A molecular switch is a protein that either enables or disables a signaling cascade by acting on one of the proteins involved in the cascade. There are two classes of proteins that can act as switches by turning a signal "on" or "off" in this way: GTPase switch proteins, which are enzymes that bind GTP and hydrolyze it to form GDP, and protein kinases, which are usually enzymes that transfer a phosphate group from ATP to a protein. Some classes of protein kinases may also use GTP.
The first type of molecular switches are GTPase switch proteins. These enzymes remove a phosphate group from GTP and convert it to GDP. When a switch protein is bound to GTP, it is active, or in the "on" position, and turns on the signaling cascade. When a switch protein is bound to GDP, it is inactive, or in the "off" position, and the signaling cascade stops. Because the switch protein is an enzyme that converts GTP to GDP, turning the switch from "on" to "off" is regulated by the length of time it takes the GTPase to hydrolyze GTP to GDP.The second type of molecular switch is a protein kinase. Protein kinases can be built into the signal receptor, or they can be separate enzymes located in the cell membrane or cytoplasm. No matter their location, protein kinases remove a phosphate group from an ATP molecule and transfer it to a tyrosine, serine, or threonine group in another protein, one that is part of the signaling cascade. The protein can only function in the signaling cascade when it contains the phosphate group. Thus, when the protein is phosphorylated, the signal is switched "on," and when the protein is dephosphorylated, the signal is "off." In contrast to GTPase switch proteins that self-regulate the switch activity, protein kinase switches are regulated by protein phosphatases, which are enzymes that remove phosphate groups from proteins. These switches drive the entire range of activities within cells.