Electron transport chain reduction potentials

Although electrons move from more negative to more positive reduction potentials in the electron transport chain, it should be emphasized that the electron carriers do not operate in a simple linear sequence. This will become evident when the individual components of the electron transport chain are discussed in the following paragraphs. 

Coenzyme QIO (21) is one of the essential enzymes in the mitochondrial electron transport chain, participating in the aerobic respiration cycle. The role of Co-QlO as a cardioprotective substance and an antioxidant are well studied. Recently, it was found that Co-QlO is also capable of attenuating the intracellular deposition of Ap in transgenic AD mouse models. Additionally, the same group reported that Co-QlO administration also led to reduction of preexisting plaque burden in the same model. Such properties are suggestive of a potential therapeutic role for Co-QlO in AD. 

Tyr 143, 36 293 Tyr 254, 36 291-293 NMR spectroscopy, 36 271-272 pH dependence, 36 274-275 primary stmcture, 36 261-263 prosthetic groups structure, 36 258 quaternary structure, 36 261-262 reduction potentials, 36 268-269 short electron transport chain, 36 258-259 site-directed mutagenesis, 36 289-290 substrate specificity, 36 272-274 Flavocytochrome C552 electrochemistry, 36 365-367, 369... 

In this chapter we will look at the processes by which reduced carriers such as NADH and FADH2 are oxidized within cells. Most familiar to us, because it is used in the human body, is aerobic respiration. Hydrogen atoms of NADH, FADH2, and other reduced carriers appear to be transferred through a chain of additional carriers of increasingly positive reduction potential and are finally combined with 02 to form H20. In fact, the hydrogen nuclei move freely as protons (or sometimes as H ions) it is the electrons that are deliberately transferred. For this reason, the chain of carriers is often called the electron transport chain. It is also referred to as the respiratory chain. 

TPhe properties of Cytochrome c, a vital component of the electron transport chain, have been studied widely (1). The reduction potential (E° ) of Cytochrome c has been measured by a variety of techniques and under different conditions, namely in the presence of various salts, as a function of temperature, bound to phospholipid vesicles and in the presence of other components of the electron transport chain. These studies have yielded a wide range of values for E° vs. the standard... 

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,... 

PS II is responsible for the oxidation of water and the reduction of a stable acceptor at the potential of ca. 0.0 to -0.2 V, while PS I transfers electrons from a donor of = 0.45 V to an acceptor of of ca. -0.65 V. An electron transport chain connects the reducing side of PS II to the oxidizing side of PS I, down the electrochemical gradient. At the reducing side of PS I NADP is reduced, while at the oxidizing side of PS II water is oxidized and Oj is evolved. 

Fig. 9. Proposed function of electrochemical and Na potentials in energy conservation coupled to methanol disproportionation to CH4 and CO2- It is assumed that prior to oxidation methanol binds first to coenzyme M and that the oxidation is mechanistically and energetically the reversal of CO2 reduction to methyl-coenzyme M. The Na /H" antiporter is involved in the generation of A/iNa Ifom AjlVt. CHO-MFR, formyl-methanofuran CH2=H4MPT, methylene-tetrahydromethanopterin CH3-H4MPT, methyl-tetrahydromethanopterin CH3-S-C0M, methyl-coenzymeM. The hatched boxes indicate membrane-bound electron transport chains or membrane-bound methyltransferase catalyzing either Na or H translocation (see Figs. 5, 6 and 12). ATP is synthesized via membrane-bound H -translocating ATP synthase. The stoichiometries of Na and of translocation were taken from refs. [105,107,167]. x, y and z are unknown stoichiometric factors.

Standard Oxidation-Reduction Potential (E" ) of Components of the Electron-Transport Chain... 

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. 

The electron transport chain (ETC) or electron transport system (ETS) shown in Figure 16-1 is located on the inner membrane of the mitochondrion and is responsible for the harnessing of free energy released as electrons travel from more reduced (more negative reduction potential, E to more oxidized (more positive carriers to drive the phosphorylation of ADP to ATP. Complex 1 accepts a pair of electrons from NADH ( = -0.32 V)... 

Fig, 4. The structure of the mitochondrial respiratory electron transport chain. Values in millivolts are standard (pH 7) reduction potentials for the components indicated. 

King, personal communication). As befits its position just before c in the electron transport chain, the standard reduction potential of is slightly lower, -)-225 mV at pH 7. The standard midpoint potentials as measured in the mitochondria are even closer than the solution values +235 mV for c and +225 mV for Ci (7). 

A troublesome aspect of Fig. 41 is the low standard reduction potential of the hydrogen couple at the right. By the criterion of standard potentials, C3 should not be able to donate electrons to H. But these standard potentials assume equal amounts of reduced and oxidized C3, at pH 7, and 1 atm partial pressure of Ha, all of which may be unrealistic. If the ferredoxin and C3 were predominantly in the reduced state, and if the partial pressure of Ha was low, then the ferredoxin and C3 potentials would be shifted down, and the hydrogen potential would be raised. The issue is not whether this electron transport chain works it does. The problem is only to reconcile this with the standard reduction potentials by an appropriate adjustment of concentrations. 

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