Succinate redox potential

Two reactions of the reverse electron transfer are of physiological significance. 1 mean (i) oxidation of succinate (redox potential, +0.03 V) by NAD (redox potential, -0.32 V) and (ii) oxidation of NADH by NADPH responsible for maintenance of [NADPH]/NADP ] [NADH]/[NAD ] in spite of the fact that redox potential of the NADPH/NADH pair is almost equal to that of NADH/NAD pair. The former process includes a reversal of NADH-CoQ reductase (Complex I of the respiratory chain). Usually it operates as a A 1h+ generator catalysing the downhill electron transfer from NADH to CoQ. However, when NAD is reduced by succinate, the same complex acts as a ApH consumer carrying out the uphill transfer of electrons from C0QH2 to NAD" [5]. 

Boxes indicate electron-transport chain complexes, whereas ovals represent the electron transporters UQ, RQ and cytochrome c. The open boxes represent complexes involved in the classical aerobic respiratory chain, whereas grey boxes represent complexes involved in malate dismutation. The vertical bar represents a scale for the standard redox potentials in mV. Translocation of protons by the complexes is indicated by H+ +. Abbreviations Cl, Clll and CIV, complexes I, III and IV of the respiratory chain cyt c, cytochrome c FRD, fumarate reductase Fum, fumarate SDH, succinate dehydrogenase Succ, succinate RQ, rhodoquinone UQ, ubiquinone. 

Fig. 5.3. The major components involved in mitochondrial NADH oxidation in facultative anaerobic mitochondria. In anaerobically functioning mitochondria, NADH is oxidized either by soluble enzymes (left) or by membrane-bound complexes of the electron-transport chain (middle). Under aerobic conditions, a classic respiratory chain is used to oxidize NADH (right). Proton translocation is indicated by H with arrows. Ovals represent the electron transporters RQ, UQ and cytochrome c (cyt. c), and electron transport is indicated by dashed arrows. The vertical bar represents a scale for the standard redox potentials in millivolts. Fum fumarate, NADH-DH NADH dehydrogenase, NADH-ECR soluble NADH enoyl-CoA reductase, RQH2 rhodoquinol, Succ succinate, UQH2 ubiquinol...

Most anaerobically functioning mitochondria use endogenously produced fumarate as a terminal electron-acceptor (see before) and thus contain a FRD as the final respiratory chain complex (Behm 1991). The reduction of fumarate is the reversal of succinate oxidation, a Krebs cycle reaction catalysed by succinate dehydrogenase (SDH), also known as complex II of the electron-transport chain (Fig. 5.3). The interconversion of succinate and fumarate is readily reversible by FRD and SDH complexes in vitro. However, under standard conditions in the cell, oxidation and reduction reactions preferentially occur when electrons are transferred to an acceptor with a higher standard redox potential therefore, electrons derived from the oxidation of succinate to fumarate (E° = + 30 mV) are transferred by SDH to ubiquinone,... 

Adenosine triphosphatase cytochrome c oxidase and, 321 transhydrogenase and, 79 Adenosine triphosphate choline dehydrogenase and, 263 cytocrome c oxidase and, 300, 302, 343 glyceraldehyde-3-phosphate dehydrogenase and, 25, 26, 48, 49 lipoamide dehydrogenase and, 125 NADH dehydrogenase and, 207 redox potentials and, 215, 216 succinate dehydrogenase and, 247, 248, 249... 

Flavocytochrome Fumarate Reductase. The flavocytochrome fumarate reductase (Fff) is a soluble periplasmic protein from Shewanella spp. that reduces fumarate but does not oxidize succinate, in contrast to the membrane-bound fumarate reductases that are related to succinate dehydrogenases, and transfer electrons from quinol to fumarate. It is a monomeric protein of 63.8 kDa that is composed of three domains. The N-terminal domain contains four c-type hemes, and the flavin domain contains noncovalently bound FAD and is related to flavoprotein subunits of membrane-bound fumarate reductases and succinate dehydrogenases. There is also a third domain in the flavocytochromes that has considerable flexibility and may be involved in controlling access of substrate to the active site. The macroscopic redox potentials of the fom hemes of Ffr are —102, —146, —196, and -23 8 mV, while that of FAD is —152 mV. The low redox potential of FAD in Ffr compared to that in membrane-bound fumarate reductase (—55 mV) may explain why it is unable to oxidize succinate. 

The respiratory chain catalyses transfer of reducing equivalents from NADH generated in the mitochondrial matrix or M space, to dioxygen (Fig. 2.1A). Fig. 2.1B shows a thermodynamic view, giving the operational redox potentials ( ,) for the main individual components (for details, see below). The total redox span is about 1.11 V for oxidation of NADH, and about 760 mV for oxidation of ubiquinol (or succinate). 

A simplified representation of the mitochondrial respiratory chain in terms of the oxidation of NADH and succinate by oxygen is illustrated in Figure 4.9 together with Table 4.2 showing the twelve iron sulphur proteins identified, their g values and mid-point redox potentials. The g values found are consistent from a variety of different preparations, though some changes in line share are found. The mid-point potentials are variable, with values from -20 mV to -265 mV for centre N-2. 

Besides non-enzymatic 02 generation. Of can be enzymatically formed as a result of the ApH+-consuming reverse electron transfer from succinate to O2. In fact, standard redox potential of fumarate/succinate is slightly positive whereas that of OfOf is negative. It was found that ApH+ generated by succinate oxidation via Complexes III and IV can be used to reduce O2 to 02 (eq. 4) ... 

The individual electron carriers of the four complexes of the respiratory chain, shown in Figure 14-4, are arranged in accordance with their redox potentials, with the transfer of electrons from NADH to oxygen associated with a potential drop of 1.12 V, and that of succinate to oxygen of 0.8 V. In the electron transport system, the electrons can be transferred as hydride ions (H ) or as electrons (e.g., in the cytochromes). 

The addition of ATP to anaerobic or terminally inhibited mitochondria or submitochondrial particles containing succinate Eo = 0.03 V at pH 7) induces reduction of cjdiochrome bj 16,17,65 see also 6 6). The original concept of the possible mechanism of this phenomenon described by Wilson and Dutton 19) was that the Eo of cytochrome f T changes because of the formation of a high energy derivative which is the primary intermediate for site 2 energy conservation reaction in oxidative phosphorylation. However, there has been another possible mechanism presented in which ATP can induce reduction of cytochrome bx by the decrease in the effective redox potential Ek) of the cytochrome because of reversed electron flow 57) or of the abolition of an accessibility barrier between the substrate and the cytochrome 58). The former explanation would be favored by the chemical hypothesis of oxidative phosphorylation, while the latter is favorable for the chemiosmotic hypothesis. 

Figure 31. An electron transport system and redox potentials in mitochondria. FMN refers to Flavin mononucleotide in NADH2 dehydrogenase, FAD refers to Flavin adenine dinucleotide in succinate dehydrogenase, I, II, and III correspond to the reaction processes which may be involved in phosphorylation, Fe—S non-heme iron, Cyt Heme in cytochromes (after ref. 171).

Reversed electron transport reversal of Oxidative phosphorylation (see) in which NAD is reduced by an ATP-dependent reverse transport of electrons. R.e.t. occurs in organisms that oxidize hydrogen donors whose redox potential (see Oxidation) is more positive than that of the pjmdine nucleotide coenzymes, and it operates in the oxidation of substrates not specific for NAD (see Respiratory chain), e.g. Succinate + NAD - Fumate + NADH + H. Tlie redox system succinate/fumarate (E o = -tO - 031V) is 325 mV more positive than the redox system NAJD / NADH + H (E o =-0.32 V) electrons are passed firom succinate to flavoprotein in the respiratory chain, then via NADH-dehydrogenase to NAD. R.e.t. has been shown in nitrate bacteria (Nitrobac-ter), insect flight muscle mitochondria and kidney mi-... 

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