Active Transport of Na + and K + through Animal Cell Membranes

The transport of Na + out of the cell and K + into the cell against a concentration gradient is catalyzed by a (Na + + K + )‐activated ATPase. The way in which the cations pass through the cell membrane has not yet been elucidated. Studies on the ATP hydrolysis revealed a Na + ‐dependent phosphorylation of the enzyme protein; the conformation of the enzyme also appears to change. The energy required for transport of the cations against their concentration gradients is probably provided by K + ‐dependent hydrolysis of the enzyme‐bound phosphate. The enzyme can synthesize ATP from inorganic phosphate and ADP on reversal of the cation concentration gradient. By keeping the enzyme in a particular conformation, the cardiac glycoside ouabain specifically inhibits the Na + pump.