Nicotinamide adenine dinucleotide energy metabolism

Four of the B vitamins are essential in the citric acid cycle and therefore in energy-yielding metabolism (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor in the a-ketoglutarate dehydrogenase complex and in succinate dehydrogenase (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD),... 

Rice bran is the richest natural source of B-complex vitamins. Considerable amounts of thiamin (Bl), riboflavin (B2), niacin (B3), pantothenic acid (B5) and pyridoxin (B6) are available in rice bran (Table 17.1). Thiamin (Bl) is central to carbohydrate metabolism and kreb s cycle function. Niacin (B3) also plays a key role in carbohydrate metabolism for the synthesis of GTF (Glucose Tolerance Factor). As a pre-cursor to NAD (nicotinamide adenine dinucleotide-oxidized form), it is an important metabolite concerned with intracellular energy production. It prevents the depletion of NAD in the pancreatic beta cells. It also promotes healthy cholesterol levels not only by decreasing LDL-C but also by improving HDL-C. It is the safest nutritional approach to normalizing cholesterol levels. Pyridoxine (B6) helps to regulate blood glucose levels, prevents peripheral neuropathy in diabetics and improves the immune function. 

Two important implications of the reactions described in Equations (5.1) and (5.2) are (i) that redox reactions play an important role in metabolic transformations, with the cofactors nicotinamide adenine dinucleotide (NAD+) acting as electron acceptor in catabolic pathways and nicotinamide adenine dinucleotide phosphate (NADPH) as electron donor in anabolism, and (ii) that energy must be produced by catabolism and used in biosyntheses (almost always in the form of adenosine triphosphate, ATP). 

In the second stage, the building blocks are degraded by various pathways in tissues to a common metabolic intermediate, acetyl CoA. Most of the energy contained in metabolic fuels is conserved in the chemical bonds (electrons) of acetyl CoA. A smaller portion is conserved in reducing nicotinamide adenine dinucleotide (NAD) to NADH or flavin adenine dinucleotide (FAD) to FADH. Reduction indicates the addition of electrons that may be free, part of a hydrogen atom (H), or a hydride ion (H ). 

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. 

Fig. 1. Energy metabolism in the normal myocardium (ATP adenosine-5 -triphosphate, ADP adenosine-5 -diphosphate, P phosphate, PDH pyruvate dehydrogenase complex, acetyl-CoA acetyl-coenzyme A, NADH and NAD" nicotinamide adenine dinucleotide (reduced and oxidized), FADH2 and FAD flavin adenine dinucleotide (reduced and oxidized).

Several reactions in metabolism are oxidation-reduction (or redox) reactions. Two of the principal redox carriers are nicotinamide adenine dinucleotide (NAD+) and coenzyme Q. Remember that we live in an oxidizing world, so species that are in the reduced form are frequently high-energy compounds that react exothermically with oxygen. Also recall that organic molecules are reduced by adding bonds to hydrogen. 

In contrast to plant cells, which normally get their cellular energy from photosynthesis, animal cells need a carbohydrate source, usually glucose, and the amino acid glutamine. The catabolism of these substrates allows the production of two coenzymes (ATP and NADH - nicotinamide adenine dinucleotide), which are essential for maintaining the viability of the cells. These coenzymes can be used for the maintenance, metabolism and/or for the synthesis of particular desired products (Wagner, 1997). 

Nucleotides are stmctural units of deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and cofactors such as coenzyme A (CoA), flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), with important roles in energy transfer, metabolism and intracellular signalling. 

Most of the electrons removed from fuels during energy metabolism are transferred via nicotinamide adenine dinucleotide (NAD). NAD collects electrons from many different energy fuels in reactions catalyzed by specific enzymes. These enzymes are dehydrogenases. Reduced NAD, in turn, shuttles the electrons to the respiratory chain. Flavin adenine dinucleotide (FAD) also acts as an electron shuttle. In each reaction involving NAD (or FAD), two electrons are transferred that is, two electrons are carried or shuttled. NAD and FAD are small molecules with molecular weights of 663 and 785 and are manufactured in the body from the vitamins niacin and riboflavin, respectively. These molecules are called N.A.D. and F.A.D., not nad" or Jad. ... 

Co-enzyme I (nicotinamide-adenine dinucleotide NAD) and Co-enzyme II (nicotinamide-adenine dinucleotide phosphate NADP) are required by all living cells. They enable both the conversion of carbohydrates into energy as well as the metabolism of proteins and fats. Both nicotinamide and nicotinic acid are building blocks for these co-enzymes. The common name for the vitamin is niacin and, strictly speaking, refers only to nicotinic acid. 

Nicotinamide adenine dinucleotide (NAD) is the coenzyme form of the vitamin niacin. Most biochemical reactions require protein catalysts (enzymes). Some enzymes, lysozyme or trypsin, for example, catalyze reactions by themselves, but many require helper substances such as coenzymes, metal ions, and ribonucleic acid (RNA). Niacin is a component of two coenzymes NAD, and nicotinamide adenine dinucleotide phosphate (N/kDP). NAD (the oxidized form of the NAD coenzyme) is important in catabolism and in the production of metabolic energy. NADP (the oxidized form of NADP) is important in the biosynthesis of fats and sugars. 

In the energy metabolism of cells, an intermediate carrier of the electrons (and protons) is commonly required. Such an electron carrier is, for instance, NAD+ (nicotinamid adenin dinucleotide), which formally accepts two electrons and one proton and is thereby reduced to NADH. The NADH may give off the electrons again to specialized electron acceptors and the protons are released in the cell sap. Thereby, the NADH, which must be used repeatedly, is recycled. 

Anabolic (biosynthetic) reactions are driven by ATP energy, and the process of reduction is mediated by enzymes most of which utilize nicotinamide adenine dinucleotide phosphate (NADP or NADP+) as cofactor. The specificity of catabolism for NAD+ and anabolism for NADP+ is an example of chemical compartmentation and enables some degree of metabolic regulation to be exerted through control of the levels of the two cofactors. The relative concentrations of the oxidized and reduced forms of a particular cofactor may also serve a regulatory role. 

Today s systems are in most cases based on the bioluminescence with ATP and luciferase from the firefly. As an alternative system it is also possible to use a colour test nicotinamide adenine dinucleotides (NAD/NADH) and nicotinamide adenine dinucleotide phosphates (NADP/NADPH), which are also compounds used for the energy transfer in the metabolism in living cells or compounds found in food debris. 

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