NADPH is the dominant form of this coenzyme.^[17] 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

  Most organisms synthesize NAD⁺ from simple components.^[2] The specific set of reactions differs among organisms, but a common feature is the generation of quinolinic acid (QA) from an amino acid - either amidated to a nicotinamide (Nam) group, forming nicotinamide adenine dinucleotide.^[2]

In a further step, some NAD⁺ is converted into NADP⁺ by NAD+ kinase, which phosphorylates NAD⁺.^[19] In most organisms, this enzyme uses ATP as the source of the phosphate group, although in bacteria such as Mycobacterium tuberculosis and in archaea such as Pyrococcus horikoshii, inorganic polyphosphate is an alternative phosphate donor.^[20][21]

Salvage pathways

Besides assembling NAD⁺ de novo from simple amino acid precursors, cells also salvage preformed compounds containing nicotinamide. Although other precursors are known, the three natural compounds containing the nicotinamide ring and used in these salvage metabolic pathways are nicotinic acid (Na), nicotinamide (Nam) and nicotinamide riboside (NR).^[22] The precursors are fed into the NAD(P)⁺ biosynthetic pathway, shown above, through adenylation and phosphoribosylation reactions.^[2] These compounds are can be taken up from the diet, where the mixture of nicotinic acid and nicotinamide are called vitamin B₃ or niacin. However, these compounds are also produced within cells, when the nicotinamide group is released from NAD⁺ in ADP-ribose transfer reactions. Indeed, 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.^[23]

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.^[24] 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.^[2]

The salvage pathways used in pathogen Chlamydia trachomatis, which lacks recognizable candidates for any genes involved in the salvage or biosynthesis of both NAD⁺ and NADP⁺, and may instead salvage these coenzymes from its host.^[28]

Functions

Nicotinamide adenine dinucleotide has several essential roles in sirtuins that use NAD⁺ to remove acetyl groups from proteins.

Oxidoreductases

Further information: Oxidoreductases

The main role of NAD⁺ in metabolism is the transfer of electrons from one redox reaction to another. This type of reaction are catalyzed by a large group of enzymes called coenzyme Q.^[30] 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.^[31]

When bound to a protein, NAD⁺ and NADH are usually held within a amino acid metabolism have recently been discovered that bind the coenzyme, but lack this motif.^[34]

Despite this similarity in how proteins bind coenzymes, enzymes almost always show a high level of specificity for either NAD⁺ or NADP⁺.^[35] This specificity reflects the distinct metabolic roles of the two coenzymes, and is the result of distinct sets of ionic bond is formed between a basic amino acid side chain and the acidic phosphate group of NADP⁺. Conversely, 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 aldose reductase, glucose-6-phosphate dehydrogenase, and methylenetetrahydrofolate reductase can use both coenzymes in some species.^[36]

Role in redox metabolism

Further information: 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 chloroplasts.^[38]

Since both the oxidized and reduced forms of nicotinamide adenine dinucleotide are used in these linked sets of reactions, the cell maintains approximately equal concentrations of NAD⁺ and NADH; the high NAD⁺/NADH ratio allows this coenzyme to act as both an oxidizing and a reducing agent.^[39] In contrast, the main function of NADP⁺ is as a reducing agent in photosynthesis. Since NADPH is needed to drive redox reactions as a strong reducing agent, the NADP⁺/NADPH ratio is kept very low.^[39]

Although it is important in catabolism, NADH is also used in anabolic reactions, such as proton-motive force to run part of the electron transport chain in reverse, generating NADH.^[42]

Non-redox roles

The coenzyme NAD⁺ is also consumed in ADP-ribose transfer reactions. For example, enzymes called telomere maintenance.^[47] In addition to these functions within the cell, a group of extracellular ADP-ribosyltransferases has recently been discovered, but their functions remain obscure.^[48]   Another function of this coenzyme in cell signaling is as a precursor of ryanodine receptors, which are located in the membranes of organelles, such as the endoplasmic reticulum.^[51]

NAD⁺ is also consumed by transcription through deacetylating histones and altering nucleosome structure.^[53] These activities of sirtuins are particularly interesting due to their importance in the regulation of aging.^[54]

Other NAD-dependent enzymes include bacterial ATP to form the DNA-AMP intermediate.^[56]

Pharmacology

The enzymes that make and use NAD⁺ and NADH are important in both current enzyme inhibitors or activators based on its structure that change the activity of NAD-dependent enzymes, and by trying to inhibit NAD⁺ biosynthesis.^[57]

The coenzyme NAD⁺ is not itself currently used as a treatment for any disease. However, it is potentially useful in the therapy of neurodegenerative diseases such as Alzheimer's and Parkinson disease.^[2] Evidence for these applications is mixed; studies in mice are promising,^[58] whereas a placebo-controlled clinical trial failed to show any effect.^[59] NAD⁺ is also a direct target of the drug free radical form.^[60] This radical then reacts with NADH, to produce adducts that are very potent inhibitors of the enzymes enoyl-acyl carrier protein reductase,^[61] and dihydrofolate reductase.^[62]

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 surprizing.^[63] However, this can be possible: for example, inhibitors based on the compounds resveratrol increase the activity of these enzymes, which may be important in their ability to delay aging in both vertebrate,^[65] and invertebrate model organisms.^[66][67]

Due to the differences in the antibiotics.^[68][69] 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.^[25]

History

Further information: History of biochemistry

  The coenzyme NAD⁺ was first discovered by the British biochemists Arthur Harden and William Youndin in 1906.^[70] They noticed that adding boiled and filtered nucleotide sugar phosphate by Hans von Euler-Chelpin.^[71] 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.^[72]

The source of nicotinamide was identified in 1938, when Arthur Kornberg made another important contribution towards understanding NAD⁺ metabolism, by being the first to detect an enzyme in the biosynthetic pathway.^[75] 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.^[76] Finally, in 1959, Jack Preiss and Philip Handler discovered the intermediates and enzymes involved in the biosynthesis of NAD⁺;^[77][78] consequently, de novo synthesis is often called the Preiss-Handler pathway in their honor.

The non-redox roles of NAD(P) are a recent discovery.^[1] 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.^[79] Later studies in the 1980s and 1990s revealed the activities of NAD⁺ and NADP⁺ metabolites in cell signaling - such as the action of sirtuins in 2000, by Shin-ichiro Imai and coworkers at the Massachusetts Institute of Technology.^[81]

See also

• List of oxidoreductases
• Enzyme catalysis

References

Further reading

Function

• Nelson DL; Cox MM (2004). Lehninger Principles of Biochemistry, 4th ed, W. H. Freeman. ISBN 0-716-74339-6. 
• Bugg, T (2004). Introduction to Enzyme and Coenzyme Chemistry, 2nd ed, Blackwell Publishing Limited. ISBN 1-40511-452-5. 
• Lee HC (2002). Cyclic ADP-Ribose and NAADP: Structure, Metabolism and Functions. Kluwer Academic Publishers. ISBN 1-40207-281-3. 

History

• New Beer in an Old Bottle: Eduard Buchner and the Growth of Biochemical Knowledge, edited by Athel Cornish-Bowden and published by Universitat de València (1997): ISBN 84-370-3328-4, A history of early enzymology.
• Williams, Henry Smith, 1863–1943. A History of Science: in Five Volumes. Volume IV: Modern Development of the Chemical and Biological Sciences, A textbook from the 19th century.