The Sequence of Electron Carriers

FIGURE 19-6 Method for determining the sequence of electron carriers. This method measures the effects of inhibitors of electron transfer on the oxidation state of each carrier. In the presence of an electron donor and 02/ each inhibitor causes a characteristic pattern of oxidized/reduced carriers those before the block become reduced (blue), and those after the block become oxidized (pink). 

A Bucket Brigade of Molecules Carries Electrons from the TCA Cycle to 02 The Sequence of Electron Carriers Was Deduced from Kinetic Measurements Redox Potentials Give a Measure of Oxidizing and Reducing Strengths... 

The Sequence of Electron Carriers Was Deduced from Kinetic Measurements... 

Chem. 217 395, 1955. One of a series of papers developing kinetic techniques for elucidating the sequence of electron carriers in the respiratory chain. 

Briefly, the sequence of electron carriers within the PS 1 reaction centre can be summarised as ... 

Oxidative phosphorylation is susceptible to inhibition at all stages of the process. Specific inhibitors of electron transport were invaluable in revealing the sequence of electron carriers in the respiratory chain. For example, rotenone and amytal block electron transfer in NADH-Q oxidoreductase and thereby prevent the utilization of NADH as a substrate (Figure 18.43). In contrast, electron flow resulting from the oxidation of succinate is unimpaired, because these electrons enter through QH2, beyond the block. AntimycinA interferes with electron flow from cytochrome h Q-cytochrome c... 

Figure 9.6 Sequence of electron carriers in the electron transfer chain. The positions of entry into the chain from metabolism of glucose, glutamine, fatty acyl-CoA, glycerol 3-phosphate and others that are oxidised by the Krebs cycle are shown. The chain is usually considered to start with NADH and finish with cytochrome oxidase. FMN is flavin mononucleotide FAD is flavin adenine dinucleotide.

Reoxidation of the reduced carriers NADH and FADH2 actually involves a sequence of electron carriers, the electron transport chain, whose function is indicated below the circle near the center of Fig. 10-1. The oxidation of reduced NADH by 02 (Eq. 10-7) is a highly exergonic process and is accompanied by the... 

Many of the reactions of the plastocyanins and azurins with other redox proteins follow Marcus behaviour.946 These reactions all show a single mechanism of electron transfer, with no kinetic selectivity and no specific interactions between the proteins. The notable exception to this behaviour is cytochrome / (c552), where a specific interaction occurs,934 appropriate for its natural redox partner. Equation (48) represents a probable sequence of electron carriers, although it is difficult to extrapolate conclusions to the membrane-bound proteins. 

Sometimes electron transfer via the transport of electron carrier and via electron exchange reactions occur simultaneously. Co-existence of both these channels was observed for dark electron transfer across the viologen-containing vesicle membrane [169, 201]. To illustrate this let us turn back to the experiments shown schematically in Fig. 5 a. In accordance with the reaction sequence (34)-(37) and... 

Figure 18.9. Sequence of Electron Carriers in the Respiratory Chain.

Figure 18.6 Sequence of electron carriers in the respiratory chain. Notice that the electron affinity of the components increases as electrons move down the chain.

Electron transport chain (1) A series of compounds that pass electrons to oxygen (the final electron acceptor). (2) A sequence of electron carriers of progressively higher reduction potential in a cell that is linked so that electrons can pass from one carrier to the next. The chain captures some of the energy released by the flow of electrons and uses it to drive the synthesis of ATP. 

The elucidation of this sequence of electron carriers has been achieved, for the most part, by the use of mitochondria isolated from cells in such a way as to maintain for a time much of their in vivo activity. 

In a final confirmation, agents that inhibit the flow of electrons through the chain have been used in combination with measurements of the degree of oxidation of each carrier. In the presence of 02 and an electron donor, carriers that function before the inhibited step become fully reduced, and those that function after this step are completely oxidized (Fig. 19-6). By using several inhibitors that block different steps in the chain, investigators have determined the entire sequence it is the same as deduced in the first two approaches. 

Many approaches have been used to deduce the sequence of carriers through which electron flow takes place (Fig. 18-5). In the first place, it seemed reasonable to suppose that the carriers should lie in order of increasing oxidation-reduction potential going from left to right of the figure. However, since the redox potentials existing in the mitochondria may be somewhat different from those in isolated enzyme preparations, this need not be strictly true. 

The b cytochromes and cytochrome c, fit into this scheme between reducing substrates and cytochrome c. The idea thus developed that the respiratory apparatus includes a chain of cytochromes that operate in a defined sequence. The next question was whether the cytochromes are all bound together in a giant complex, or whether they diffuse independently in the membrane. Before we address this point, we need to consider three other types of electron carriers that participate in the electron-transport chain flavo-proteins, iron-sulfur proteins, and ubiquinone. 

The sequence of carriers in the respiratory chain should be generally consistent with the relative E° values of the carriers because, given equal concentrations of reactants and products, electrons flow from a carrier with a more negative E° value to one with a more positive value. The sequence of flavoproteins, UQ, and cytochromes that we presented in equation (4) agrees with this expectation. This proposition is demonstrated by plotting the E° values as a function of the carriers suggested positions in the chain (fig. 14.7). However, the E° values do not dictate the de-... 

The overpotential 77 is required to overcome hindrance of the overall electrode reaction, which is usually composed of the sequence of partial reactions. There are four possible partial reactions and thus four types of rate control charge transfer, diffusion, chemical reaction, and crystallization. Charge-transfer reaction involves transfer of charge carriers, ions or electrons, across the double layer. This transfer occurs between the electrode and an ion, or molecule. The charge-transfer reaction is the only partial reaction directly affected by the electrode potential. Thus, the rate of charge-transfer reaction is determined by the electrode potential. 

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