ATPases (or ATP synthases) are membrane-bound enzyme complexes/ion transporters that combine ATP synthesis and/or hydrolysis with the transport of protons across a membrane. ATPases can harness the energy from a proton gradient, using the flux of ions across the membrane via the ATPase proton channel to drive the synthesis of ATP. Some ATPases work in reverse, using the energy from the hydrolysis of ATP to create a proton gradient. There are different types of ATPases, which can differ in function (ATP synthesis and/or hydrolysis), structure (F-, V- and A-ATPases contain rotary motors) and in the type of ions they transport [PMID:15473999, PMID:15078220].

• F-ATPases (F1F0-ATPases) in mitochondria, chloroplasts and bacterial plasma membranes are the prime producers of ATP, using the proton gradient generated by oxidative phosphorylation (mitochondria) or photosynthesis (chloroplasts).
• V-ATPases (V1V0-ATPases) are primarily found in eukaryotic vacuoles, catalysing ATP hydrolysis to transport solutes and lower pH in organelles.
• A-ATPases (A1A0-ATPases) are found in Archaea and function like F-ATPases.
• P-ATPases (E1E2-ATPases) are found in bacteria and in eukaryotic plasma membranes and organelles, and function to transport a variety of different ions across membranes.
• E-ATPases are cell-surface enzymes that hydrolyse a range of NTPs, including extracellular ATP.

Synonym(s): ATP synthase, bacterial Ca2+/Mg2+ ATPase, chloroplast ATPase, coupling factors (F0,F1 and CF1), F0F1-ATPase, F1-ATPase, F1F0H+-ATPase, H+-ATPase, H+-translocating ATPase, H+-transporting ATPase, mitochondrial ATPase, proton-ATP.

The H(+)-transporting two-sector ATPase (EC: 3.6.3.14) is a component of the cytoplasmic membrane of eubacteria, the inner membrane of mitochondria, and the thylakoid membrane of chloroplasts. The ATP synthase complex is composed of a nine-subunit (A-G, F6, F8) transmembrane channel through which protons are pumped (F0-complex), and a five-subunit (alpha, beta, gamma, delta, epsilon) catalytic core for ATP synthesis (F1-ATPase). The F1-ATPase uses the transmembrane proton motive force generated by photosynthesis or oxidative phosphorylation to drive the synthesis of ATP from ADP and phosphate. The F1-ATPase has been shown to be a rotary motor in which the central gamma subunit rotates inside the cylinder made of alpha3beta3 subunits, using the ATP as a driving force via an ATP binding and hydrolysis cycle [PMID:11309608].

F-ATPases (also known as F1F0-ATPase, or H(+)-transporting two-sector ATPase) (EC: 3.6.3.14) are composed of two linked complexes: the F1 ATPase complex is the catalytic core and is composed of 5 subunits (alpha, beta, gamma, delta, epsilon), while the F0 ATPase complex is the membrane-embedded proton channel that is composed of at least 3 subunits (A-C), nine in mitochondria (A-G, F6, F8). Both the F1 and F0 complexes are rotary motors that are coupled back-to-back. In the F1 complex, the central gamma subunit forms the rotor inside the cylinder made of the alpha(3)beta(3) subunits, while in the F0 complex, the ring-shaped C subunits forms the rotor. The two rotors rotate in opposite directions, but the F0 rotor is usually stronger, using the force from the proton gradient to push the F1 rotor in reverse in order to drive ATP synthesis [PMID:11309608]. These ATPases can also work in reverse to hydrolyse ATP to create a proton gradient.

This entry represents subunit A (or subunit 6) found in the F0 complex of F-ATPases. This subunit is a key component of the proton channel, and may play a direct role in the translocation of protons across the membrane. Catalysis in the F1 complex depends upon the rotation of the central stalk and F0 c-ring, which in turn is driven by the flux of protons through the membrane via the interface between the F0 c-ring and subunit A. The peripheral stalk links subunit A to the external surface of the F1 domain, and is thought to act as a stator to counter the tendency of subunit A and the F1 alpha(3)beta(3) catalytic portion to rotate with the central rotary element [PMID:16045926].

More information about this protein can be found at Protein of the Month: ATP Synthases [PUB00020719].