Coenzymes nicotinamide

FIGURE 15 5 Structure of NAD the oxidized form of the coenzyme nicotinamide adenine dinucleotide The functional part of the coen zyme is framed in red... 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

A coenzyme is an organic compound that activates the primary enzyme to a catalytically active form. A coenzyme may act as a cofactor (see footnote 2), but the converse is not necessarily true. For example, the coenzyme nicotinamide adenine dinucleotide, in either its oxidized or reduced forms (NAD+ or NADH), often participates as a cofactor in enzyme reactions. 

The nicotinamide coenzymes nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH) are associated with a wide variety of enzymes involved in oxidation-reduction reactions (Fig. 21). NADH is typically involved in oxidative catabolic reactions, while NADPH is primarily used in biosynthetic pathways [58]. 

A special metabolic task carried out by the nucleus is biosynthesis of NADT The immediate precursor of this coenzyme, nicotinamide mononucleotide (NMN""), arises in the cytoplasm and is then transported into the nucleolus, where it is enzymatically converted into the dinucleotide NADT Finally, NAD"" then returns to the cytoplasm. 

Nicotinate and nicotinamide, together referred to as niacin, are required for biosynthesis of the coenzymes nicotinamide adenine dinucleotide (NAD"") and nicotinamide adenine dinucleotide phosphate (NADP" ). These both serve in energy and nutrient metabolism as carriers of hydride ions (see pp. 32, 104). The animal organism is able to convert tryptophan into nicotinate, but only with a poor yield. Vitamin deficiency therefore only occurs when nicotinate, nicotinamide, and tryptophan are all simultaneously are lacking in the diet. It manifests in the form of skin damage (pellagra), digestive disturbances, and depression. 

The oxidation of 1,2- and 1,4-dihydropyridines has been extensively studied. This is due in large part to the occurrence of the 1,4-dihydropyridine ring system in the reduced forms of the coenzymes nicotinamide adenine di- and tri-phosphate (NADH and NADPH). These redox couples are responsible for a number of biological oxidations and reductions (B-70MI20701). 

The alkaloids are also relevant to drug design. Alkaloids are complex heterocyclic compounds that contain nitrogen and thus have base-like (hence the term alkaloid ) properties they are extremely structurally diverse. Nicotine is one of the simplest alkaloids. Oxidation of nicotine produces nicotinic acid, a vitamin that is incorporated into the important coenzyme nicotinamide adenine dinucleotide, commonly referred to as NAD" (oxidized form). The neurotransmitter serotonin is an alkaloid containing the aromatic indole ring system. 

It is converted to coenzymes, nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP). These coenzymes are bound to hydrogenases, function as oxidants by accepting hydrogen and electrons from substrates and become reduced. 

Niacin, a water-soluble vitamin vital for oxidation by living cells, functions in the body as a component of two important coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). NAD and NADP are involved in the release of energy from carbohydrate, fat, and protein, and in the synthesis of protein, fat, and pentoses for nucleic acid formation. Milk is a poor source of preformed niacin, containing about 0.08 mg per 100 g. However, milk s niacin value is considerably greater than indicated by its niacin content (Horwitt et al. 1981). Not only is the niacin in milk fully available, but the amino acid tryptophan in milk can be used by the body for the synthesis of niacin. For every 60 mg of tryptophan consumed, the body synthesizes 1 mg of niacin. Therefore, the niacin equivalents in 100 g milk equal 0.856 mg including that from pre-... 

Functioning of the enzyme requires the presence of a coenzyme, nicotinamide adenine dinucleotide which exists in its oxidized (NAD+) or reduced (NADH) forms. The structure of NADH is shown in (177). Reduction or oxidation occurs by transfer of the pro-R C-4 hydrogen atom of the nicotinamide stereospecifically to or from the substrate. The reaction is therefore a ternary one, with the substrate and coenzyme necessarily within the active site for the reaction to occur.l46Sa... 

Exercise 9-20 A solution containing the two forms of the important coenzyme nicotinamide adenine dinuclaotide (abbreviated, NAD and NADH see Section 15-6C for structures) has an absorbance in a 1-cm cell of 0.311 at 340 nm and 1.2 at 260 nm. Both NAD and NADH absorb at 260 nm, but only NADH absorbs at 340 nm. The molar extinction coefficients are... 

The niacin vitamers in foods include nicotinic acid and nicotinamide (Fig. 4), which occur in limited quantities in the free form, and their coenzymes, nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) (93,96). The nicotinic acid analog of NAD as well as nicotinamide and nicotinic acid mononucleotides also occur in nature. In addition, niacin occurs as nicotinyl esters bound to polysaccharides, peptides, and glycopep-tides, which are known as niacytin and niacynogens, respectively. In general, the niacin vitamers in cereal grains and other seeds are principally the nicotinic acid forms, whereas those in meat and fish are primarily the nicotinamide forms (94,95). 

We shall start the discussion with a classical experiment related to the stereochemistry of oxidation of ethanol and reduction of acetaldehyde mediated by the enzyme yeast alcohol dehydrogenase in the presence of the oxidized (NAD+) and reduced (NADH) forms, respectively, of the coenzyme nicotinamide adenine dinucleotide (Fig. 54). The stereochemically interesting feature of this reaction stems from the fact that the methylene hydrogens in CH3CH2OH and the faces of the carbonyl in CH3CH = 0 are enantiotopic. The question thus arises which of the CH2-hydrogens... 

Ethylene glycol is used as a freezing-point depressant in automotive antifreeze. It is highly toxic because the enzyme alcohol dehydrogenase and the coenzyme nicotinamide adenine dinucleotide (NAD) oxidize ethylene glycol to much more liver-toxic compounds like glyoxal, hydroxyacetaldehyde, glyoxylic... 

A similarity between the location of zinc(n) in yeast alcohol dehydrogenase (YADH) and that of the metal in LADH has been established.240 NADH and substrate proton relaxation rates using the paramagnetic iodoacetamide analogue complex of YADH show the coenzyme nicotinamide moiety to be situated ca. 7 A from the zinc ion. This is virtually identical with the position of the metal in LADH determined by X-ray crystallography.241 The substrate resides between the NADH and the metal ion, i.e. it is co-ordinated to the zinc. 

NAD A Coenzyme Nicotinamide adenine dinucleotide (NAD) is one of the principal oxidation-reduction reagents in biological systems. This nucleotide has the structure of two D-ribose rings (a dmucleotide) linked by their 5 phosphates. The aglycone of one ribose is nicotinamide, and the aglycone of the other is adenine. A dietary deficiency of nicotinic acid (niacin) leads to the disease called pellagra, caused by the inability to synthesize enough nicotinamide adenine dinucleotide. 

It is not strictly correct to regard niacin as a vitamin. Its metabolic role is as the precursor of the nicotinamide moiety of the nicotinamide nucleotide coenzymes, nicotinamide adenine dinucleotide (NAD) and NADP, and this can also be synthesized in vivo from the essential amino acid tryptophan. At least in developed countries, average intakes of protein provide more than enough tryptophan to meet requirements for NAD synthesis without any need for preformed niacin. It is only when tryptophan metabolism is disturbed, or intake of the amino acid is inadequate, that niacin becomes a dietary essential. 

A pulse of 2-10 Mev electrons from a linear accelerator, passed through aqueous solutions of pyridinium ions leads rapidly to substantial concentrations of pyridinyl radicals , including those derived from the coenzyme, nicotinamide adenine dinucleotide . The pyridinyl radicals disproportionate, are protonated or dimerize. The reducing agent is a solvated electron or the COj" radical anion (Eq. 4) . 

In some cases, energy is also conserved as energy-rich hydrogen atoms in the coenzyme nicotinamide adenine dinucleotide phosphate in the reduced form of NADPH. The NADPH can then be used as a source of high-energy hydrogen atoms during certain biosynthetic reactions of anabolism. 

Nicotinamide is a part of the important coenzyme. Nicotinamide Adenine Dinucleotide (NAD). This NAD+ coenzyme is important during biological oxidations and is discussed in detail in a later page. 

Hydrogen resulting from the photolysis of water to be used eventually for the reduction of CO2 is first bound to the coenzyme nicotinamide adenine dinucleotide phosphate (NADP). 

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