^{Structure and action of the Na+ /K+  -ATPase}

The Na⁺ /K⁺ -ATPase is an integral membrane protein consisting of α55 kDa β and 110 kDa subunits.  The functional unit is either a heterotetramer (α2β2) or, more likely, a heterodimer. Upon hydrolysis of one molecule of ATP to ADP and Pi  the Pi  transiently binds to an aspartyl residue in the protein, the protein undergoes a conformational  change and three Na⁺ ions are pumped out of the  cell  across  the  plasma  membrane  and  two  K⁺ ions  are  pumped  in the opposite  direction  into the cell. Both ions are being moved up their concentration  gradients  across  the  membrane;  hence  the  requirement   for  an  input  of energy.   No   transport   occurs   unless   ATP   is   hydrolyzed,   and   no   ATP   is hydrolyzed if there is no Na+ and K+ to transport (i.e. it is a coupled system).