Nicotinamide adenine dinucleotide phosphate NADP -NADPH reduction

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... 

An important aspect of enzymatic oxidation-reduction reactions involves the transfer of hydrogen atoms. This transfer is mediated by coenzymes (substances that act together with enzymes) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). These two species pick up H atoms to produce NADH and NADPH, respectively, both of which can function as hydrogen atom donors. Another pair of species involved in oxidation-reduction processes by hydrogen atom transfer consists of flavin adenine triphosphate (FAD) and its hydrogenated form FADH2. The structural formulas of NAD and its cationic form, NAD+, are shown in Figure 4.7. 

There are two other cofactors that can participate in redox processes these are /lavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide phosphate (NADP+). both of which are shown in Fig. 11-2. FAD accepts 2H s and is thereby reduced to FADH2, whereas NADP+ accepts H and is reduced to NADPH and H +. Both of these reduced cofactors can be oxidized, thereby donating their H s (or reducing equivalents), similar to the oxidation of NADH. The enzymes that catalyze those reactions involving an oxidation or a reduction are usually very selective toward a particular cofactor (NAD or NADP) in a particular oxidation state. 

Photosynthesis comprises a light-induced and a dark reaction. The first, called photophosphorylation, involves the two-electron reduction of nicotinamide adenine dinucleotide phosphate (NADP+) by water, to produce NADPH and oxygen. The redox reaction is coupled to the generation of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) ... 

The electron donor in most reductive biosyntheses is NADPH, the reduced form of nicotinamide adenine dinucleotide phosphate (NADP see Figure 14.13). NADPH differs from NADH in that the 2 -hydroxyl group of its adenosine moiety is esterified with phosphate. NADPH carries electrons in the same way as NADH. However, NADPH is used almost exclusively for reductive biosyntheses, whereas NADH is used primarily for the generation of ATP. The extra phosphoryl group on NADPH is a tag that enables enzymes to distinguish between high-potential electrons to be used in anabolism and those to be used in catabolism. 

Upon reduction of both nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) to NADH and NADPH, respectively, a strong absorption peak appears at 340 nm. The millimolar absorption coefficient at this wavelength for both reduced coenzymes is 6.22 mM" cm". As these compounds function as coenzymes of many dehydrogenases, a large number of enzyme reactions can be followed (either directly, or using coupled assays) by monitoring the change of absorbance at 340 nm (8). 

In vivo, most electrons from reduced ferredoxin are passed onto nicotinamide adenine dinucleotide phosphate cation (NADP ), via ferredoxin-NADP reductase, to generate the NADPH needed to drive carbon dioxide fixation by the Calvin cycle. Thus electrons from photosystem I can pass through at least three routes (Figure 1), of which route C is preferred (II). However, if the supply of NADP were limited, for example, because of a poor supply of carbon dioxide causing a slow turnover of the Calvin cycle, the electron flow rate along pathway C would be expected to be decreased and more 02" should be made by route B and, to a lesser extent, by route A (15-17), Some oxygen reduction takes place even when carbon dioxide is present in ample amounts (18). 

The answer is a. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121—138. Wilson, pp 287-320.) The major contributor of electrons in reductive biosynthetic reactions is nicotinamide adenine dinucleotide phosphate (NADPH -I- H ), which is derived by reduction of NAD. NAD is formed from the vitamin niacin (also called nicotinate). Niacin can be formed from tryptophan in humans. In the synthesis of NAD, niacin reacts with 5-phosphoribosyl-l-pyrophosphate to form nicotinate ribonucleotide. Then, AMP is transferred from ATP to nicotinate ribonucleotide. Finally, the amide group of glutamate is transferred to the niacin carboxyl group to form the final product, NAD. NADP is derived from NAD by phosphorylation of the 2 -hydroxyl group of the adenine ribose moiety. The reduction of NADP to NADPH -I- H occurs primarily through the hexose monophosphate shunt. 

This reaction fixes carbon but there is no net change in oxidation number. The CO2 is reduced to carboxyl but one of the carbon atoms in the RuBP is oxidized to yield the second carboxyl group. In subsequent steps, each mole of PGA reacts with a mole of NADPH in order to produce two moles of 3-phosphoglyceraldehyde, a product in which average oxidation number of carbon is 0. NADPH is the reduced form of nicotinamide adenine dinucleotide phosphate (see any biochemistry text for structures and further details). In biosynthetic processes, it functions as a hydride donor or reductant. A typical reaction is shown below. Note that NADPH + H is equivalent to NADP + H2. 

Fig. 10. Hydrogen donor systems for ribonucleotide reduction. Enzyme reactions are I thioredoxin reductase (EC 1.6.4.5) II ribonucleotide reductase (EC 1.17.4) III glutathione reductase (EC 1.6.4.2). GSH, GSSG reduced and oxidized glutathione NADPH, NADP reduced and oxidized nicotinamide adenine dinucleotide phosphate coenzymes. The hydrogen transfer chain is continued in Fig. II...

The free hydrogen is used for carbon reduction according to Equation (2.42). The process is complex (called the Cabin cycle) and the starting point is the transfer of H onto NADP" (nicotinamide adenine dinucleotide phosphate C2iH29N70i7P3), which is the oxidized form of NADPH (which is the reduced form of NADP-") ... 

Superoxide production from paraquat in a pig pulmonary microvascular endotheUal cell suspension was demonstrated by Tampo et al. (1999) using 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazol[l,2-a]pyrazin-3-one, a chemiluminescence probe, to detect superoxide anions. Increased rates of superoxide production from paraquat, which were sensitive to superoxide dismutase, required the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in the reaction medium, and occurred instantaneously after the addition of NADPH, which is impermeable to cell membranes. NADH as an electron donor was not as effective, and xanthine or succinate had no influence. Paraquat was anaerobically reduced in the presence of NADPH and 2-methyl-6-(p-metho-xyphenyl) - 3,7 - dihydroimidazol[l,2 - ajpyrazin- 3 -one to yield a one-electron reduced radical, and the reduction was inhibited by NADP". Diphenyleneio-donium, an inhibitor of flavoprotein reductase, also markedly inhibited both paraquat reduction and superoxide production. 

EC 2.7.1.33) to N-(i )-4 -phosphopantothenate. Activation of the carboxylate results from the addition of P-alanine (P-alanine is formed on decarboxylation of aspartate [Asp, D], EC 4.1.1.11), by cytidylate formation (cytosine triphosphate [CTP] is followed by coupling to L-cysteine (Cys, C) (EC 6.3.2.S) to produce N-[(i )-4 -phosphopantothienoyl]-L-< steine. Oxidation to the thioaldehyde with flavin mononucleotide (EMN FMNH2) allows decarboxylation of the latter with the formation of the corresponding enol. Then, reduction with nicotinamide adenine dinucleotide phosphate (NADPH/IT NADP) (EC 4.1.1.36) leads to 4-phosphopantetheine (pantetheine 4 -phosphate). 

The vast majority of alcohol dehydrogenases require nicotanimide cofactors, such as nicotinamide adenine dinucleotide (NADH) and its respective phosphate NADPH. The structure of NAD/NADP is shown in Fig. 3.39. Hydrogen and two electrons are transferred from the reduced nicotinamide to the carbonyl group to effect a reduction of the substrate (see Fig. 3.39). 

Niacin is a water-soluble vitamin. The RDA of niacin for the adult man is 19 mg. Niacin is converted in the bi>dy to the cofactor nicotinamide adenine dinucleotide (NAD). NAD also exists in a phosphorylated form, NADP The phosphate group occurs on the 2-hydrr>xyl group of the AMP half of the coenzyme, NAD and NADP are used in the catalysis of oxidation and reduction reactions. These reactions are called redox reactions. NAD cycles between the oxidized form, NAD, and the reduced form, NADH + H. The coenzyme functions to accept and donate electrons. NADP behaves in a similar fashion. It occurs as NADP and NADPH + HT The utilization of NAD is illustrated in the sections on glycolysis, the malatc-aspartate shuttle, ketone body metabolism, and fatty acid oxidation. The utilization of NADP is illustrated in the sectirrns concerning fatty acid synthesis and the pentose phosphate pathway. 

For contrast and completeness, NAD and NADH should be introduced here. NAD is the oxidized form of nicotinamide adenine dinucleotide. It is structurally identical to NADP except that it lacks a phosphate group at a key point. Order and control are brought to biochemical oxidations and reductions by this seemingly trivial distinction. NAD is generally an oxidant. NADPH is generally a reductant. Each is present within a cell at only microscopic concentrations. Specialized mechanisms exist for the oxidation... 

The direct reduction of glutamate 5-phosphate (Scheme 12.3) with the nicotinamide adenine diphosphate (NADPH)/NADP+-dependent glutamate-5-semialde-hyde dehydrogenase (EC 1.2.1.41) as shown in Scheme 12.5 produces glutamate 5-semialdehyde. The aldehyde spontaneously undergoes cyclization to (5)-3,4-dihydro-2//-pyrrole-2-carboxylate, which is then reduced to L-proline (Pro, P).The reduction is accomplished again with the phosphorylated nicotinamide adenine dinucleotide being oxidized while in the presence of the enzyme pyrroline-5-car-boxylate reductase (EC 1.5.1.2). 

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