The electron transfer chain

Metabolism of the major fuels, described above, generates hydrogen atoms or electrons. These are oxidised, not directly, but via a series of oxidations and reductions (oxido-reduction reactions or, alternatively, redox reactions) that 

These enzymes, together with the proteins that generate ATP from ADP and P, (phosphate), constitute 30-40% of the total protein of the inner mitochondrial membrane. 

In summary, the electron transfer chain consists of a series of membrane-bound enzymes that possess different prosthetic groups which become alternately reduced and oxidised as they transfer electrons (or hydrogen atoms), from one carrier to the next in sequence, to oxygen to produce H2O. 

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NDO can be classified as class III dioxygenase the electron transfer chain involves a Rieske-type ferredoxin. Electrons enter NDO through the Rieske-type cluster of the dioxygenase. Kauppi et al. (11) have suggested that the binding site of NDO for the ferredoxin involves the 6 strands 10 and 12 of the Rieske domain as well as residues from the catalytic domain that form a depression in the protein surface close to Cys 101, which is a ligand of the Rieske cluster. In Rieske proteins from be complexes, access to this side of the cluster is blocked by an acidic surface residue (Asp 152 in the ISF, Glu 120 in RFS). 

Midpoint potential values are useful quantitites for defining the role of the various centers in the system. In some instances, these values have even been used to predict the location of the centers in the electron transfer chain, assuming that the potential increases along the chain from the electron donor to the electron acceptor. In several oxidoreductases, however, the measured potential of some centers was found to be clearly outside the range defined by the donor and the acceptor, which raised an intriguing question as to their function. This was observed, for instance, in the case of the [4Fe-4S] (Eni = -320 mV) center in E. coli fumarate reductase (249), the [3Fe-4S] + (Era = -30 mV) center in D. gigas hydrogenase (207), and the low-potential [4Fe-4S] + + (E, = 200 and -400 mV) centers in E. 

Table 6.2 Apparent formal redox potentials of systems present in the electron-transfer chain (pH = 7). It should be noted that the potential values were obtained in the homogeneous phase. Due to stabilization in a membrane, the oxidation-reduction properties vary so that the data listed below are of orientation character...

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

Fds with conventional [Fe2-S2] clusters can undergo a one-electron transfer to a deeply valence-trapped FemFen species. For proteins of known structure (and presumably others) one iron atom is closer to the surface (by about 0.5 nm) and it has been established that the added electron resides on that atom. No instances are known where an [Fe2-S2] centre acts as a physiological two-electron donor or acceptor. In addition to the conventional [Fe2-S2] ferredoxins, the electron-transfer chains of mitochondria and photosynthetic bacteria contain Rieske proteins which have a cluster with the composition [(Cys.S)2FeS2Fe(N.His)2], in which the two imidazole groups are bound to the same iron atom (Figure 2.9). This atom is the site... 

By 1949 low temperature spectroscopy had been introduced. With this technique Keilin and Hartree detected a further component in the electron transfer chain which had a sharp band at 552 nm. They later showed it to be identical with cytochrome cj, which had first been observed by Yakushiji and Okunuki (1940) during succinate oxidation by cyanide-inhibited beef heart muscle. As the oxidation of cytochrome C was accelerated by cytochrome c, Okunuki and Yakushiji (1941) had placed C] in the chain in the order... 

The electron-transfer chain (ETC) catalytic process (or, electrocatalysis) is the catalysis of a reaction triggered by electrons (through a minimal quantity of an oxidizing or reducing agent) without the occurrence of an overall change in the oxidation state of the reagent. 

Cytochrome P-450 has been characterized in four stable states [Fe, Fe " RH, Fe RH, (O2—Fe ) RH (metastable)] during its oxygenase reaction cycle. In the complete native system a flavoprotein and a redoxin (putidaredoxin) act as electron donors but also as effectors that complement the cytochrome. In the more complex microsomal system the sequence and intermediates are less well defined the electron-transfer chain contains two flavoproteins and one cytochrome, whose interactions with cytochrome P-450 are still the subject of great controversy. 

Cytochrome c and cytochrome/both are involved in the electron transfer chain from glucose metabolites to molecular oxygen in aerobic organisms. From values of the half-cell potentials of cytochrome c and of cytochrome/at 30°C, it is possible to calculate that is 0.11 V for the reaction in Equation (12.12) [4]. Hence... 

The enzyme is present in the mitochondria so that the FADH2 is oxidised by the electron transfer chain. The enzyme is worthy of note since it catalyses the initiating... 

The connections between the phases are provided by coenzymes, which become reduced in glycolysis, P-oxidation and the Krebs cycle and, subsequently, transfer hydrogen atoms or electrons into the electron transfer chain. These are ultimately oxidised by oxygen, and ATP is generated. 

The Krebs cycle will only operate when the hydrogen atoms and electrons produced in the cycle enter the electron transfer chain, ultimately to react with oxygen that is, the two processes must take place simultaneously. A metabolic pathway is defined as a sequence of reactions that is initiated by a flux-generating step. In the cycle, citrate synthase catalyses the flux-generating reaction (Table 9.2) but there is no such reaction in the electron-transfer chain. Consequently, the cycle can be considered to be the first part of a longer pathway, which includes the electron transfer chain (Figure 9.3). 

Figure 9.3 k summary of the Physiological pathway of the Krebs cycle. The pathway starts with acetyl-CoA, since citrate synthesis is the flux-generating step. The physiological pathway includes the electron transfer chain, since there is no flux-generating step in this chain. The pathway is indicated by the broader lines. The pathway, therefore, starts with acetyl-CoA and finishes with CO2 and H2O, which are lost to the environment. Acetyl-CoA is formed from a variety of precursors glucose and fatty acids are presented in this figure. 

Figure 9.5 A summary of pathways of the three main fueb and the positions where they enter the cycle. The figure also shows the release of hydrogen atoms/electrons and their transfer into the electron transfer chain for generation of ATP and formation of water. Glutamine is converted to glutamate by deamidation and glutamate is converted to oxoglutarate by transamination or deamination. The process of glycolysis also generates ATP as shown in the Figure.

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.

Figure 9.7 The sequence of electron transfer complexes in the electron transfer chain. The regions enclosed by broken lines indicate the association of cam ers in complexes. Their constituents are listed in Table 9.4. Note that electrons may enter at the level of ubiquinone from sources other than succinate. Complex V is FoFi ATPase (Table 9.4).

Figure 9.14 Simple diagram illustrating the transport of the major fuels for the Krebs q/cle, and hydrogen atoms for the electron transfer chain, across the inner mitochondrial membrane.

Under aerobic conditions, the hydrogen atoms of NtUDH are oxidised within the mitochondrion pyruvate is also oxidised in the mitochondrion (Figure 9.15). However, NADH cannot be transported across the inner mitochondrial membrane, and neither can the hydrogen atoms themselves. This problem is overcome by means of a substrate shuttle. In principle, this involves a reaction between NADH and an oxidised substrate to produce a reduced product in the cytosol, followed by transport of the reduced product into the mitochondrion, where it is oxidised to produce hydrogen atoms or electrons, for entry into the electron transfer chain. Finally, the oxidised compound is transported back into the cytosol. The principle of the shuttle is shown in Figure 9.16. 

Figure 9.15 Fate of NADH produced in glycolysis. In hypoxic or anoxic conditions, pyruvate is converted to lactate with oxidation of NADH. In aerobic conditions, NADH is oxidised as shown in Figure 9.17 or 9.18 and pyruvate is oxidised via the Krebs cycle and the electron transfer chain.

The hydrogen atoms from NADH are then transferred along the electron transfer chain to be oxidised by oxygen. 

Citrate synthase There are three properties of citrate synthase that are relevant to regnlation. The prodnct of the reaction, citrate, is an allosteric inhibitor of the enzyme. The concentration of acetyl-CoA in mnscle is weU above the ATm of citrate synthase for acetyl-CoA. Conseqnently, the activity of this enzyme is flnx-generating for the cycle pins the transfer of electrons along the electron transfer chain, i.e. the process from acetyl-CoA to molecnlar oxygen can be considered as a transmission seqnence , as defined in Chapter 3. In contrast, the concentration of oxaloacetate is weU below the ATm, so that variations in its concentration can regnlate the enzyme activity and therefore, the flnx throngh the cycle. 

Control of flow of electrons along the electron transfer chain... 

An important point in the regulation of these processes is that all the reactions from mitochondrial NADH, to and including cytochrome c, are near-equilibrium (Figure 9.26(a)) (Appendix 9.8) that is, there is only one reaction in the electron transfer chain that is non-equihbrium - the terminal reaction catalysed by cytochrome oxidase. There is some similarity with the process of glycolysis in which the initial reaction and the terminal reactions are the nonequilibrium reactions (Figure 9.26(b)). 

Figure 9.26 (a) Near-equUibiium and non-equilibrium reactions in the electron transfer chain. The electron transfer chain is considered to be the Latter part of the physiological Krebs cycle (see above). The non-equilibrium processes are the Krebs cycle and the terminal reaction cytochrome oxidase. All other reactions are near-equilibrium, including the ATP-generating reactions. These are not shown in the figure, (b) The similarity of electron transfer chain and glycolysis in the position of near-equilibrium/non-equilibrium reactions, in the two pathways. In both cases, non-equilibrium reactions are at the beginning and at the end of the processes (see Chapters 2 and 3 for description of these terms and the means by which such reactions can be identified). 

The number of molecules of ATP that can be generated from the transfer of a given number of hydrogens (or electrons) along the electron transfer chain to oxygen is... 

Oxidation of two electrons transferred along the electron transfer chain from NADH to oxygen generates 2.5 molecules of ATP. 

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