Electron transport chain standard redox potential

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,... 

The arrangement of components of the electron transport chain was deduced experimentally. Since electrons pass only from electronegative systems to electropositive systems, the carriers react according to their standard redox potential (Table 14-2). Specific inhibitors and spectroscopic analysis of respiratory chain components are used to identify the reduced and oxidized forms and also aid in the determination of the sequence of carriers. 

NADH and NADPH are equivalent in terms of their standard redox potentials, but because redox enzymes are usually selective for one or the other of them, two distinct pools of reductants exist. NADH is used as a source of reducing equivalents for the electron transport chain (ETC) while NADPH provides reducing equivalents for many biosynthetic reactions. Hence, even within a single spatial compartment such as the cytoplasm, the NADH to NAD+ ratio can be very low, favoring oxidation of fuels, while simultaneously the NADPH to NADP ratio can be very high, facilitating biosynthesis. 

The sequence of the carriers in the respiratory chain has been deduced from their redox potentials, the use of inhibitors of electron transport (Section 13.7) and enzyme specificities. Since electrons normally flow from more electronegative to more electropositive values, the standard redox potentials of the carriers should become progressively more positive towards oxygen. Figure 13.3 shows the established order of the complexes but within some complexes the order of carrier participation requires elucidation. 

ATP is synthesized from ADP and phosphate during electron transport in the respiratory chain. This type of phosphorylation is distinguished from substrate-level phosphorylation, which occurs as an integral part of specific reactions in glycolysis and the TCA cycle. The free energy available for the synthesis of ATP during electron transfer from NADH to oxygen can be calculated from the difference in the value of the standard potential of the electron donor system and that of the electron acceptor system. The standard potential of the NADH/NAD+ redox component is —0.32 V and that of H2O/5O2 is -1-0.82 V therefore, the standard potential difference between them is... 

Each of the coupled redox reactions in biological electron transport involves the transfer of electrons from one redox couple to another couple of higher reduction potential. Thus, each individual redox reaction in the sequence is exergonic under standard conditions. For electrons entering the respiratory chain as NADH, the overall reaction sequence is given by the following equation ... 

Pumps require energy to function. The "pumps" of the ETS chain derive their energy to transport protons from oxidation/reduction (called redox) reactions that occur as electrons move from one complex to another. To quantitate the amount of energy in these transfers, the standard reduction potential can be used. 

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