Niacin nicotinamide ring

Niacin was discovered as a nutrient during studies of pellagra. It is not strictly a vitamin since it can be synthesized in the body from the essential amino acid tryptophan. Two compounds, nicotinic acid and nicotinamide, have the biologic activity of niacin its metabolic function is as the nicotinamide ring of the coenzymes NAD and NADP in oxidation-reduction reactions (Figure 45-11). About 60 mg of tryptophan is equivalent to 1 mg of dietary niacin. The niacin content of foods is expressed as mg niacin equivalents = mg preformed niacin + 1/60 X mg tryptophan. Because most of the niacin in cereals is biologically unavailable, this is discounted. 

Nicotinamide adenine dinucleotide (NAD+ in its oxidized form) and its close analog nicotinamide adenine dinucleotide phosphate (NADP+) are composed of two nucleotides joined through their phosphate groups by a phosphoanhydride bond (Fig. 13-15a). Because the nicotinamide ring resembles pyridine, these compounds are sometimes called pyridine nucleotides. The vitamin niacin is the source of the nicotinamide moiety in nicotinamide nucleotides. 

Figure 8.1). Although the amino acid tryptophan is quantitatively the major precursor of the nicotinamide ring of the coenzymes (Section 8.3), it is not considered to he a niacin vitamer.

Nicotinamide adenine dinucleotide is a major electron carrier in the oxidation of fuel molecules (Figure 14.13). The reactive part of NAD+ is its nicotinamide ring, a pyridine derivative synthesized from the vitamin niacin. In the oxidation... 

The nicotinamide ring of NAD+ is derived from the vitamin niacin (nicotinic acid). It is also produced to a limited extent from the amino acid tryptophan. 

A 70-B. Although dietary niacin is the major source of the nicotinamide ring of NAD, it may also be produced from tryptophan. 

Nicotinamide adenine dinudeotide is a major electron carrier in the oxidation of fuel molecules (Figure 15.13). The reactive part of NAD is its nicotinamide ring, a pyridine derivative synthesized from the vitamin niacin. In the oxidation of a substrate, the nicotinamide ring of NAD a hydrogen ion and two electrons, which are equivalent to a hydride ion (H ). The reduced form of this carrier is called NADH, In the oxidized form, the nitrogen atom carries a positive charge, as indicated by NAD. the electron acceptor in many reactions of the type... 

The enzyme lactate dehydrogenase, which catalyzes the transfer of electrons from lactate to NAD, illustrates these principles (Fig. 8.14). The coenzyme nicotinamide adenine dinucleotide (NAD ) is synthesized from the vitamin niacin (which forms the nicotinamide ring), and from ATP (which contributes an AMP). The ADP portion of the molecule binds tightly to the enzyme and causes conformational... 

The electron density map of pyridine (see IOC) clearly shows a high concentration of electron density (more red) on the nitrogen. Note that pyridine has been implicated in male sterility, although some claim this is a myth. Nonetheless, it is reasonable to exercise caution when pyridine is used. Several important pyridine derivatives have substituents on the aromatic ring, including 2,6-lutidine (11) and picolinic acid (12). Many derivatives of pyridine are found in pharmaceutically active compounds. One is nicotinic acid (niacin, 13), which is vitamin Bg and is found in liver, yeast, and meat. A deficiency in this vitamin can lead to pellagra (a wasting disease). Nicotinamide (14, niacinamide) is one of the two principal forms of the B-complex vitamin niacin. Nicotinamide may be useful for individuals with type 1 (insulin-dependent) diabetes. 

NAD" " (nicotinamide adenine dinucleotide) is an important coenzyme in which the vitamin niacin provides the nicotinamide group, which is bonded to ribose and ADP (see Figure 18.6). The oxidized NAD+ undergoes reduction when a carbon in the nicotinamide ring reacts with 2H (two hydrogen ions and two electrons), leaving one H+. 

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. 

Fig. 39.19. Degradation of tryptophan. One of the ring carbons produces formate. The nonring portion forms alanine. Kynurenine is an intermediate, which can be converted to a number of urinary excretion products (e.g., xanthurenate), degraded to CO2 and acetyl CoA, or converted to the nicotinamide moiety of NAD and NADP, which also can be formed from the vitamin niacin.

Figure A6.1 Structures of three of the vitamin-derived cofactors. NAD, like several other cofactors, has a handle consisting of adenine (blue) and ribose phosphate (black), but the actual chemistry of oxidation and reduction is done by the nicotinamide (red), which is derived from the vitamin nicotinic add or niacin. Note the positive charge on the ring nitrogen in the oxidised form, NAD+, which is shown here. Pyridoxal phosphate, derived from vitamin carries out its chemical contribution via the aldehyde group (mauve). Biotin likewise contributes its own characteristic chemistry, with the nitrogen atom, shown in blue, readily able to pick up CO2 as a carboxyl group.

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