Nicotinamide adenine dinucleotide phosphate transfer

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... 

DHFR catalyses the hydride-ion transfer between the nicotinamide adenine dinucleotide phosphate (NADPH) cofactor and a substrate molecule (S) according to... 

The second type of biological electron transfer involves a variety of small molecules, both organic and inorganic. Examples of these are (a) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) as two electron carriers and (b) quinones and flavin mononucleotide (FMN), which may transfer one or two electrons. The structure of NAD and its reduced counterpart NADH are shown in Figure 1.12. 

For the formation of one 02 molecule four electrons have to be transferred. This requires a "quantum storage device". In the photosynthetic system of green plants this is achieved with two photosystems that are linked through an electron transport chain, Fig. 10.2, and by means of the thylakoid-membrane that enables the separation of the photoproducts 02 and the reduced form of nicotinamide adenine dinucleotide phosphate, NADPH. 

Dugan, R. E., Porter, J. W. Stereospecificity of the transfer of hydrogen from reduced nicotinamide adenine dinucleotide phosphate, in each of the two reductive steps catalyzed by /S-hydroxy-jS-methylglutaryl coenzyme A reductase. J. Biol. Chem. 246, 5361—5364 (1971). 

I I 3. The answer is c. (Hardman, pp 1243-1247.) Antimetabolites of folic acid such as methotrexate, which is an important cancer chemotherapeutic agent, exert their effect by inhibiting the catalytic activity of the enzyme dihydrofolate reductase. The enzyme functions to keep folic acid in a reduced state. The first step in the reaction is the reduction of folic acid to 7,8-dihydrofolic acid (FH2), which requires the cofactor nicotinamide adenine dinucleotide phosphate (NADPH). The second step is the conversion of FH2 to 5,6,7,8-tetrahydrofolic acid (FH ). This part of the reduction reaction requires nicotinamide adenine dinucleotide (NADH) or NADPH. The reduced forms of folic acid are involved in one-carbon transfer reactions that are required during the synthesis of purines and pyrimidine thymidylate. The affinity of methotrexate for dihydrofolate reductase is much greater than for the substrates of folic acid and FH2. The action of... 

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

The hepatic endoplasmic reticulum possesses oxidative enzymes called mixed-function oxidases or monooxygenase with a specific requirement for both molecular oxygen and a reduced concentration of nicotinamide adenine dinucleotide phosphate (NADPH). Essential in the mixed-function oxidase system is P-450 (Figure 1.12). The primary electron donor is NADPH, whereas the electron transfer involved P-450, a flavoprotein. The presence of a heat-stable fraction is necessary for the operation of the system. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

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. 

Figure 5-1. Schematic representation of the three stages of photosynthesis in chloroplasts (1) The absorption of light can excite photosynthetic pigments, leading to the photochemical events in which electrons are donated by special chlorophylls. (2) The elections are then transferred along a series of molecules, causing the oxidized form of nicotinamide adenine dinucleotide phosphate (NADP+) to become the reduced form (NADPH) ATP formation is coupled to the electron transfer steps. (3) The biochemistry of photosynthesis can proceed in the dark and requires 3 mol of ATP and 2 mol of NADPH per mole of C02 fixed into a carbohydrate, represented in the figure by (CH20).

A particular half-cell reaction, such as Equation 6.15, can accept or donate electrons. We quantitatively describe this by the redox potential for that reaction, as expressed by Equation 6.9 [Ej = E u — (RT/qF) In (reduced))/(oxidized))]. We will use (NADPH) to represent the activity of all of the various ionization states and complexed forms of the reduced nicotinamide adenine dinucleotide phosphate, and (NADP+) has an analogous meaning for the oxidized component of the NADP+-NADPH couple. For redox reactions of biological interest, the midpoint (standard) redox potential is usually determined at pH 7. By using Equation 6.9, in which the number q of electrons transferred per molecule reduced is 2, we can... 

Fig. 2.1. Examples from The Energy Hall of Fame. These molecules not only deliver energy, but transfer special groups in the process. Acyl, RCO— ADP, adenosine diphosphate ATP, adenosine triphosphate dUMP deoxyuridine monophosphate FAD, flavin adenine dinucleotide GTP, guanosine triphosphate NADH, nicotinamide adenine dinucleotide NADP, nicotinamide adenine dinucleotide phosphate P, phosphate TMP, thymidine monophosphate UDP, uridine diphosphate UTP, uridine triphosphate.

When one of the substrates is water (i.e., when the process is one of hydrolysis), with the reaction taking place in aqueous solution, only a fraction of the total number of water molecules present participates in the reaction. The small change in the concentration of water has no effect on the rate of reaction and these pseudo-one substrate reactions are described by one-substi ate kinetics. More generally the concentrations of both substrates may be variable, and both may affect the rate of reaction. Among the bisubstrate reactions important in clinical enzymology are the reactions catalyzed by dehydrogenases, in which the second substrate is a specific coenzyme, such as the oxidized or reduced forms of nicotinamide adenine dinucleotide, (NADH), or nicotinamide adenine dinucleotide phosphate, (NADPH), and the amino-group transfers catalyzed by the aminotransferases. 

Transketolase is a TPP-dependent enzyme found in the cytosol of many tissues, especially hver and blood cells, in which principal carbohydrate pathways exist. In the pentose phosphate pathway, which additionally supplies reduced nicotinamide-adenine dinucleotide phosphate (NADPH) necessary for biosynthetic reactions, this enzyme catalyzes the reversible transfer of a glycoaldehyde moiety from the first two carbons of a donor ketose phosphate to the aldehyde carbon of an aldose phosphate. 

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