The body produces many different hormones, but each hormone affects only its target cells. Target cells are specific cells to which a hormone travels to produce a specific effect. Target cells have receptors—proteins that bind specific signal molecules that cause the cell to respond. Each receptor binds to a specific hormone. When a hormone binds to a receptor, the binding triggers events that lead to changes within the cell. Receptors can be found on the cell membrane, in the cytoplasm, or in the nucleus of a cell.
Most amino acid-based hormones bind to receptor proteins on the cell membrane. Thus, the hormone acts as a "first messenger." As the example in Figure 50-1 shows, the resulting hormone-receptor complex activates an enzyme that converts ATP to cyclic AMP (cAMP). Cyclic AMP, in turn, activates additional enzymes and proteins inside the cell. Thus, the hormone acts as a "first messenger" and cAMP acts as a "second messenger." A second messenger is a molecule that initiates changes inside a cell in response to the binding of a specific substance to a receptor on the outside of a cell. In addition to cAMP, cells have other second messengers.
Because steroid and thyroid hormones are fat soluble, they diffuse through the cell membranes of their target cells and bind to receptors in the cytoplasm or nucleus. The hormone-receptor complexes cause the cells to activate existing enzymes or to initiate synthesis of new enzymes or proteins. Figure 50-2 shows how a hormone-receptor complex binds to DNA, activates transcription of mRNA, and stimulates production of new proteins. The proteins cause changes in the target cell.
Receptor protein for glucagon
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