Electron transport chain redox potential

NADH and reduced substrate dehydrogenase-flavoproteins (FPH2) must be continually reoxidized for mitochondrial oxidations to proceed. This is achieved by the electron transport chain (respiratory chain) which is a series of redox carriers of graded redox potential in the inner mitochondrial membrane (Appendix 1) that catalyzes the net reactions ... 

Redox potentials were also used to arrange the electron carriers in their correct order. This procedure was applied to the cytochromes by Coolidge (1932). There were however serious difficulties. Electrochemical theory applies to substances in solution the values obtained are significantly affected by pH and the concentrations of the different components. Of the members of the electron transport chain only the substrates NAD+, NADP+, and cytochrome c are soluble. The other components were difficult to extract from tissue particles without altering their properties. Further, it was hard to determine their concentration and to decide on appropriate values for pH and oxygen concentration. Nevertheless, mainly from work by Ball (1938), at the time in Warburg s laboratory, an approximate order of redox potentials was drawn up ... 

Another kind of experiment is to equilibrate the electron transport chain with an external redox pair of known potential using uncoupled mitochondria. 

A high content of linolenate in the thylakoid membranes would, most probably, make them more fluid and also provide a medium of low dielectric constant. In this medium, the electron-transport chains that are inhibited by water can function well.384-386 It was found that photoreduction of cytochrome C is increased by the addition of MGDG and DGDG.387 A complex that contained 12% of manganese, DGDG, and a flavine was isolated from a variety of leaves388 this was found to have a high redox potential, and thus, it may participate as an oxidizer. 

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. 

Bipyridyliums with redox potentials in the range of -300 to -500 mV, such as diquat and paraquat, can accept electrons in competition with the acceptor of photosystem I (Figure 2, site 4) and have herbicidal activity. Interception of electron flow from photosystem I essentially shunts the electron transport chain. 

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,... 

Figure 17.6 Electron transport chain showing redox potentials at each step. Note that electrons travel from components with a very negative redox potential (e.g., -0.32 V for NAD+/NADH) to one with a very high one (0.82 V for Oz). With each step, the AEq increases. The overall AE 0 is 1.14 V. (Reproduced by permission from Hall DO, Rao KK, Cammack R. Science Prog Oxford 62 285-317, 1975.)...

Figure 6-8. Components of the mitochondrial electron transport chain with midpoint redox potentials in parentheses. Also indicated are the four protein complexes (I-IV) involved. Spontaneous election flow occurs toward couples with higher (more positive) redox potentials, which is downward in the figure.

Cytochrome Ch is similar in most respects to other typical bacterial cytochromes c and C2- However its X-ray structure (Read et al., 1999) shows a number of unusual features it bears a closer gross resemblance to mitochondrial cytochrome c than to the bacterial cytochrome C2 and the left hand side of the haem cleft is unique. In particular it is highly hydrophobic, the usual water is absent, and the iconservediTyr67 is replaced by tryptophan. A number of features of the structure demonstrate that the usual hydrogen bonding network involving water in the haem channel is not essential, and that other mechanisms for modulation of redox potentials may exist in this cytochrome. It should be emphasised that the unique character of this cytochrome does not appear to be related in any way to its special involvement in oxidising cytochrome Ci in the methylotroph electron transport chain. 

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. 

The oxidized P700 is reduced by the copper protein, plastocyanin, present in the lumenal space ofthe thylakoid. Reflecting the overall reaction it supports, the PS-1 reaction center is sometimes called plastocyanin ferredoxin oxidoreductase. Note that in cyanobacteria the corresponding electron donor is cytochrome c552 rather than plastocyanin and some algal species when in a copper-deficient medium synthesize a c-type cytochrome as a replacement for plastocyanin. The complete electron-transport chain ofphotosystem 1 is shown in Fig. 2. The approximate redox potentials and halftimes for forward electron transfers at ambient temperature are also indicated. Perhaps to assure wasteful back reactions are mini-... 

Electron transport chain Present in the mitochondrial membrane, this linear array of redox active electron carriers consists of NADH dehydrogenase, coenzyme Q, cytochrome c reductase, cytochrome c, and cytochrome oxidase as well as ancillary iron sulfur proteins. The electron carriers are arrayed in order of decreasing reduction potential such that the last carrier has the most positive reduction potential and transfers electrons to oxygen. 

Recent research work has shown that there are in fact two different such reaction centres each receiving energy from two different pigment systems. The situation is further complicated by the fact that flow of electrons from H20 to NADPH2 involves the co-operation of both pigment systems in a sequence of events worked out by Hill and Bendall in 1960 and known as the Z scheme. The Z scheme bears, once again, a remarkable similarity to the respiratory electron transport chain, although it is even more complex and still poorly understood in parts, particularly the identity of many of the electron carriers. It is known, however, that they include plastoquinone, two cytochromes (f and b) and ferredoxin. The photosynthetic phosphorylation of ADP to ATP occurs if the difference in redox potential between the electron carriers is sufficient to allow this endothermic reaction to take... 

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