ATPase
 
 Any enzyme that catalyzes the hydrolysis of ATP to ADP and inorganic phosphate (Pi) is classified as an ATPase. In some cases, the phosphate is transiently transferred to the protein before its release as a product. The hydrolysis of ATP liberates much energy, so this reaction is usually coupled to another, energetically unfavorable reaction. Three major types of ATPase are associated with membranes that couple ATP hydrolysis to the translocation of specific ions across a membrane: P- V-, and F-ATPases.
P-ATPases have a relatively simple polypeptide composition (one or two subunits) and are phosphorylated as part of their catalytic cycle. Examples of P-ATPases are (1) the Na⁺, K⁺ – ATPase of the plasma membrane of animal cells; (2) the H⁺ – ATPase of the plasma membranes of yeast, fungi, and plants; and (3) the Ca²⁺ – ATPase of the sarcoplasmic reticulum (the endoplasmic reticulum of muscle). The plasma membrane Na⁺, K⁺ – ATPase and H⁺ – ATPase function to generate and maintain the plasma membrane electrical potential difference (inside negative), as well as the ionic disequilibria across that membrane. Ca²⁺ – ATPases function in Ca²⁺ homeostasis.
V-ATPases have a much more complicated polypeptide composition than do P-ATPases and are not phosphorylated during catalysis. V-ATPases are found on the membranes within the interiors of eukaryotic cells (endomembranes), including membranes from vacuoles (from which the “V” is derived), lysosomes, Golgi, secretory vesicles, clathrin-coated vesicles, and, in some instances, plasma membranes. V-ATPases are H⁺ – ATPases that show some similarity to the proton-linked ATP synthases. The function of V-ATPases is to catalyze proton transport into the endomembrane interior compartments at the expense of ATP hydrolysis. Acidification of the interior of endomembranes is required for some of their functions. In archaebacteria, an enzyme with similarity to V-ATPases functions as an ATP synthase.
F-ATPases (also known as “F₁–F₀”) have a complex polypeptide composition and, as in V-ATPases, there is no phosphorylated intermediate in the reaction mechanism. In nonphotosynthetic eukaryotes, the F-ATPase is found exclusively on the inner membrane of mitochondria, whereas in green plants and algae there are two distinct F-ATPases; one in mitochondria and the other on the thylakoid membrane of chloroplasts. In bacteria, F-ATPase is present in the plasma membrane. ATPases couple the flow of protons (or Na⁺ in some cases) to ATP hydrolysis and synthesis. The activity of the F-ATPases is very tightly regulated. Several mechanisms combine to prevent wasteful ATP hydrolysis by F-ATPase, but to allow rapid ATP synthesis at the expense of electrochemical proton (or Na⁺) potentials generated by proton translocation linked to electron transport. F-ATPases operate in vivo as ATP synthases; consequently, the term ATP synthase is preferred to F-ATPase when referring to the entire enzyme. The catalytic portion of the ATP synthase may be readily removed from coupling membranes and purified as a large 400-kDa water-soluble protein. Such preparations may have high ATPase activity and are called “F₁” or “F₁-ATPase”.