Please be patient while the structures in the left frame load. In order to display all of the structures in the tour properly, press 'View' buttons below in order (from 1 to the end).
Pumps are the active transporters: they require energy to catalyze the transport of cations through the cell membrane. The energy required for the pump function can come from light (for example, photosynthetic reaction centers and proton pumping), from a redox process (complexes I to III in mitochondrial membrane) or from hydrolysis of ATP (ATPase pumps). If the mechanism of ATPases involves a phosphorylated enzyme intermediate, then the ATPase belongs to a P-type ATPase family.
The sodium-potassium (Na +-K+) pump is an example of P-type ATPase pump that moves three Na+ ions out and two K+ ions into the cell for each ATP hydrolyzed. The action of Na +-K+ pump maintains a resting membrane potential of -30 mV to -70 mV in mammalian cells. (The potential is negative on the inside of the membrane.)
During the pumping cycle, the pump alternates between two major conformations E1 and E2 (E stands for enzyme). In the E1 conformation, the metal binding sites have high affinity for the metal cations and are open to the cytoplasm. The E2 conformation opens the same metal binding sites to the extracellular environment and changes the metal binding affinity to low. The metal ions are not transported through the membrane but are held at fixed positions within the protein structure while the protein exposes the binding site alternatively to the extracellular and intracellular sides of the membrane. The pump adopts several different states (also known as cycle intermediates or pump forms) in each conformation that differ based on phosphorylation and cations bound. To see these different states and a proposed mechanism, click on thumbnail below.
The display in the left frame shows a ball-and-stick model of the structure of Na +-K+ pump in its E2.2K+.Pi state isolated from shark rectal glands. The protein consists of three different subunits making it an αβγ heterotrimer.
Note that the secondary structure of all subunits is almost exclusively composed of α helices. This helix-rich secondary structure provides the protein with flexibility necessary for achieving two distinct conformations.
The α - Subunit
The α-subunit of this Na +-K+ pump consist of four distinct domains.
nucleotide binding domain (or N-domain) is found in the cytoplasm. It is in charge of binding the ATP and of phosphorylation of P-domain.
actuator domain (or A-domain) is the protein phosphatase. It is connected to the upper parts of the α subunit through several very flexible hinges (upper part of the domain). This connection allows the A-domain to move relatively freely relative to the rest of the subunit.
phosphorylation domain (or P-domain). This domain is highly conserved among all P-type ATP-ases.
4]2- is found in close proximity to Asp376. This anion is frequently used as a mimic for free inorganic phosphate (Pi) in protein crystallography.
+/K+ binding site is located approximately in the middle of T-domain.
+ cation closer to the surface of the protein is coordinated by three mainchain carbonyls (Ala330, Val332 & Val329) and three side chain oxygens (Asn783, Glu786 & Asp811). The geometry at this K+ is distorted octahedral.
+ center is distorted square pyramidal. Note that oxygens from Asn783 and Asp811 carboxylate groups serve as bridging ligands between two potassium sites.
Three sodium cations bind in the same pocket, but the exact locations and coordinating residues are unknown due to the lack of crystallographic data on sodium-bound Na+-K+ pump.
The β - Subunit
+ affinity: after a complete or partial removal of this domain the affinity for the two cations drops although the pump still performs its function properly. The exact mechanism of the affinity control remains unknown.
The γ - Subunit
+/K+ and some other pumps and regulate their activity in a tissue as well as isoform specific way.
Click on the thumbnail below to see a visual summary of the Na+-K+-ATPase pump structure:
+-K+ pump: feel free to play with this and any other display in this tour.