The components of the electron transport chain differ between procaryotes and eu-caryotes, and even among bacterial systems, thus details may differ from the example outlined below. The purpose of the electron transport is the same for all systems, however, that is, the transfer of electrons from NADH/FADH2 via a series of carriers to, ultimately, oxygen. Around half of the energy released during this process is conserved as ATP.
The carrier molecules, which act alternately as acceptors and donors of electrons, are mostly complex modified proteins such as flavoproteins and cytochromes, together with a class of lipid-soluble molecules called ubiquinones (also called coenzyme Q). The carriers are arranged in the chain such that each one has a more positive redox potential than the previous one. In the first step in the chain, NADH passes electrons to flavin mononucleotide (FMN), and in so doing becomes converted back to NAD+, thereby ensuring a ready supply of the latter for the continuation of glycolysis (Figure 6.21). From FMN, the electrons are transferred to coenzyme Q, and thence to a series of cytochromes; at each transfer of electrons the donor reverts back to its oxidised form, ready to pick up more electrons. You may recall that FADH2 yields only two, rather than three molecules of ATP per molecule; this is because it enters the electron transport chain at a later point than NADH, thereby missing one of the points where export of protons occurs. The final cytochrome in the chain transfers its electrons to molecular oxygen, which, as we have seen, acts as the terminal oxygen acceptor. The negatively charged oxygen combines with protons from its surroundings to form water. Four electrons and protons are required for the formation of each water molecule:
Since two electrons are released by the oxidation of each NADH, it follows that two NADH are needed for the oxidation of each oxygen.
How does this transfer of electrons lead to the formation of ATP? The chemiosmotic theory proposed by Peter Mitchell in 1961 offers an explanation. Although it was not immediately accepted, the validity of the chemiosmotic model is now widely recognised, and in 1978 Mitchell received a Nobel Prize for his work. As envisaged by Mitchell, sufficient energy is released at three points in the electron transport chain for the transfer of protons to the outside of the membrane, resulting in a gradient of both concentration and charge (proton motive force). The protons are able to return across the membrane and achieve an equilibrium through specific protein channels within the enzyme ATP
Figure 6.22 Chemiosmosis. The active transport of protons across the membrane creates a gradient of charge and concentration (proton motive force). Special channels containing ATP synthase allow the return of the protons; the energy released is captured as ATP. From Hames, BD, Hooper, NM & Houghton, JD: Instant Notes in Biochemistry, Bios Scientific Publishers, 1997. Reproduced by permission of Thomson Publishing Services
Figure 6.22 Chemiosmosis. The active transport of protons across the membrane creates a gradient of charge and concentration (proton motive force). Special channels containing ATP synthase allow the return of the protons; the energy released is captured as ATP. From Hames, BD, Hooper, NM & Houghton, JD: Instant Notes in Biochemistry, Bios Scientific Publishers, 1997. Reproduced by permission of Thomson Publishing Services synthase. The energy released by the protons as they return through these channels enables the ATP synthase to convert ADP to ATP (Figure 6.22).
Aerobic respiration in eucaryotes is slightly less efficient than in procaryotes due to the fact that the three stages take place at separate locations (see Table 6.2). Thus the total number of ATPs generated is 36 rather than the 38 in procaryotes (Table 6.3).
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