6.3 How is electron transport coupled to proton pumping?
6.3.1 Stoichiometry of proton ejection
As pointed out above, some of the protons that are being expelled from the mitochondrion are accepted from the hydrogen carriers NADH and ubiquinone and have the privilege of traveling together with electrons for a part of the journey. However, at some point they must part company, and the protons must be expelled, whereas the electrons are retained. Also, more protons are being expelled than can be accounted for by the hydrogen carriers. In particular, complex IV (cytochrome c oxidase) does not interact with any hydrogen carriers yet expels up to four protons for each pair of electrons accepted. So, there must be mechanisms that extract energy from the transfer of proton-less electrons between electron carriers and apply it towards the expulsion of electron-less protons. How does this work?
The experimental evidence on this point is mostly not very precise. Scientists (including textbook authors) often believe that speculation is bad, and so they simply skip this question. However, I feel it is too important to be ignored, so I will present a crude and contrived conceptual model to provide an idea how things might work (Figure 6.3-1). The basic idea is that binding and release of electrons should be able to cause conformational changes to a protein. This would be entirely analogous to conformational changes caused by allosteric effectors binding to enzymes, or to phosphate groups binding to muscle protein (myosin light chain). An electron carries a charge, a charge causes a field, and a field will cause forces that act on charges on the protein, so there really is no difficulty whatsoever in assuming that migrating electrons cause conformational changes.
An obvious shortcoming of the model presented in Figure 6.3-1 is that it links the transport of a single proton to the transport of a single electron. In reality, the number of protons is higher; to account for this, we can envision multiple similar valves connected in series witin a single pump molecule.