Transmembrane ATPase

Transmembrane ATPases are integral membrane proteins that use energy from the hydrolysis of adenosine triphosphate (ATP) to transport ions and other solutes between the solutions on either side of a biological membrane and against a solute's natural direction of flow.

Transmembrane ATPases import many of the metabolites necessary for cell metabolism and export toxins, wastes, and solutes that can hinder cellular processes. An important example is the sodium-potassium exchanger (or Na⁺/K⁺ATPase), which establishes the ionic concentration balance that maintains the cell potential.

Besides exchangers, other categories of transmembrane ATPase include cotransporters and pumps (however, some exchangers are also pumps). Some of these, like the Na⁺/K⁺ATPase, cause a net flow of charge, but others do not. These are called "electrogenic" and "nonelectrogenic" transporters, respectively.

The coupling between ATP hydrolysis and transport is more or less a strict chemical reaction, in which a fixed number of solute molecules are transported for each ATP molecule that is hydrolyzed; for example, 3 Na⁺ ions inward and 2 K⁺ ions outward per ATP hydrolyzed, for the Na⁺/K⁺ exchanger.

Transmembrane ATPases harness the chemical potential energy of ATP, because they perform work: they transport solutes in a direction opposite to their thermodynamically preferred direction of movement—that is, from the side of the membrane where they are in low concentration to the side where they are in high concentration. This process is considered active transport.

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The ATP synthetase (or ATP synthase) of mitochondria and chloroplasts is an anabolic enzyme that harnesses the energy of a transmembrane proton gradient as an energy source for adding an inorganic phosphate group to a molecule of adenosine diphosphate (ADP) to form a molecule of adenosine triphosphate (ATP). ATP synthetase can also function in reverse; that is, use energy released by ATP hydrolysis to pump protons against their thermodynamic gradient.