The three-dimensional structure of the potassium channel has also revealed the presence of four K+-binding sites in the constricted region. What forces potassium ions to exit the protein on the extracellular side of the membrane?
a. The multiple sites in the constricted region overlap to some extent, which means more than one site cannot be occupied at the same time.
b. Binding of a second potassium ion to a site adjacent to the first potassium ion's binding site creates an electrostatic repulsion that helps to push the first potassium ion onto the next binding site.
c. The strength of the interaction between the protein and the potassium ions becomes weaker as the ions move through the region of the selectivity filter.
d. The potassium channel undergoes a conformational change that forces any interacting potassium ion to exit the protein into the extracellular space.