The Na+/K+ ATPase
Na+/K+ ATPase is a pump that uses the energy from the hydrolysis of ATP to actively pump sodium and potassium ions against their concentration. The binding of three sodium ions to internal binding sites on the cytoplasmic side of the pump and phosphorylation of the pump induces a conformational change that releases the Na+ ions into the extracellular space. The subsequent binding of two potassium ions to the pump within the synapse followed by dephosphorylation causes it to return to its original confirmation and release the potassium ions into the cytoplasm. This pump is found in almost all cells of the body and the ionic gradient that it creates is used to carry out numerous important cellular processes, such as the transport of other molecules across membrane eg the monoamines.
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References
Membrane transport of small molecules and the electrical properties of membranes. In: Molecular Biology of the Cell, 4th edition. Alberts B et al. New York, USA: Garland Science, 2002: 615–657.