ATP synthase is a fairly complex membrane-bound enzyme present in the inner mitochondrial membrane which uses the energy exerted by an electrochemical proton gradient to synthesise ATP.This is therefore an important enzyme in the process of oxidative phosphorylation whereby activated carriers such as NADH and FADH2 transport electrons to the Electron Transfer Chain in order to generate an electrochemical proton gradient across the inner mitochondrial membrane. The ATP synthase enzyme allows protons to flow back down their concentration gradient by serving as a hydrophilic pore in the membrane. This energetically favourable reaction is used to drive the synthesis of ATP from ADP and Pi - an energetically unfavourable reaction.
ATP synthase structure
ATP synthase is an unusual enzyme. It has the characteristic shape of a lollipop, with the lollipop head protruding into the matrix where there is a low proton concentration. This is the F1 catalytic portion consisting of several alpha and beta subunits. Each beta subunit has a binding site for ADP and Pi. These are joined to the F0 transmembrane unit by a rotating stalk and held into place by an arm connecting the catalytic unit to the proton conducting unit. The rotatory motion of the enzyme converts proton-motive force into mechanical energy that can be used to synthesise ATP.
The F0 proton conducting unit consists of identical subunits which each have an aspartate residue. Although aspartate is initially hydrophilic, it becomes hydrophobic when it accepts H+ onto its COO- side group. This is delivered through a proton half channel from the intermembrane space. This causes the subunit to move away from the aqueous half channel, where the next subunit will accept H+. This will continue until the transmembrane unit is fully rotated and H+ dissociates out of another proton half channel into the matrix. The energy from the rotating stalk is used to induce a conformational change in the beta subunits of the F1 portion which allows the release of ATP.
Alberts B., Johnson A., Lewis J., Rafi M., Roberts K., Walter P. (2007) Molecular Biology of the Cell, 5th edition, New York: Garland Science, pages 821-822