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|Section2= |Section7= }} 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== Nicotinamide adenine dinucleotide, like all ''dinucleotide''s, 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. In metabolism, the compound accepts or donates electrons in redox reactions. 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 RH is oxidized and NAD reduced to NADH by transfer of the hydride to the nicotinamide ring. :RH + 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. 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.〔 In appearance, all forms of this coenzyme are white amorphous powders that are hygroscopic and highly water-soluble. 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. 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. 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.〔 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. 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.〔 These changes in fluorescence are also used to measure changes in the redox state of living cells, through fluorescence microscopy. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nicotinamide adenine dinucleotide」の詳細全文を読む スポンサード リンク
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