The mitochondrial ATP synthase (F-ATP synthase) is an enzyme which is located in the inner mitochondrial membrane whose function is closely related to the mitochondrial respiratory chain. Because of this, some authors call it Complex V, though most do not use this nomenclature. Personally, I also think it should not be called Complex V, since the complexes are, in my opinion, those involved in the transport of electrons, and this process ends in oxygen (in the complex IV).
As its name suggests, the ATP synthase will catalyze the synthesis of ATP molecules through the process of oxidative phosphorylation. That is, to conduct the phosphorylation of ADP to ATP it is necessary an oxidation, which in this case involves the use of NADH or FADH2 in the mitochondrial respiratory chain.
The general reaction of operation of ATP synthase is:
ADP + Pi → ATP
This reaction is quite endergonic and therefore requires energy to occur. And where does this energy come from? From the H+ gradient created during the operation of the respiratory chain complexes. Therefore, the energy liberated during the transport of electrons is used to create an accumulation of H+ in the intermembrane space, and then these ions will tend to return to the matrix, causing the release of energy. This is the energy that is used to produce ATP.
ATP synthase has two different subunits:
- Fo subunit which is a transmembrane subunit with a pore through which the H+ return to the matrix. As a curiosity, the name is Fo (and not F "zero"), because the "o" derives from the fact that this subunit binds to oligomycin, which is an antibiotic.
- Subunit F1, the catalytic subunit which is responsible for ATP synthesis and is located in association with the matrix side of the inner mitochondrial membrane. Paradoxically, this subunit has an ATPase activity (ATP hydrolysis) when isolated, but when in contact with the inner mitochondrial membrane and specifically to Fo subunit, it has the activity of ATP synthesis.