Stoichiometry of the respiratory chain
All the processes described above serve primarily to obtain energy from the substrate for useful work. It is therefore understandable that we are interested in how much energy mitochondria can extract from, for example, glucose or palmitic acid molecules.
In the previous sections, we dealt with the amount of electrons (in the form of reduced cofactors) taken during glucose metabolism. Similarly, we can find out their number for other substrates. So the question of mitochondrial stoichiometry is: how many molecules of ATP can we make in the ETC for a certain number of electrons transferred? We can further divide this question into two sub-questions:
- How many protons are transported across the membrane for one pair of electrons?' We transfer ten protons for two electrons. Complexes III and IV together transport six protons, and complex I will transfer approximately four protons per electron pair.
- How many protons must be moved back into the matrix to make one molecule of ATP? Here the answer is a bit more complicated. According to current models of F1·F0-ATP synthase, the H+/ATP ratio is approximately 4.33. This means that thirteen protons need to be transferred to produce three ATP molecules – ten via F1·F0-ATP synthase and the other three protons are used for the ANT translocator to import ADP and phosphate and to export ATP.
However, the mentioned numbers can only be achieved under optimal conditions, when all components of the ETC work flawlessly and the inner mitochondrial membrane is completely impermeable to protons. Of course, these conditions are not usual.