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Respiratory chain The respiratory chain is composed of four complexes on the inner mitochondrial membrane, or cristae walls. Three of these complexes, I, III and IV, termed the respirasome, pump protons from the matrix to the intermembraneous space, generating a proton gradient which provides a proton motive force. It is this proton motive force which powers the phosphorylation of ADP to produce ATP, the major currency of energy in the cell. This energy to pump protons across the inner mitochondrial membrane against their concentration or pH gradient (here effectively the same as pH is a logarithmic function of proton concentration) comes from the redox series of reactions that occurs along the length of the respiratory chain. The respiratory chain can be summarised thus: The reducing cofactor NADH arrives at complex I, NADH-Q oxidoreductase. This reduces the first component of complex I, FMN, which goes to FMNH2. This transfers electrons to a series of Fe-S clusters and eventually to Q, ubiquinone, which becomes the reduced form QH2 and is free to move in the membrane to Q pools. At every step the energy level of these electrons decreases, as they reduce the next component, simultaneously oxidising the component they leave and leaving them ready for the next batch of electrons. Energy is released at each step, and complex 1 uses this to pump protons across the IMM. Complex II is linked to the TCA cycle as one of the TCA steps, succinate ? fumarate, occurs here as the enzyme which catalyses it, succinate dehydrogenase, is actually part of this succinate-Q reductase complex. FADH2 is formed as the electron-accepting cofactor, and it stays in the complex, transferring electrons through iron-sulphur clusters to reduce ubiquinone and add to the QH2 in the Q pools. In this way the redox reaction of the TCA cycle is harnessed to contribute to the proton motive force, although complex II itself does not directly pump protons; the QH2 is oxidised in complex III, and the electrons it provides are used to pump protons. Complex III, also known as Q-cytochrome c oxidoreductase, transfers electrons from QH2 to cytochrome c, a electron-transferring protein with a haem group; the iron in this group alternatives between its oxidisation states of +2 and +3. The reduced cytochrome c then diffuses away from this enzyme to complex IV, cytochrome oxidase. Here, in cytochrome oxidase, the electrons from the reduced cytochrome c will eventually be transferred to the final acceptor, molecular oxygen O2, to form H2O, with the concurrent usage of 4H+ from the matrix in the reaction and the pumping of 4h+ from the matrix into the intermembranous space. This reaction goes via copper ions, which alternate between their states of Cu+ and Cu2+ during the redox series. The order of function of the respiratory chain can be demonstrated in 3 ways: by inhibiting certain steps, by reducing the entire chain and then introducing o2, or by calculating standard reduction potentials for the components. The latter can be determined by experiments which compare the reducing potentials of substances; electrons will flow from substances with lower to higher potentials. This means that substances with higher reducing potentials are better reducing agents and should be at the end of the electron transport chain. Indeed, the entire ETC should be in order of increasingly positive standard reduction potential, E'o. This is a theoretical order, but it can be shown to be true by use of the other two methods. It is possible to reduce the entire electron transport chain by removing the final electron acceptor, o2, from an experimental solution. The last carrier will therefore be unable to be oxidised, so all components will be reduced and electrons will cease to travel down the chain. We can measure which
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