Cell Physiology and Molecular Biophysics
The Na,K-pump and Active Transport
The Na,K-pump is a plasma membrane-spanning protein complex that generates the electrochemical gradients for Na+ and K+ which exist in animal cells. The pump extrudes Na+ from the cell and accumulates K+ at the expense of metabolic energy derived from ATP. Indeed, Na+,K+-stimulated hydrolysis of ATP (Na,K-ATPase) is often used as a measure of pump function. Its catalytic turnover and the resulting electrochemical gradients are responsible for the potential difference across the plasma membrane and indirectly control cellular volume. The transmembrane Na+ gradient produced by the Na,K-pump is also utilized by a number of cotransport systems to regulate the intracellular concentrations of other ions and solutes (e.g., Ca++, sugars).The pump consists of two dissimilar subunits: a 110,000-dalton alpha subunit and a 55,000-dalton beta subunit. The subunits are present in equimolar amounts, and the likely structure of the enzyme complex is an (alpha-beta)2 dimer. The alpha subunit contains the binding sites for the substrates required by the enzyme and exists as a phosphorylated intermediate during the catalytic cycle. The role of the beta subunit is unknown, although it is necessary for normal pump function and may play a role in stabilizing alpha.
Jens Skou, the discoverer of Na,K-ATPase, was awarded the 1997 Nobel Prize in Chemistry.
A course outline on the Na,K-ATPase and other P-type ATPases is also available.