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Molecule of the Month. Sodium-Potassium Pump Cells continually pump sodium ions out and potassium ions in, powered by ATP Sodium-potassium pump with potassium ions green in the transport sites and a phosphate analogue yellow in the ATP-binding site.
The cell membrane is shown schematically in gray. Our bodies use a lot of energy. ATP adenosine triphosphate is one of the major currencies of energy in our cells; it is continually used and rebuilt throughout the day. Amazingly, if you add up the amount of ATP that is built each day, it would roughly equal the weight of your entire body. This ATP is spent in many ways: to power muscles, to make sure that enzymes perform the proper reactions, to heat your body.
The lion's share, however, goes to the protein pictured here: roughly a third of the ATP made by our cells is spent to power the sodium-potassium pump. The sodium-potassium pump PDB entries 2zxe and 3b8e is found in our cellular membranes, where it is in charge of generating a gradient of ions.
It continually pumps sodium ions out of the cell and potassium ions into the cell, powered by ATP. For each ATP that is broken down, it moves 3 sodium ions out and 2 potassium ions in.
As the cell is depleted of sodium, this creates an electrical gradient and a concentration gradient, both of which are put to use for many tasks. The most spectacular use of this gradient is in the transmission of nerve signals. Our nerve axons deplete themselves of sodium ions, then use special voltage-gated sodium channels to allow the ions to rush back in during a nerve impulse.
The sodium-potassium pump has the job of keeping the axon ready for the next signal. The gradient is also helps control the osmotic pressure inside cells, and powers a variety of other pumps that link the flow of sodium ions with the transport of other molecules, such as calcium ions or glucose. A traditional cure for heart failure works by blocking the sodium-potassium pump. Plant toxins like digitalis and ouabain PDB entry 3a3y and similar toxins from poisonous toads, collectively known as cardiotonic steroids, can be used in small doses to slow the pumping of ions.
As the level of sodium ions builds up inside the cell, this slows the sodium-calcium exchanger, leading to a build up of calcium, which ultimately increases the force of contraction of the heart muscle.
Recent research has revealed that our own cells make molecules similar to these toxins, but only in low concentrations to regulate the action of our sodium-potassium pumps. Several types of ion pumps powered by ATP. The sodium-potassium pump shown here from PDB entry 2zxe is one of a large class of P-type ATPase pumps, so called because they all incorporate a phosphate-linked intermediate in their mechanism. Several other examples are currently available in the PDB.
Many structures of the calcium pump are available PDB entry 1su4 is pictured here , showing how these pumps undergo large conformational changes through the pumping cycle. Other examples include the proton pump found in plant cell membranes PDB entry 3b8c , and a proton-potassium pump that acidifies the stomach PDB entry 3ixz , not shown here. The proton pump and the calcium pump are each composed of a single chain, whereas the pumps that transport potassium typically have a second smaller chain, shown here in turquoise.
The structure of the sodium-potassium pump also has a third regulatory chain, shown here in purple.
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