Nicotinamide adenine dinucleotide respiratory chain

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. 

Figure 1 The mitochondrial respiratory chain. Electron transfer (brown arrows) between the three major membrane-bound complexes (I, III, and IV) is mediated by ubiquinone (Q/QH2) and the peripheral protein c)dochrome c (c). Transfer of protons hnked to the redox chemistry is shown by blue arrows red arrows denote proton translocation. NAD+ nicotinamide adenine dinucleotide, FMN flavin mononucleotide, Fe/S iron-sulfur center bH,bi, and c are the heme centers in the cytochrome bc complex (Complex III). Note the bifurcation of the electron transfer path on oxidation of QH2 by the heme bL - Fe/S center. Complex IV is the subject of this review. N and P denote the negatively and positively charged sides of the membrane, respectively...

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. ... 

The oxidation reactions involved are catalyzed by a series of nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide (FAD) dependent dehydrogenases in the highly conserved metabolic pathways of glycolysis, fatty acid oxidation and the tricarboxylic acid cycle, the latter two of which are localized to the mitochondrion, as is the bulk of coupled ATP synthesis. Reoxidation of the reduced cofactors (NADH and FADH2) requires molecular oxygen and is carried out by protein complexes integral to the inner mitochondrial membrane, collectively known as the respiratory, electron transport, or cytochrome, chain. Ubiquinone (UQ), and the small soluble protein cytochrome c, act as carriers of electrons between the complexes (Fig. 13.1.1). 

In general, carbohydrates are broken down aerobically, i.e. with a supply of oxygen. Anaerobic glycolysis seems to be more of a bypass for cells with a poor oxygen supply. The Embden-Meyerhof pathway is significant primarily because it operates not only under anaerobic conditions but, up to the step of pyruvate, under aerobic conditions as well (at least to the extent that glucose is not oxidized directly to pentose (Section 6, Chapt. XVIlI-1). Reduced nicotinamide-adenine dinucleotide is reoxidized through the respiratory chain, and pyruvate can be metabolized further in a variety of ways. 

The mitochondrial electron transport chain (ETC) or respiratory chain comprises a series of membrane-bound redox-aetive intermediates including flavoproteins, quinones, cytochromes, and iron-sulfur elusters [9] (Fig. 1). The latter facilitate the thermodynamically controlled transfer of electrons from conjugate redox pairs of low redox potential (E°) [e.g., —320 mV for reduced nicotinamide-adenine dinueleotide/oxidized nieotinamide-adenine dinucleotide (NADH/ NAD )] to the final electron donor, O2 (with a high redox potential of -1-820... 

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