![]() ![]() ATP is then cleaved and the γ-phosphate is transferred to. Binding of three Na + ions causes a conformational change that rotates the N domain so that the γ-phosphate of ATP is positioned near the phosphorylation site of the P domain. In the E1 state, ATP is bound at the N domain and high affinity Na + binding sites are open to the cytosol. An animation by Mark Hilge illustrating this mechanism can be found here E1 and E1-P No crystal structure is available for the E1 structure, but many aspects of it have been predicted based on homology to SERCA. The crystal structure of the E2 conformation in its phosphorylated state has been reported. The sodium-potassium pump transports cations across the membrane using what is the "alternative access" model, in which the protein alternates between two conformations, E1 and E2. Regulation of ion pumping action by FXYD has been shown to be tissue and isoform specific. The FXYD subunit, sometimes known as the γ-subunit, is an accessory regulatory protein comprised of a transmembrane α-helix and an extracellular domain (which is not shown in this structure). It plays a role in providing binding specificity for potassium ions. The β-subunit has important roles in targeting the polypeptide to the membrane and in providing stability. Contact between the α and β subunits also occurs at various residues in the extracellular domains. These residues also make contact with a cholesterol molecule, the presence of which is necessary for ion transport to occur. This subunit uses a to bind to the M7 and M10 helices of the α-subunit within the lipid bilayer. The β-subunit is a single spanning membrane protein with a transmembrane α-helix and a glycosylated extracellular domain. This diversity can influence the rate ion transport and the ability to act as a signaling receptor. The majority of structural diversity among the isoforms occurs at the N-terminus, the first extracellular loop, and the third cytosolic domain. There are 4 known isoforms of the α-subunit, but even the two most divergent isoforms share 78% sequence identity. Additionally, there are located on the cytoplasmic face of the membrane: the actuator domain (A), the nucleotide-binding domain (N), and the phosphorylation domain (P). The binding sites for K + and Na + are located within the transmembrane helices. These helices are centered around a three helix bundle formed by M4-M6. This subunit is composed of 10 transmembrane α-helices (M1-M10). The α-subunit is the largest subunit and contains the binding sites for Na +, K +, and ATP. Overall, the structure of the sodium-potassium-pump is a transmembrane protein with three subunits labeled α, β, and FXYD. The Na +-K + pump is a P-type ATPase with a structure similar to the H +-K +-ATPase and the sarco(endo)plasmic reticulum Ca 2+-ATPase (SERCA). 4.2.3 Interaction with Structural Proteins.1 Introduction to Sodium-Potassium-ATPases.See also Sodium-potassium ATPase (Hebrew). įor Na/K ATPase complex with Ouabain see Ouabain. Skou and he was awarded the Nobel Prize in Chemistry in 1997 for this discovery. The sodium-potassium pump was first described in 1957 by Jens C. In addition to its role as a transport protein, the sodium-potassium-pump has also been shown to act as a receptor for cardiotonic steroid signaling. ![]() The number of expressed Na/K pumps differs by cell type but is generally between 80,000 and 30 million. Purified Na-K-ATPase has been shown to have turnover rates of 8,000 to 10,000 cycles per minute, though in cultured cells the turnover rate was decreased to between 1,500 and 5,000 cycles per minute. This enzyme not only pumps ions against their gradients, but does so rather efficiently. In each cycle of ATP hydrolysis, the protein transports three Na + ions out of the cell and two K + ions across the plasma membrane into the cell. These gradients provide energy for several cellular functions including control of membrane potential and cell size, pH homeostasis, and uptake of nutrients and water. The Na-K pump is found on the surface of all animal cells and is a major force in maintaining the concentration gradients of these ions across the membrane. The sodium-potassium-ATPase, also known as the Na-K pump or the sodium pump, is the protein responsible for the ATP-dependent, coupled transport of sodium and potassium ions across the plasma membrane. ![]()
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