Na+/K+-ATPase

Na⁺/K⁺-ATPase (sodium-potassium adenosine triphosphatase, also known as the Na⁺/K⁺ pump or sodium–potassium pump) is an enzyme (an electrogenic transmembrane ATPase) found in the plasma membrane of all animal cells. It performs several functions in cell physiology.Mechanism of the sodium-potassium exchange pump. Na⁺/K⁺-ATPase (sodium-potassium adenosine triphosphatase, also known as the Na⁺/K⁺ pump or sodium–potassium pump) is an enzyme (an electrogenic transmembrane ATPase) found in the plasma membrane of all animal cells. It performs several functions in cell physiology. The Na+/K+-ATPase enzyme is a solute pump that pumps potassium into cells while pumping sodium out of cells, both against their concentration gradients. This pumping is active (i.e. it uses energy from ATP). For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are imported; there is hence a net export of a single positive charge per pump cycle. The sodium-potassium pump was discovered in 1957 by the Danish scientist Jens Christian Skou, who was awarded a Nobel Prize for his work in 1997. Its discovery marked an important step forward in the understanding of how ions get into and out of cells, and it has particular significance for excitable cells such as nerve cells, which depend on this pump to respond to stimuli and transmit impulses. An alternative theory, Ling’s adsorption theory, posits that the membrane potential and action potential of a living cell is due to the adsorption of mobile ions onto adsorption sites of cells. All mammals have four different sodium pump sub-types, or isoforms. Each has unique properties and tissue expression patterns. The Na+/K+-ATPase helps maintain resting potential, affects transport, and regulates cellular volume. It also functions as a signal transducer/integrator to regulate the MAPK pathway, ROS, as well as intracellular calcium. In fact, all cells expend a large fraction of the ATP they produce (typically 30% and up to 70% innerve cells) to maintain their required cytosolic Na and Kconcentrations.( Voet Biochemistry section 20-3 p759)For neurons, the Na+/K+-ATPase can be responsible for up to 3/4 of the cell's energy expenditure. In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). The sodium-potassium pump mechanism moves 3 sodium ions out and moves 2 potassium ions in, thus, in total, removing one positive charge carrier from the intracellular space (please see Mechanism for details). In addition, there is a short-circuit channel for potassium in the membrane, thus the voltage across the plasma membrane is close to the Nernst-potential of potassium. Export of sodium from the cell provides the driving force for several secondary active transporters membrane transport proteins, which import glucose, amino acids, and other nutrients into the cell by use of the sodium gradient. Another important task of the Na+-K+ pump is to provide a Na+ gradient that is used by certain carrier processes. In the gut, for example, sodium is transported out of the reabsorbing cell on the blood (interstitial fluid) side via the Na+-K+ pump, whereas, on the reabsorbing (lumenal) side, the Na+-glucose symporter uses the created Na+ gradient as a source of energy to import both Na+ and glucose, which is far more efficient than simple diffusion. Similar processes are located in the renal tubular system.