Proteins

As noted above, a protein's activity can be regulated when it undergoes a change in its conformation. A dramatic and extensively studied model of protein conformational change is that of the Na⁺/K⁺ ATPase pump. This is an integral membrane protein with one side facing the exterior of the cell and the other facing the cytosol. It is used for the specific transport of sodium or potassium across the membrane, and one of its most important functions is the repolarization of a nerve fiber after it "fires."

The first step in the transport process is the binding of three Na⁺ (sodium) ions to the inside face of the protein. This is followed by protein phosphorylation using ATP, which causes the protein to change its conformation. This moves the sodium ions from the cytosol to the exterior. This conformational change also opens up exterior binding sites, which tightly bind two potassium ions outside the cell. Following the potassium binding, the protein is dephosphorylated, losing its recently added phosphate group. This dephosphorylation then changes the protein back to the original conformation, causing the protein to loosen its binding of potassium and deliver those two ions to the cytosol. This process demonstrates that protein structure can be reversibly changed. The net result is that the inside of the cell develops a slight negative charge compared to the outside. The disruption of this "polarized" state constitutes nerve cell firings, which allow the cells of the nervous system to communicate with one another.