ATP synthetase (ATPase, F0F1-ATPase, H+-ATPase) is a large protein complex. Its mass is about 500000 dalton. It penetrates through the whole width of the inner membrane of mitochondria and consists of 9 different polypeptide chains. It carries out interconversion of energy of the electrochemical proton gradient and chemical (phosphate) bonds. Five polypeptide chains form the spherical head going into the matrix. It is called F1-ATPase. The part of the ATP synthetase that is built into the membrane is correspondingly called F0-ATPase.

ATP synthetase contains transmembrane channels for protons. When protons move through these channels "down" the electrochemical gradient, synthesis of ATP is carried out. Proton energy is used for phosphorylation of ADP and converting it into ATP. In the matrix protons meet oxygen and form water. Thus, oxidation of hydrogen is accompanied by phosphorylation of ADP, that is why the process took the name oxidative phosphorylation.

The action of ATP synthetase is reversible: it can use the energy of the ATP hydrolysis for pumping protons through the inner mitochondrial membrane.