ATP synthase

Definition
noun, plural: ATP synthases
An enzyme that catalyzes the formation of ATP from the phosphorylation of ADP with inorganic phosphate, using a form of energy, such as the energy from a proton gradient.
Supplement
This enzyme consists of two major segments: F_o portion, the transmembrane proton channel and F₁ portion, the catalytic component. The proton channel component allows the diffusion of protons (hydrogen ions) from an area where there are more hydrogen ions to an area where there are less hydrogen ions due to a proton gradient. As the proton (H+ ion) moves down the concentration gradient this moves the enzyme in a spinning motion, which brings ADP and inorganic phosphate together to form a bond, thus creating ATP molecule. The resulting ATP molecule is released so that a new ADP molecule can enter for another phosphorylation.
This enzyme can also work in reverse using the energy release from ATP hydrolysis to pump protons against their thermodynamic gradient. Since it can also use ATP as a source of energy it is also aptly called ATP synthetase.
See also: chemiosmosis