Coenzyme electron transfer from NADH

Complex I mediates electron transfer from NADH + H+ to coenzyme Q. The consequent reduction of coenzyme Q is coupled to vectorial proton transport the H+/e stoichiometry (ratio) has been variously estimated to be 1.0, 1.5, and 2.0. 

NADH-coenzyme Q (CoQ) oxidoreductase, transfers electrons stepwise from NADH, through a flavoprotein (containing FMN as cofactor) to a series of iron-sulfur clusters (which will be discussed in Chapter 13) and ultimately to CoQ, a lipid-soluble quinone, which transfers its electrons to Complex III. A If, for the couple NADH/CoQ is 0.36 V, corresponding to a AG° of —69.5 kJ/mol and in the process of electron transfer, protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes). 

Figure 7-4. The electron transport chain. Electrons enter from NADH to complex I or succinate dehydrogenase, which is complex II. Electrons derived from glycolysis through the glycerol-3-phosphate shuttle, complex I, and complex II join at coenzyme Q and are transferred to oxygen as shown. As electrons pass through complexes I, III, and IV, protons are transported across the membrane, creating a pH gradient.

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

Coenzyme Q passes electrons through iron-sulfur complexes to cytochromes b and ch which transfer the electrons to cytochrome c. In the ferric Fe3+ state, the heme iron can accept one electron and be reduced to the ferrous state Fe2+. Since the cytochromes carry one electron at a time, two molecules on each cytochrome complex are reduced for every molecule of NADH that is oxidized. The electron transfer from coenzyme Q to cytochrome c produces energy, which pumps protons across the inner mitochondrial membrane. The proton gradient produces one ATP for every coenzyme Q-hydrogen that transfers two electrons to cytochrome c. Electrons from FADH2, produced by reactions such as the oxidation of succinate to fumarate, enter the electron transfer chain at the coenzyme Q level. 

We already encountered the respiratory electron transfer chain in Chapter 5, and in the present context. Figure 13.8 serves as a reminder that the structures of many of the components have been determined (Hosier, Ferguson-Miller, Mills, 2006). Electrons flow from NADH/NAD" " and succinate (Complexes I and II) via Coenzyme Q to the cytochrome bc complex (Complex III) and are then transferred via cytochrome c to cytochrome c oxidase (CcO) (Complex IV). We will discuss the electron transport cytochromes in the next section. 

The direct electron transfer from the coenzyme NADH to the electrode can be facilitated using organic conducting salts such as NMP+TCNQ" (see 3.3.l.f). These salts reduce the cofactor redox potential required, which limits any interference phenomena and fouling up of the electrodes [43]. These problems can also be overcome using an electrode with a hydrophobic membrane (p02 electrode) on which alcohol oxidase and catalase [170] can be immobilized. These enzymes catalyse the following reactions ... 

As its name implies, this complex transfers a pair of electrons from NADH to coenzyme Q a small, hydrophobic, yellow compound. Another common name for this enzyme complex is NADH dehydrogenase. The complex (with an estimated mass of 850 kD) involves more than 30 polypeptide chains, one molecule of flavin mononucleotide (FMN), and as many as seven Fe-S clusters, together containing a total of 20 to 26 iron atoms (Table 21.2). By virtue of its dependence on FMN, NADH-UQ reductase is a jlavoprotein. 

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 first of these new, electron transferring components was coenzyme Q (CoQ). Festenstein in R.A. Morton s laboratory in Liverpool had isolated crude preparations from intestinal mucosa in 1955. Purer material was obtained the next year from rat liver by Morton. The material was lipid soluble, widely distributed, and had the properties of a quinone and so was initially called ubiquinone. Its function was unclear. At the same time Crane, Hatefi and Lester in Wisconsin were trying to identify the substances in the electron transport chain acting between NADH and cytochrome b. Using lipid extractants they isolated a new quininoid coenzyme which showed redox changes in respiration. They called it coenzyme Q (CoQ). CoQ was later shown to be identical to ubiquinone. 

Most compounds oxidized by the electron transport chain donate hydrogen to NAD+, and then NADH is reoxidized in a reaction coupled to reduction of a flavoprotein. During this transformation, sufficient energy is released to enable synthesis of ATP from ADP. The reduced flavoprotein is reoxidized via reduction of coenzyme Q subsequent redox reactions then involve cytochromes and electron transfer processes rather than hydrogen transfer. In two of these cytochrome redox reactions, there is sufficient energy release to allow ATP synthesis. In... 

The nicotinamide coenzymes are biological carriers of reducing equivalents (electrons). The most common function of NAD+ is to accept two electrons and a proton (H equivalent) from a substrate undergoing metabolic oxida-tion to produce NADH, the reduced form of the coenzyme. This then diffuses or is transported to the terminal-electron transfer sites of the cell and reoxidized by terminal-electron acceptors, 02 in aerobic organisms, with the concomitant formation of ATP (chapter 14). Equations (8), (9), and (10) are typical reactions in which NAD+ acts as such an acceptor. 

The biochemical importance of flavin coenzymes ap-pears to be their versatility in mediating a variety of redox processes, including electron transfer and the activation of molecular oxygen for oxygenation reactions. An especially important manifestation of their redox versatility is their ability to serve as the switch point from the two-electron processes, which predominate in cytosolic carbon metabo-lism, to the one-electron transfer processes, which predomi-nate in membrane-associated terminal electron-transfer pathways. In mammalian cells, for example, the end products of the aerobic metabolism of glucose are C02 and NADH (see chapter 13). The terminal electron-transfer pathway is a membrane-bound system of cytochromes, nonheme iron proteins, and copper-heme proteins—all one-electron acceptors that transfer electrons ultimately to 02 to produce H20 and NAD+ with the concomitant production of ATP from ADP and P . The interaction of NADH with this pathway is mediated by NADH dehydrogenase, a flavoprotein that couples the two-electron oxidation of NADH with the one-electron reductive processes of the membrane. 

Electrons enter the ETC at respiratory Complexes I and II. The electrons from NADH enter at respiratory Complex I (RC I, NADH dehydrogenase) with the concomitant oxidation of NADH to NAD+. The electrons carried by FADH2 are transferred to RC II (succinate dehydrogenase) as the FADH2 is oxidized to FAD and succinate is reduced to fumarate. These electrons from RC I and II are transferred to the quinone form of coenzyme Q (CoQ), which delivers them to RC III (UQ-cytochrome c reductase). Cytochrome c then accepts the electrons from RC III, and the reduced cytochrome c is reoxidized as it delivers the electrons to RC IV, cytochrome c oxidase. The electrons are then used by RC IV to reduce molecular oxygen to water. 

Electron transport has three major stages (1) transfer of electrons from NADH to coenzyme Q, (2) electron transport from coenzyme Q to cytochrome c, and (3) electron transport from cytochrome c to oxygen. These stages are briefly described below. 

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. 

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