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  Section: General Biochemistry » Ion Transport Across Biological Membranes
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Inorganic Ion Transport Against A Concentration Gradient At Expense of Atp Hydrolysis


Most cells have a high concentration of K+ and a low concentration of Na+ inside the cell membrane relative to the concentration of these ions bathing the cell. As discussed, the rapid flow of these ions along their concentration gradient across the cell membrane is used in signal transduction between the cells of an organism, and plays an important role in life. To maintain the concentration gradient of sodium and potassium ions and thus the resting membrane potential of the cells, energy is needed. In 1979, Skou and Norby discovered an enzyme that reestablishes the original concentration gradient. It is called the Na+–K+ ATPase where ATP stands for adenosine triphosphate, an energy source. Sodium ions are moved from inside of the cell to the outside and potassium ions aremoved in the opposite direction. With each cycle of the enzyme, one molecule of ATP is hydrolyzed and two potassium ions are moved into the cell and three sodium ions moved out.

Na+–K+ ATPase is a membrane protein with two subunits spanning the membrane. The current hypothesis is that the enzyme reacts with ATP to give a phosphorylated enzyme and ADP (adenosine diphosphate). The conversion of ATP to ADP (adenosine diphosphate) and Pi (inorganic phosphate has been formulated as follows:

ATP + Enzyme → ADP +P-Enzyme (phosphorylated enzyme) P-Enzyme + H2O → Enzyme + Pi

The resulting enzyme has a high affinity for K+ and a low affinity for Na+. Hydrolysis of the phosphorylated enzyme results in the liberation of inorganic phosphate and the regeneration of the enzyme form with high affinity for Na+ and low affinity for K+. The net result is the movement of two K+ ions into the cell and three Na+ ions out of the cell for each ATP hydrolyzed. About 25% of the energy consumption of a human at rest is used to maintain the resting concentration of sodium and potassium ions in cells.


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