Nicotinamide adenine dinucleotide electron 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... 

Reduced nicotinamide-adenine dinucleotide (NADH) plays a vital role in the reduction of oxygen in the respiratory chain [139]. The biological activity of NADH and oxidized nicotinamideadenine dinucleotide (NAD ) is based on the ability of the nicotinamide group to undergo reversible oxidation-reduction reactions, where a hydride equivalent transfers between a pyridine nucleus in the coenzymes and a substrate (Scheme 29a). The prototype of the reaction is formulated by a simple process where a hydride equivalent transfers from an allylic position to an unsaturated bond (Scheme 29b). No bonds form between the n bonds where electrons delocalize or where the frontier orbitals localize. The simplified formula can be compared with the ene reaction of propene (Scheme 29c), where a bond forms between the n bonds. 

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. 

Isotope effects have been used to determine whether the hydride transfer from the enzyme cofactor nicotinamide-adenine dinucleotide (NADH) (reaction (43)) takes place as a hydride ion transfer in a single kinetic step or in a multistep reaction via an uncoupled electron and hydrogen transfer. 

RGURE 7 An oxidation-reduction reaction. Shown here is the oxidation of lactate to pyruvate. In this dehydrogenation, two electrons and two hydrogen ions (the equivalent of two hydrogen atoms) are removed from C-2 of lactate, an alcohol, to form pyruvate, a ketone. In cells the reaction is catalyzed by lactate dehydrogenase and the electrons are transferred to a cofactor called nicotinamide adenine dinucleotide. This reaction is fully reversible pyruvate can be reduced by electrons from the cofactor. In Chapter 13 we discuss the factors that determine the direction of a reaction. 

The soluble hydrogenase from the hydrogen-oxidizing bacterium N. opaca is one of a class of hydrogenases that contain flavin and use nicotinamide adenine dinucleotide (NAD) as electron acceptor. The protein consists of four dissimilar subunits and contains approximately four atoms of nickel, one FMN, three [Fe-4S] clusters, one [2Fe-2S] cluster, and up to one [3Fe-xS] cluster (82). Two of the nickel atoms were readily removed by dialysis, in contrast to the nickel in most hydrogenases. The enzyme would only catalyze electron transfer from hydrogen to NAD if cations, of which Ni2+ is the most effective, were added. In the absence of the cations, the enzyme could be separated as... 

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. 

Figure 17.4 The electron transport chain of mitochondria. Triangles indicate sites of inhibition by various compounds. Cyt, cytochrome ETF, electron transfer flavoprotein. (Reproduced with permission from Moreadith RW, Batshaw ML, Ohnishi T, Kerr D, Knox B, Jackson D, Hruben R, Olson J, Reynafarje B, Lehninger AL. Deficiency of the iron-sulfur clusters of mitochondrial reduced nicotinamide-adenine dinucleotide-ubiquinone oxidoreductase (complex I) in an infant with congenital lactic acidosis J Clin Invest 74 685-697, 1984.)...

Nicotinamide adenine dinucleotide is a coenzyme which is only loosely bound to the active site of the enzymes with which it interacts and is free therefore, to dissociate from the enzyme during the catalytic cycle. The role of the dehydrogenase enzyme is to bring together the substrate and the NAD+ in the correct orientation for the two to react. These NAD+-dependent enzymes are known as dehydrogenases. They work in conjunction with NAD+ to oxidise substrates by the transfer of 1H+ and 2e from the substrate to the 4-position of the nicotinamide ring of the NAD+ (see Fig. 2.1). The overall reaction is the equivalent of a hydride transfer and is commonly referred to as such. NAD+-dependent enzymes are primarily involved in respiration (NAD+ occurs in significant amounts in mitochondria), whereas, NADP+-dependent coenzymes are primarily involved in the transfer of electrons from intermediates in catabolism. 

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). 

There are many carrier molecules for electrons one is called the nicotinamide adenine dinucleotide (NAD+) and another is the flavin adenine dinucleotide FAD+. The reduced cofactors NADH and FADH2 transfer electrons to the electron transport chain. FMN receives electrons from NADH and passes them to coenzyme Q through Fe-S systems. Coenzyme Q receives electrons from FMN and FADH2 through Fe-S systems. Cytochromes receive electrons from the reduced form of coenzyme Q. Each cytochrome consists of a heme group, and the iron of the heme group is reduced when the cytochrome receives an electron Fe3+ Fe2+. At the end of the electron transfer chain, oxygen is reduced to water. 

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).

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