The Na⁺/K⁺ pump:

The Na⁺/K⁺ pumping between inside and outside a cell is assisted by a membrane-bound Na+/K+ - ATPase enzyme that catalyzes the movement of ions in the both direction across a cell. Through this pumping process the concentration difference of the ions inside and outside a cell is maintained and hence, a constant cell potential achieved.

Figure 7.4. A general principle for Na+/K+ pumping carried out by Na+/K+ - ATPase enzyme.

The Na+/K+ - ATPase enzyme is a tetrameric protein with two α- subunits and two β- subunits.

The α- sub units are close and contact to each other, while β- subunits are situated apart from each other (Figure). The ions exchange proceeds via following steps;

Step 1. Inside the cell three Na⁺ ions bind to the α- subunits of the enzyme which is consisting of six oxide units as binding site.

Step 2. The attachment of Na⁺ ions to the α- subunits changes the local polarity that helps an ATP to bind with a α- subunit. The bound ATP is hydrolyzed to ADP and acovalently bound with a phosphate ester (P). Due to this phosphorylation a conformation change (the inside cavity is closed and outside cavity is opened) took place in the α- subunit.

Step 3. The weakly bound three Na⁺ ions are released outside the cell.

Step 4 and Step 5. Two K⁺ ions outside the cell bind to the open cell cavity and simultaneously dephosphorylation takes place. Because of this a configurational change happens and two K+ ions move inside the cell with the help of a - subunits.

Step 6. The bound two K⁺ ions are released inside the cell and initial configuration is achieved by the enzyme.

Use: In the pumping process three positively charged Na⁺ ions are released from the cell and two positively charged K⁺ ions are accepted inside the cell. Hence, a charge difference is produced by the process. This charge difference creates potential gradient across the cell. For good example, signal transmission across neuron cells.